Physics A-Level

Physics A-Level glossary

2228 key terms for Physics A-Level, A to Z, each linked to the topic pages that use it.

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2228 terms found. Open a term to try a short memory check.

A

A-scan: Displays the amplitude of reflected ultrasound pulses against time or depth on a single axis, producing a one-dimensional trace with peaks corresponding to…AQA, OCR
Absolute and percentage uncertainties: Absolute uncertainty carries the same unit as the quantity, while percentage uncertainty is the absolute uncertainty divided by the measured value, multiplied…OCR
Absolute electric potential: The work done per unit positive charge in bringing a small positive test charge from infinity to that point, so V = W/Q.AQA
absolute magnitude: A logarithmic measure of luminosity shown on the vertical axis, typically spanning roughly –10 at the top to +15 at the bottom, with values increasing…AQA
absolute motion: Motion relative to an aether filling space, which provides a preferred frame against which such motion could be measured.AQA
Absolute scale of temperature: Also called the thermodynamic scale, it does not depend on property of any particular substance.OCR
Absolute temperature: Thermodynamic temperature in kelvin, always converting T to K by adding 273 rather than using degrees Celsius.OCR
Absolute temperature and mean kinetic energy: Raising T increases mean molecular kinetic energy, so molecules hit the walls harder and more often.AQA
Absolute uncertainty: The value added to and subtracted from a plotted value to give the upper and lower limits of a bar.AQA
Absolute zero: The temperature at which pressure would be zero for an ideal gas, found as the intercept on the T-axis when the line is extrapolated to p = 0 (0 K or −273 °C).AQA, OCR
Absorber: Material placed between source and detector, such as paper or thin card for alpha, aluminium sheets of varying thickness for beta, and lead sheets for gamma.OCR
Absorption: The loss of optical power as light is absorbed by the fibre material or impurities, converting energy to other forms such as thermal energy.AQA
Absorption and shielding: Beta particles need a few millimetres of a low-Z absorber like aluminium, as high-Z materials like lead increase bremsstrahlung X-ray production; gamma rays…AQA
Absorption characteristics: Absorption of beta and gamma is exponential with thickness, while alpha has a definite range.OCR
Absorption experiment: A source is placed near a detector with an absorber between them; counts are recorded with and without the absorber, and the background count is subtracted.AQA
Absorption of radiation: Process in which the detected count rate depends on how much material the radiation passes through, so more material absorbs more radiation.AQA
Acceleration: The vector rate of change of velocity, produced by a resultant force, and it can be positive or negative relative to a chosen direction.AQA
Acceleration due to gravity: The acceleration g of a body in free fall, equal to about 9.81 m s⁻² near the Earth's surface and directed vertically downwards.OCR
Acceptance angle: The angle at the entrance face that determines whether an incoming ray can be guided; rays entering too steeply are refracted out through the cladding rather…AQA
Accommodation: A change in lens focal length or power, with ciliary muscles relaxing for distant objects and contracting for near objects.AQA
Accretion: A process through which supermassive black holes grow over billions of years, alongside mergers, emitting intense radiation from accreting matter.AQA
Accretion discs and black hole mass: A quasar is powered by accretion onto a supermassive black hole of roughly 10⁶ to 10¹⁰ solar masses at the centre of a galaxy.AQA
Accuracy: Describes how close a measurement is to the true value.OCR
Accuracy and precision: Considering accuracy, resolution and zero error, and recording the value with the correct unit and an appropriate number of significant figures.OCR
Accurate scientific drawing: Drawing symbols correctly, such as checking the direction of the triangle and the position of the bar when drawing a diode.OCR
Acoustic impedance: Echoes return from boundaries where this changes between tissues, and its difference between two media determines the amplitude of the returning echo.AQA, OCR
Acoustic impedance and reflection: Echo amplitude depends on the acoustic impedance difference at a boundary, so larger mismatches produce stronger echoes.AQA
Acoustic impedance mismatch: A difference between the acoustic impedances Z₁ and Z₂ of two media; a large mismatch gives strong reflection, and if Z₁ = Z₂ the numerator is zero so no…OCR
Acoustic matching: The technique of choosing a coupling medium whose acoustic impedance is close to that of the skin, so that ultrasound passes efficiently from the transducer…OCR
Action at a distance: Non-contact forces explained by the concept of a field, which acts through empty space without needing a medium.OCR
Action–reaction pairs: If body A exerts a force on body B, then B exerts an equal-magnitude force on A in the opposite direction, acting on a different body.AQA, OCR
Active galactic nuclei: Extremely luminous objects powered by supermassive black holes accreting matter, of which quasars are an example.AQA
Activity: The number of nuclear decays per second, measured in becquerels (Bq).OCR
Addition and subtraction rule: For addition and subtraction, combine the absolute uncertainties by adding them, keeping the unit of the quantity.AQA
Adiabatic: A process in which no heat enters or leaves, so work done changes internal energy and temperature.AQA
Adiabatic change: No heat flows, so pV^γ is constant, where γ = C_p/C_V; the curve is steeper than an isotherm.AQA
aether: A hypothesised medium for light, support for which was removed by the null result.AQA
Age of the universe: Estimated as t ≈ 1/H under the assumption that H is constant, since d/v = d/(Hd) = 1/H.AQA, OCR
Age-related deterioration: Raises thresholds most at high frequencies.AQA
Air resistance: A resistive force exerted by the air on any object moving through it, acting opposite to the direction of motion.AQA
Airy pattern: The pattern produced when diffraction at the circular aperture spreads each point source.AQA
Algebraic modelling of collisions: The conservation equation is written as the sum of initial momenta equals the sum of final momenta, then solved as a linear equation.AQA
Algebraic rearrangement of equations: Rearranging an equation such as v = u + at to isolate an unknown before substituting values.OCR
Algebraic subtraction of scalars: Because potential is a scalar, the difference is found by simply subtracting the two potential values algebraically.AQA
Allowed wavelengths: For a string of length L fixed at both ends, the ends must be nodes, so λ = 2L/n where n is a positive integer.AQA
Alpha and beta decay: Alpha emission reduces nucleon number by 4 and charge by 2, beta-minus leaves nucleon number unchanged and increases charge by 1, and beta-plus leaves nucleon…OCR
Alpha decay: Emission of an alpha particle (2 protons and 2 neutrons, a helium nucleus), reducing proton number by 2 and nucleon number by 4.AQA, OCR
Alpha emission: A helium nucleus is emitted, so the daughter nuclide has Z decreased by 2 and N decreased by 2, with mass number A decreased by 4.AQA
Alpha particle charge: A charge of 2e, substituted as q in the energy equation alongside Q = Ze for the nucleus.AQA
Altitude: Found by calculating the orbital radius r from r³ = GMT²/(4π²) and then subtracting the planet's radius.AQA
Amount of substance: The quantity for which the mole (mol) is the S.I.OCR
Ampere (A): The unit of current, where 1 A = 1 C s⁻¹.OCR
Amplification: Increases signal power to overcome attenuation and noise before or after transmission.AQA
Amplitude: Obtained by halving the peak-to-peak voltage, which must be distinguished from peak-to-peak voltage when interpreting an oscilloscope trace.OCR
Amplitude and total energy: The total energy E = ½kA² = ½mω²A² remains constant, where A is amplitude.OCR
Amplitude dependence of energy: The graphs show that total energy is proportional to A².OCR
Amplitude modulation: The carrier's amplitude varies at the information frequency while the carrier frequency stays constant, so the envelope of the displayed waveform rises and…AQA
Amplitude modulation (AM): A technique in which the amplitude of the carrier is varied in step with the information signal while its frequency stays constant.AQA
Amplitude variation: A progressive wave has the same amplitude at all points, whereas a stationary wave has zero amplitude at nodes and maximum at antinodes.OCR
Amplitude–driving frequency graph: A plot of the steady-state amplitude of a forced oscillator against the driving frequency, with one curve for each level of damping.OCR
Analogue and digital signals: Digital signals use discrete levels, commonly two voltage levels labelled 0 and 1, whereas analogue signals vary continuously.AQA
Analogue data collection: Sensors convert a physical quantity into an electrical signal, usually a voltage, that varies continuously and is therefore analogue.AQA
Analogue measurement: Using analogue instruments, such as metre rules, vernier callipers, micrometers, thermometers, ammeters, voltmeters and stopwatches with dials, to record a…OCR
Analogy between electrical and mechanical systems: Charge is treated as displacement and current as velocity, so the capacitor's voltage corresponds to the spring's restoring force.AQA
AND: A dot means AND, so the output is 1 only when both inputs are 1.AQA
Angle between field and normal: The angle θ in Φ = BA cos θ, measured between the field direction and the normal to the surface.OCR
Angle dependence of Doppler shift: The measured frequency shift depends on cos θ, so the beam should be as parallel as possible to the flow for maximum sensitivity.OCR
Angle to the normal: The angle i is measured from the normal, not the surface.OCR
Angular acceleration: Links to linear acceleration at a point by a = rα, allowing rotational and linear dynamics to be combined.AQA
Angular frequency: The quantity ω in the solutions, linked to period and frequency by ω = 2πf = 2π/T.AQA, OCR
Angular frequency and its units: A quantity with units of rad s⁻¹ that must be checked before substituting into the velocity equation.OCR
Angular impulse: The product of torque and the time for which it acts, with units N m s.AQA
Angular magnification: The ratio of the angle subtended at the eye by the final image to the angle subtended at the unaided eye by the object, M = β/α.AQA
Angular measure: An angle linked to the arc it subtends, where a full circle is 2π rad and a right angle is π ÷ 2 rad.AQA
Angular momentum: The product of moment of inertia I and angular velocity ω, so L = Iω, with units kg m² rad s⁻¹ or kg m² s⁻¹.AQA
Angular resolution: Given by θ ≈ λ ÷ D in radians, and a larger D produces a smaller θ, meaning finer detail can be resolved.AQA
Angular separation: The tiny angle between star and planet, about 0.1 arcsecond at 10 parsecs, which is below the resolution of most telescopes.AQA
Angular velocity: A quantity in rad s⁻¹, converted between rev s⁻¹ and rad s⁻¹ using ω = 2πf before substituting into L = Iω.AQA, OCR
Annihilation: The process in which a positron and an electron convert the total rest mass into two gamma photons of equal energy, each about 511 keV, emitted in almost…AQA, OCR
Annihilation energy calculations: Calculations in which the total rest energy of the particle and antiparticle is converted into the total energy of the photons produced.AQA
Anode: The p-type terminal of the Zener diode, which is positive relative to the cathode in forward bias.AQA
Anomalous: A description of a point whose status can be judged using error bars.AQA
Anomaly: A reading that does not fit the pattern of the other measurements.OCR
Anti-quark charge reversal: Anti-quarks carry opposite charge to their corresponding quarks, so anti-up has −2/3 e, anti-down has +1/3 e and anti-strange has +1/3 e.OCR
Antibaryons: Particles made from three antiquarks, with the antiproton given as ūūd̄ and the antineutron as ūd̄d̄.AQA
Antineutrino: Antiparticle of the neutrino, differing in lepton number rather than by charge.AQA
Antinode: A point of maximum displacement, where the waves always reinforce because they arrive in phase.AQA, OCR
Antiparticle: For every type of particle there is a corresponding one with the same rest mass but opposite electric charge and opposite quantum numbers such as baryon…AQA, Pearson Edexcel
Antiparticles: Each has the same mass as its particle but opposite electric charge and opposite lepton number.AQA
Antiproton: Antiparticle of the proton, with charge -1 rather than +1.AQA
Antiquarks: Particles that have the opposite sign of every quantum number, so ū has charge −⅔e, d̄ has charge +⅓e and s̄ has charge +⅓e, each with baryon number −⅓.AQA
Aperture stop: Reduces spherical aberration, which arises when a spherical mirror brings marginal rays to a different focus from paraxial rays.AQA
Apparatus selection: Choosing equipment appropriate to the quantity being measured and the precision required, such as a micrometer, vernier calliper, top-pan balance, ammeter,…OCR
Apparent forces: Effects produced in a non-inertial frame that are not caused by physical interactions, such as a loose object appearing to lurch forward.AQA
Apparent magnitude: The objective, logarithmic scale astronomers use instead, comparing the intensity of radiation received from a star with a reference value so measurements are…AQA
Apparent versus absolute magnitude: Apparent magnitude m describes observed brightness, while absolute magnitude M describes intrinsic luminosity at a standard distance of 10 parsecs.AQA
Application of W = VIt to real components: Multiply all three quantities V, I and t together, not just two of them, to find the energy transferred.OCR
Application outside the body: The expression applies outside the body, and a uniform sphere behaves as a point mass at its centre for external points.OCR
Application to sound, light and microwaves: The principle applies to all wave types, including sound, light and microwaves, and underlies interference patterns.OCR
Applications in particle physics: This explains mass spectrometers, cyclotrons and the helical paths of cosmic rays.AQA
Applications of superconductivity: Include the production of strong magnetic fields and the reduction of energy loss in transmission of electric power.AQA
Applying W = VQ and W = EQ: W = VQ gives the energy transferred when charge Q passes through a potential difference V, and W = EQ gives the energy transferred when charge Q passes…OCR
Approximation and accuracy in models: For the model to be accurate, the time interval Δt must be significantly smaller than the half-life.OCR
Archimedes’ principle and weight of fluid displaced: The upthrust equals the weight of fluid displaced by the object.OCR
Area under a force–time graph: A signed quantity representing impulse; for a constant force it is FΔt, and for a varying force it is found by splitting into strips or counting squares.AQA
Area under a graph: For a varying force, split the area into rectangles, triangles or trapezia, or count squares, rather than using a single force value.AQA, OCR
Area under a graph as a product: The area between the velocity–time graph and the time axis gives displacement, with areas below the axis counted as negative.AQA
Area under a graph as work done: Because the graph is a triangle of base Q and height V, the area is ½QV, which links directly to the energy formulae and explains why the factor ½ appears.OCR
Area under force–extension graph: For a material obeying Hooke's law, the stored energy equals this, which for a straight-line region is ½FΔL or ½k(ΔL)².AQA
Area under graphs: The area between the line and the time axis gives displacement on a velocity–time graph and velocity change on an acceleration–time graph.OCR
Assumptions of ideal-gas behaviour: Ideal-gas molecules are modelled as having no intermolecular potential energy, so no potential term is included in the internal energy.OCR
Assumptions of kinetic theory: The volume of the molecules is negligible compared with the container, collisions are perfectly elastic, and intermolecular forces are negligible except…OCR
Astronomical calculations: Radian measure is applied in angular resolution and the small-angle approximation θ ≈ λ ÷ D, where θ must be in radians.AQA
Astronomical distance units: The parsec is the distance unit used throughout OCR A Level Physics A cosmology, and it can be converted between parsecs, metres and light-years.OCR
Astronomical unit (AU): The average Earth–Sun distance, about 1.50 × 10¹¹ m, convenient for distances within the Solar System.OCR
Asynchronous output changes: Combinational outputs can change asynchronously if external inputs change between pulses, even though flip-flop states update only on clock edges.AQA
Asynchronous reset: A reset that may clear the counter before the next clock edge, so the decoded state may appear only briefly.AQA
Atomic and nuclear density: The mean density of a nucleus is much greater than that of an atom because the mass of an atom is concentrated in a very small nucleus.OCR
Atomic and nuclear dimensions: The ratio of atomic diameter to nuclear diameter is about 10⁵, meaning the atom is roughly one hundred thousand times wider than its nucleus.OCR
Atomic and nuclear structure: Later work showed the nucleus contains protons and neutrons, and scattering experiments revealed that protons and neutrons are themselves made of quarks.AQA
Atomic mass unit: A non-SI unit of mass defined as one twelfth of the mass of a carbon-12 atom in its nuclear and electronic ground state.AQA
Atomic spacing: The regular spacing between carbon atoms acts like a diffraction grating for the electron waves and is of the order of 10⁻¹⁰ m.OCR
Atomic structure: The atom is mostly empty space, with a tiny central nucleus that is positively charged and contains most of the atom's mass.OCR
Atomic-resolution surface imaging: Recorded height or current variations build up a map of the surface at atomic resolution.AQA
Attenuation: The loss of intensity with depth from absorption, scattering and reflection, which increases with frequency.AQA
Attenuation and penetration: Higher frequency gives finer detail but greater attenuation, limiting penetration; strong reflection occurs at boundaries with large acoustic impedance…AQA
Attenuation coefficient: The quantity μ in I = I₀e^(−μx), measured in m⁻¹, where a larger value or a thicker absorber gives a smaller transmitted intensity.AQA
Attenuation measurements along many paths: Detectors measure the transmitted intensity after the beam has passed through the body, and comparing this with the incident intensity gives the attenuation…AQA
Attitude monitoring: Sensors measure components of Earth's magnetic field along three orthogonal axes, from which the orientation of a vehicle or device relative to the magnetic…AQA
Attractive force between masses: Gravitational force is always attractive, so the vector form includes a negative sign.OCR
Attractive versus attractive-or-repulsive forces: Masses always attract, but charges may attract or repel.AQA
Audible sound: Sound for humans that typically spans 20 Hz to 20 kHz, so ultrasound lies above this range.OCR
Authority in science: Newton was held in enormous scientific esteem, and this made his particle model highly influential.AQA
Automatic and remote monitoring: Sensors allow automatic, continuous and remote monitoring, which is more practical than manual reading in many experiments.AQA
Average binding energy per nucleon: The total binding energy of the nucleus divided by the nucleon number A, measured in MeV per nucleon.AQA
Average force: The total momentum transferred to the wall in time Δt divided by Δt, given by F = Δp/Δt.AQA
Average power: For a resistor, P = I_rms²R = V_rms²/R = I_rms V_rms.AQA
Average speed: Total distance divided by total time, a scalar.OCR
Average versus instantaneous motion: Average values use total change divided by total time, whereas instantaneous values describe a single moment.AQA
Averaging: Repeating measurements and calculating a mean, which reduces random errors because deviations in opposite directions partially cancel, but does not remove…AQA, OCR
Avogadro constant: The number of particles in one mole, NA = 6.02 × 10²³ mol⁻¹, by which the molar gas constant is divided.AQA, OCR
Axis: An angle in degrees that identifies the orientation of the cylindrical lens, with its refractive power acting perpendicular to this axis.AQA
Axis labelling: Each axis must be labelled with the quantity and its unit, for example 'Time / s' or 'Current / A'.OCR

B

B-scan: Displays the intensity of reflections as brightness on a two-dimensional image, building up a cross-sectional picture by scanning the transducer across the…AQA, OCR
Background count rate: A reading measured first with no source and subtracted from all readings in absorption studies.OCR
Background radiation: Radiation corrected for by measuring the count rate without the source and subtracting it from readings taken with the source.AQA, OCR
Background radiation and correction: Radiation always present, so a background count is recorded with the source removed and then subtracted from each reading.AQA
Background radiation correction: Background count rate is measured with the source removed and subtracted from each reading.OCR
Balance of half-life against patient dose: The half-life must be long enough to allow production, labelling, administration and imaging, but short enough that activity falls quickly and patient dose…AQA
Balance of neutron production and absorption: The moderator, control rods and coolant together balance neutron production and loss so the chain reaction remains controlled.AQA
Balanced nuclear equation: An equation for the reaction in which nucleon number and proton number are conserved.AQA
Balancing: The total nucleon number and total proton number are equal on both sides of a nuclear transformation equation.OCR
Band gap and photon energy: The spectral response peaks where photon energy is just above the band gap, and falls at long wavelength once photons cannot free electrons.AQA
Bandwidth: The range of frequencies a channel must carry, limiting how much information can be transmitted.AQA
Barium: Usually swallowed as a suspension of barium sulfate, it is used for the gastrointestinal tract because it is dense and effectively insoluble, so it coats the…OCR
Barium meal: A technique in which the barium coats the oesophagus, stomach and intestine, and those coated surfaces attenuate the beam far more than soft tissue.AQA
Barium sulfate: A heavy, high-atomic-number compound that is chemically inert and not absorbed, so it can be swallowed as a suspension.AQA
Baryon number: A bookkeeping property that is +1/3 for every quark and −1/3 for every antiquark, so three quarks give a baryon number of 1.AQA
Baryon number and strangeness: Anti-quarks carry opposite baryon number and strangeness to their quarks, and quark composition determines a hadron's baryon number and strangeness.OCR
Baryon number of antibaryons: Each antibaryon, such as an antiproton, is assigned a baryon number of −1.AQA
Baryon number of leptons and mesons: Both leptons and mesons have baryon number 0.AQA
Baryons: Particles made from three quarks, with the proton given as uud and the neutron as udd.AQA, OCR
Baryons and antibaryons: Baryons contain three quarks, for example the proton (uud) and neutron (udd); antibaryons contain three antiquarks, for example the antiproton and antineutron.AQA
Base units: Each of the seven S.I.AQA, OCR
Baseline: The unobscured stellar flux, from which a transit appears as a temporary decrease below it.AQA, OCR
Basilar membrane: Movement of this structure bends hair cells in the organ of Corti, generating electrical impulses in the auditory nerve.AQA
BCD counter: Counts only from 0000 to 1001 (0 to 9) and then resets, making it suitable for driving decimal displays.AQA
Beta-minus decay: A neutron changes into a proton, so Z increases by 1 and N decreases by 1 while A stays constant; an electron and an antineutrino are emitted.AQA, OCR
Beta-plus decay: A proton changes into a neutron, so Z decreases by 1 and N increases by 1 while A stays constant; a positron and a neutrino are emitted.AQA, OCR
Big Bang nucleosynthesis: In the first minutes, protons and neutrons fused mainly into helium, giving the observed relative abundance of hydrogen and helium.AQA, OCR
Big Bang theory: The universe began from an extremely hot, dense state about 1.4 × 10¹⁰ years ago and has been expanding and cooling ever since.OCR
Binary counter: Counts in natural binary, so an n-bit counter has 2ⁿ states and recycles after reaching its maximum.AQA
binary pattern: An n-bit pattern can represent 2ⁿ distinct values, and can be converted to decimal by summing place values.AQA
Binary representation: A sequence of 0s and 1s can encode a number or sample value, with each binary digit corresponding to a power of two.AQA
Binding and saturation of the strong force: Because it acts only over short distances, a nucleon interacts only with near neighbours, so the force saturates and cannot easily stabilise a whole large…AQA
Binding energy: The energy needed to separate a nucleus into its individual protons and neutrons, or released when the nucleus forms.AQA, OCR
Binding energy per nucleon: The binding energy divided by the nucleon number A, measuring how tightly each nucleon is held.OCR
Binding energy per nucleon against nucleon number curve: A graph that rises steeply for light nuclei, peaks near iron-56 at about 8.8 MeV per nucleon, then falls slowly for heavy nuclei.OCR
Binding energy per nucleon comparisons: Reading the graph means comparing the starting and finishing heights on the vertical axis, since energy is released when the products have a higher average…AQA
bit: A binary digit with two possible states, 0 or 1, and the smallest unit of digital information.AQA
bit depth: The number of bits per sample; increasing it increases the number of quantisation levels and reduces the step size between adjacent levels.AQA
Bit rate: Equals samples per second multiplied by bits per sample, so more levels or a higher sampling frequency increase the data rate.AQA
Black body: Absorbs all incident radiation and emits a continuous spectrum whose shape depends only on absolute temperature.AQA
Black hole: A region where gravity is so strong that the escape velocity exceeds the speed of light, so nothing, not even light, can escape from inside the event horizon.AQA, OCR
Black-body modelling of stars: An estimate that assumes black-body behaviour, which is an approximation since real stellar spectra deviate from a perfect black body.OCR
Black-body radiation: Radiation absorbed and emitted across all wavelengths by a black body, with a spectrum depending on temperature.AQA
Black-body radiation and temperature: The spectrum of the cosmic microwave background radiation matches that of a black body at about 2.7 K.OCR
Black-body radiator: The model assumed for the star, so that the law gives the total power radiated across all wavelengths.AQA
Block diagram: Shows the stages as labelled boxes connected by arrows indicating the direction of signal flow.AQA
Blocking condition: The wheel rotates by half a tooth spacing during the light's round trip, so returning light meets a tooth.AQA
Blood flow measurement: The speed v of the blood can be found using Δf/f = (2v cos θ)/c, where c is the speed of ultrasound in blood.OCR
Blue shift: A source moving towards the observer has its wavelength compressed, producing a blue shift.OCR
Boltzmann constant: The gas constant per single particle, given by k = R / NA and approximately 1.38 × 10⁻²³ J K⁻¹.AQA, OCR
Boolean algebra: Describes logic circuits using variables that take only 0 or 1.AQA
Boolean expression: An expression that can be derived or interpreted from a gate network and checked against the final output for consistency.AQA
Boundary conditions and allowed patterns: The boundary conditions of the medium, such as fixed ends of a string, determine which stationary wave patterns can form.OCR
Boundary conditions for stretched strings: On a stretched string fixed at both ends, boundary conditions dictate that nodes form at the ends.OCR
Boyle's law: For a fixed mass of gas at constant temperature, pV = constant, so p is inversely proportional to V and a graph of p against 1/V is a straight line through…AQA
Brake power: Output power measured with a dynamometer, given by P = Tω where ω = 2πf.AQA
Braking distance: The distance travelled while the brakes are applied, depending on the braking force, the mass of the vehicle and the road conditions.OCR
Breakdown of classical mechanics at relativistic speeds: Kinetic energy does not follow the classical formula at relativistic speeds, and the classical prediction diverges from the data at high speed.AQA
Breaking stress: The stress at which a material fractures, depending on the material and on flaws, and not the same as the ultimate tensile stress.AQA
Bremsstrahlung: Arises from deceleration of electrons in the target nucleus's electric field, producing a continuous range of photon energies.AQA
Brightness: A subjective scale of measurement that depends on the observer, viewing conditions and the eye's response, so it is not a reliable scientific quantity.AQA
Brightness and focus controls: Adjust the visible spot and affect visibility only, not the voltage or time measurements.AQA
Brittle: A material that is stiff and strong but fractures with little plastic strain, so its graph is almost linear to breaking.OCR
Brittle versus ductile force–extension graphs: Brittle behaviour means a material fractures with little or no plastic deformation, which is distinct from stiffness, whereas ductile copper shows a large…AQA
Brownian motion: The random, jerky movement of small particles such as smoke particles or pollen grains suspended in a fluid, seen as unpredictable zigzag paths under a…AQA, OCR
byte: A fixed group of 8 bits, so 1 byte = 8 bits and 1 kB = 8 kbit when using the same prefix convention.AQA

C

Calculations involving e: The charge on ions or charged objects is calculated by multiplying e by the number of excess or deficit electrons.OCR
Calculations with integer multiples of e: Using Q = n e, where n is an integer (positive, negative, or zero), and dividing the net charge by e to determine the number of excess or deficit electrons.OCR
Calibration and compensation: The sensor must be calibrated and may need temperature compensation to account for temperature, offset and interference.AQA
Capacitance: The charge stored per unit potential difference, given by C = Q/V, where Q is the magnitude of charge on one plate and V is the potential difference across…OCR
Capacitance as charge per unit potential difference: Capacitance is defined by C = Q/V, the charge stored per unit potential difference.OCR
Capacitance from gradient: A graph of V against Q is a straight line through the origin with gradient 1/C.OCR
Capacitor charging and discharging: Charging curves rise from zero and approach the supply value asymptotically, while discharging curves fall from their initial values towards zero…AQA, OCR
Carbon-14 and half-life: While alive, organisms keep a roughly constant carbon-14 proportion; after death it decays with a half-life of about 5730 years.OCR
Carrier wave: A high-frequency sinusoidal oscillation that is transmitted through space and can be modified to carry information.AQA
Carrier waves and information transfer: Modulation superimposes information onto a carrier wave.AQA
Cathode: The n-type terminal of the Zener diode, marked by a band on the package, and positive relative to the anode in reverse bias.AQA
Cathode rays: Emitted electrons accelerated towards the anode, some passing through a hole to form a beam.AQA
Celsius scale: Scale defined by the ice point (0 °C) and steam point (100 °C) of water at standard atmospheric pressure.OCR
Central maximum: The brightest and widest feature of the pattern, with narrower, fainter secondary maxima on each side separated by minima.AQA
Central obstruction: The secondary mirror causes some obstruction of the incoming light.AQA
Centre of gravity: The point where the entire weight of the object acts.OCR
Centre of mass: The point where the entire mass of an object may be considered to be concentrated.OCR
Centre-of-mass separation: The force acts along the line joining the centres of mass, and r is the centre-to-centre distance for uniform spheres.AQA
Centripetal acceleration: The acceleration of a body moving in a circle at constant speed, acting towards the centre of the circle and perpendicular to the instantaneous velocity.AQA, OCR
Centripetal acceleration magnitude and direction: For a body moving at speed v on a circle of radius r the magnitude is a = v²/r, directed towards the centre.OCR
Centripetal force: The magnetic force on a charge moving at right angles to a uniform field is perpendicular to velocity and acts as the centripetal force needed for circular…AQA, OCR
Centripetal force from a magnetic field: The magnetic force is always perpendicular to velocity and has constant magnitude Bqv, so it provides the centripetal force.AQA
Chain reactions: Because each fission releases more neutrons, these can induce further fissions, giving a self-sustaining chain reaction.AQA, OCR
Chandrasekhar limit: The maximum mass a white dwarf can have, approximately 1.4 solar masses, above which degeneracy pressure cannot support the star.AQA, OCR
Change in angular momentum: Written as ∆(Iω), which may involve changes in I, ω, or both.AQA
Change in momentum: For a constant force, mΔv, equal to the area FΔt under the force–time graph.AQA
Change in momentum and direction: The change in momentum is calculated as final momentum minus initial momentum, and the net force is in the same direction as this change.OCR
Change of +1: Occurs when a strange quark is removed or an anti-strange quark is created, for example in Λ → p + π⁻.AQA
Change of phase: A process such as melting or boiling in which a substance absorbs or releases energy while its temperature stays constant.OCR
Change of state: A process in which energy is supplied or removed while the temperature stays constant, changing the potential energy of the particle ensemble.AQA
Changes of state and constant temperature: During a change of state the temperature stays constant, so the energy supplied changes the arrangement of particles rather than their kinetic energy.OCR
Changing flux linkage: Occurs when the magnetic flux linkage through a circuit changes, inducing an e.m.f.AQA
Characteristic X-rays: Arise when an accelerated electron removes an inner-shell electron and an outer-shell electron drops into the vacancy, emitting a photon of energy equal to…AQA
Characteristic X-rays from shell transitions: Photons arise from electron transitions between inner atomic shells, giving discrete photon energies equal to shell energy differences.OCR
Charge and baryon number conservation: Charge and baryon number are always conserved, so the totals before and after an interaction are equal.AQA
Charge and electron emission: Emitted electrons are attracted back to a positive plate, so a positively charged electroscope does not discharge.OCR
Charge and neutrality: Protons are positively charged, neutrons have no charge and electrons are negatively charged; in a neutral atom the number of electrons equals the number of…OCR
Charge and potential difference in capacitors: The charge Q stored is directly proportional to the potential difference V across it, so the graph is a straight line through the origin.AQA
Charge and quantum number symmetry: A particle and its antiparticle have opposite electric charges and opposite signs for quantum numbers such as baryon number, lepton number and strangeness.AQA
Charge carriers: Released in greater numbers by thermal energy, so that their number density increases in an ntc thermistor.AQA
Charge carriers and current direction: In a metal, electrons drift through the lattice; in an electrolyte, positive and negative ions move in opposite directions, and conventional current direction…AQA
Charge carriers and scattering: In a metal, increased lattice vibrations increase electron scattering; in an NTC thermistor, heating releases more charge carriers.AQA
Charge carriers in electrolytes: Positive and negative ions, which move in opposite directions under the applied field.OCR
Charge carriers in metals: Delocalised electrons, while the positive metal ions form a fixed lattice and do not translate.OCR
Charge from quark content: Anti-quarks carry opposite charge to their quarks, and quark composition determines the hadron's charge, so these properties can be deduced from the quark…OCR
Charge neutrality: Bulk matter is almost exactly this, so the much weaker gravitational attraction accumulates without cancellation and gravity dominates on astronomical scales.AQA
Charge-carrier release in semiconductors: Increasing temperature releases more charge carriers (electrons and holes) across the small energy gap, so the number density n rises sharply.OCR
Charging: Electrons flow from the negative terminal of the supply onto one plate, while electrons leave the opposite plate towards the positive terminal.OCR
Charging circuit: In a charging circuit, τ is the time to reach (1 − 1/e) (about 63%) of the final value.OCR
Charging time constant: The time to reach 1 − 1/e of the final value, about 63%.AQA
Chemical form and labelling: Technetium-99m is eluted as sodium pertechnetate with saline and can be labelled to many pharmaceuticals.AQA
Chemical labelling and organ specificity: The isotope must be capable of being chemically bound to a compound that has an affinity for the organ of interest, so the tracer accumulates at the target…AQA
Choice of axis: The same object can have different moments of inertia about different axes, so the axis of rotation must always be stated.AQA
Choice of measuring instruments: A micrometer measures the diameter d, a metre rule measures L, an ammeter in series measures I and a voltmeter in parallel measures V.OCR
Choice of radiation source: Beta from a suitable source is used for thin sheets such as paper and aluminium foil, while gamma is used for thicker or denser steel.AQA
Choice of suitable isotope: A source such as protactinium-234 is used because it has a conveniently short half-life.OCR
Choosing appropriate units for large distances: Selecting the unit that keeps numbers manageable and stating it clearly.OCR
Choosing axes to simplify a problem: On an inclined plane, choose axes parallel and perpendicular to the slope rather than horizontal and vertical.AQA
Chromatic aberration: Arises in refractors because the focal length of a lens depends on wavelength, so different colours focus at different points.AQA
Circuit analysis: Analysing circuits with components in both series and parallel requires applying Kirchhoff’s laws and the rules for total resistance.OCR
Circuit application: A simple regulator in which a series resistor drops the difference between the input voltage and the Zener voltage while the Zener holds the output at its…AQA
Circuit construction and meter placement: The ammeter is connected in series with the wire and the voltmeter in parallel across the measured length of wire.AQA
Circuit design for measurement: Connect the component in series with an ammeter and a variable d.c.OCR
Circuit diagram interpretation: Reading a diagram that uses standard symbols joined by straight lines to show how components are connected, identifying series and parallel branches and the…OCR
Circuit equations and algebraic rearrangement: Substituting V = IR into E = I(R + r) gives E = V + Ir, and rearranging gives r = (E − V)/I.OCR
Circuit setup for capacitor investigations: The capacitor, resistor and switch are connected in series with a suitable supply, and either the current through the circuit or the potential difference…OCR
Circuit symbols and component recognition: Reading the diagram to identify each component and its symbol, then matching each symbol to the correct physical component.OCR
Circular motion and centripetal force: The magnetic force provides the centripetal force, so BQv = mv²/r and r = mv/(BQ).AQA
Circular motion of charged particles: A particle moving perpendicular to a uniform magnetic field follows a circular path of radius r given by Bqv = mv²/r.AQA
Circular orbital motion: A planet moving around the Sun follows a curved path, so it must have a centripetal acceleration directed towards the Sun.OCR
Classical Galilean velocity addition: The classical view in which velocities simply add together as vectors, contradicted by the invariance of the speed of light.AQA
Classification of fundamental and composite particles: The statement is about baryons only, so it does not claim that leptons or mesons are stable.AQA
Classification of physical quantities: Deciding whether a quantity is scalar or vector by stating whether direction is needed to define it.OCR
Classification of waves: Requires both the direction of displacement and the direction of energy propagation to be stated and compared, not just one of them.AQA
Clinical diagnosis from count maps: Diagnosis depends on comparing the pattern of uptake with normal expectations, and the clinician may quantify counts in regions of interest.OCR
Clinical suitability and practical constraints: Practical factors such as cost, portability, availability, scan time and patient comfort or safety, related to a specific clinical purpose.AQA
Clinical workflow and data storage: There is no film to load, process, handle or archive physically, so workflow is faster and storage is compact, and electronic images can be shared across…AQA
Clock: The continuous square-wave signal whose active edge triggers each state change; without it the circuit holds its current state.AQA
Clock pulses: Supplied by the astable as an oscillator to advance a sequential logic circuit.AQA
Clock rate: The number of pulses each second, measured in hertz, where f = 1/T.AQA
Clock-edge triggering: Flip-flop states update on clock edges, so Q changes only on the active clock edge rather than immediately with its input.AQA
Closed system: One in which no energy enters or leaves, so the sum of all energy stores is constant.AQA
Closed-triangle method: Drawing the forces tip-to-tail so that if they form a closed triangle, the resultant is zero.AQA
Closest approach: The point at which all the alpha particle's initial kinetic energy has become electrostatic potential energy, allowing an estimate of the distance and hence…AQA
Coalescence and explosions: Bodies may separate in collisions, move together with a common final velocity in coalescence, or have initial total momentum of zero in explosions.AQA
Coefficient of performance: A measure of useful energy transfer per unit work input, given by Q_C/W for a refrigerator and Q_H/W for a heat pump.AQA
Coherence: Waves have a constant phase difference and the same frequency, which is needed for a stable interference pattern.AQA, OCR
Coherent: Describes sources that emit waves with a constant phase difference, as the two slits do in Young's experiment.AQA
Coherent fibre bundle: Keeps the same relative positions of fibres at both ends, so each fibre carries one pixel of the image and the pattern is preserved.AQA
Coherent monochromatic sources: A laser emits monochromatic light of a single wavelength and coherent light with a constant phase difference, so the pattern remains stable.AQA
Coherent SI units in calculation: When a quantity is calculated, its unit follows from the defining equation, for example C = A s from Q = I t.AQA
Coherent sources: Sources that emit waves of the same frequency with a constant phase difference, so their superposition is stable in time.AQA
Coincidence detection: Two detectors on opposite sides of a ring surrounding the patient register photons within a very short coincidence time window, so the annihilation event is…OCR
Collecting power: Proportional to the area of the objective, so it is proportional to D², and a larger diameter gathers more radiation per second.AQA
Collecting power and aperture area: The collecting power of either telescope increases with the area of the dish or mirror, so a larger aperture gathers more radiation per second.AQA
Collimator: A thick lead block pierced by many narrow parallel channels that absorbs oblique gamma photons so only near-parallel photons reach the crystal.AQA, OCR
Collision analysis: Masses and velocities before and after are measured so that total momentum can be calculated and conservation tested.OCR
Collisions and resultant forces: Unequal collisions between the suspended particle and fast-moving fluid molecules produce a fluctuating resultant force on the particle.AQA
Collisions, decays and annihilation processes: Interactions in which the total energy and the total momentum of the system remain constant, whether a collision, decay or annihilation.AQA
Combination of absolute and percentage uncertainties: When a quantity is calculated from several measurements, their uncertainties must be combined using the rule that matches the arithmetic.AQA
Combinational circuit construction: Gates are combined by connecting the output of one to the input of another to form larger circuits.AQA
Combinational gates: Gates that respond only to their present inputs and are not assumed to depend on past inputs.AQA
Combining components to find resultant velocity: The components at any instant are combined to find the resultant velocity, using Pythagoras' theorem for the magnitude.AQA
Combining gravitational, viscous and electric forces: When the droplet is held stationary, the electric force balances the weight, giving QV/d = mg, which rearranges to QV = mgd.AQA
Combining uncertainties in calculations: Adding absolute uncertainties for sums and differences, and adding percentage uncertainties for products and quotients.OCR
Comet: A small icy body, often on a highly eccentric orbit, that develops a coma and tail when heated near the Sun.OCR
Common time of flight: The time is common to both directions and can be used to link horizontal and vertical calculations.AQA
Communication of assumptions and uncertainty: Stating the assumptions made, such as treating a room as a rectangular box, and commenting on whether the estimate is likely to be too high or too low.AQA
Comparison of nuclear and atomic sizes: Nuclear radii are about 10⁻⁵ to 10⁻⁴ times atomic radii, which are about 10⁻¹⁰ m.AQA
Comparison of proton and electron properties: The electron has a much larger specific charge than the proton because the electron has a far smaller mass.AQA
Comparison of transmission media: Weighing media such as copper, optical fibre, and wireless against each other in terms of data transmission rate, cost, and security.AQA
Comparison with a photodetector: The eye has a narrow spectral range and a logarithmic response to intensity.AQA
Comparison with bar magnet field: Outside a solenoid the field pattern resembles that of a bar magnet.OCR
Comparison with X-ray, CT and MRI: Advantages over X-rays and CT include no ionising radiation, real-time moving images, low cost and portability; disadvantages include poorer resolution than…AQA
Component identification: Being asked to identify a symbol, match a symbol to its name, or spot an incorrectly drawn symbol in a multiple-choice question.OCR
Component polarity: The correct orientation of cells and diodes, with the long line as the positive terminal and the diode's triangle pointing in the direction of conventional…OCR
Component protection: Keep the current small with a protective resistor, and avoid overheating the component by switching off between readings.OCR
Composition of the universe: The universe is made up of dark energy, dark matter, and a small percentage of ordinary matter.OCR
Compressions and rarefactions: The regions of high and low particle density in a longitudinal wave, where particles are close together and spread apart.AQA, OCR
Compressive deformation: Occurs when forces squash the material, producing a compression, which is a decrease in length.OCR
Compton effect: A photon scatters off a loosely bound or free electron, transferring some energy and producing a recoil electron and a lower-energy scattered photon.OCR
Computer: Compares pulse heights across tubes to calculate the interaction position, building a two-dimensional image of tracer distribution.OCR
Computer modelling: Using ICT to simulate a physical system, for example projectile motion, to investigate how changing a variable affects the outcome.OCR
Computer reconstruction of attenuation data: Computer software reconstructs the attenuation data into a cross-sectional image, combining many projections from different angles.OCR
Computer reconstruction of cross-sectional images: A computer processes the large set of path attenuation values, using reconstruction mathematics, to calculate the attenuation at each point in the slice and…AQA
Concurrent forces: Forces whose lines of action meet at a single point, as required for three coplanar forces to be in equilibrium.OCR
Conditions for floating and sinking: An object floats when upthrust balances its weight.OCR
Conductors, semiconductors and insulators: Conductors have a very high number density of charge carriers, insulators a very low one, and semiconductors an intermediate value.OCR
Cone cells: Detect colour and detail in brighter light, dominating at high intensity and peaking near 555 nm.AQA
Consequences for simultaneity, time and length: The postulates lead to time dilation, length contraction and the relativity of simultaneity.AQA
Conservation laws: Lepton number conservation is used alongside conservation of charge, baryon number and energy to test whether a proposed reaction is allowed.AQA
Conservation laws at vertices: At each vertex, charge, baryon number and lepton number are conserved, and the diagram must be consistent with these conservation rules.AQA
Conservation laws in particle decays: Charge, baryon number and lepton number are each conserved, with the electron having lepton number +1 and the antineutrino having lepton number −1.AQA
Conservation laws in particle interactions: Conservation of charge, energy and momentum are used to check whether a proposed particle reaction is possible.AQA, OCR
Conservation laws in particle physics: Particle interactions must satisfy conservation laws, and comparing totals decides whether a proposed interaction is allowed or forbidden.AQA
Conservation of angular momentum: If the resultant external torque on a system is zero, its total angular momentum remains constant, so I₁ω₁ = I₂ω₂.AQA
Conservation of baryon number: In any particle interaction or decay, the total baryon number before must equal the total baryon number after.AQA
Conservation of charge: Charge is conserved overall, for example the down quark charge of −1⁄3 changes to an up quark charge of +2⁄3 with the electron carrying away −1.AQA, OCR
Conservation of charge in parallel: The total charge stored is the sum of the charges on the individual capacitors.OCR
Conservation of charge in series: Each capacitor in series stores the same magnitude of charge.OCR
Conservation of energy: Each charge carrier that completes the circuit transfers energy from the source to the components, and the sum of potential differences around any closed loop…AQA, OCR
Conservation of energy in electrical circuits: Charge returning to its starting point must have the same total energy, so energy gained from sources equals energy lost in components.OCR
Conservation of energy in mechanical systems: The total energy in a closed system stays constant: energy may transfer between stores but is never created or destroyed.AQA
Conservation of energy in photon absorption: The photon energy is used partly to escape the surface and the remainder becomes kinetic energy, for a single one-to-one photon-electron interaction.OCR
Conservation of energy in photon–electron collisions: The maximum kinetic energy is the photon energy minus the work function: Ek(max) = hf − φ.AQA
Conservation of energy in simple harmonic motion: In an undamped simple harmonic oscillator, total energy is conserved while kinetic and potential energy interchange continuously.OCR
Conservation of energy including rest energy: The total energy of the system remains constant in any interaction, and this includes rest energy, so a decrease in rest mass appears as an increase in…AQA
Conservation of kinetic energy in elastic collisions: Applied with conservation of momentum to a head-on elastic collision between a neutron and a stationary nucleus.AQA
Conservation of lepton number: Lepton number is conserved, with the electron and antineutrino balancing lepton number in β⁻ decay and the positron and neutrino balancing it in β⁺ decay.AQA, OCR
Conservation of linear momentum: In a closed system with no resultant external force, total momentum before an interaction equals total momentum after it.AQA
Conservation of momentum: The wall and a molecule are treated as an isolated system during the collision, so total momentum is conserved.AQA, OCR
Conservation of momentum as a vector quantity: The total momentum of the system remains constant in any interaction, and because momentum is a vector you must consider direction as well as magnitude.AQA
Conservation of momentum in collisions: Applied with conservation of kinetic energy to a head-on elastic collision between a neutron and a stationary nucleus.AQA
Conservation of nucleon number: The total nucleon number must be the same on both sides of a decay equation, so equations must balance nucleon number.AQA, OCR
Conservation of strangeness: In strong and electromagnetic interactions strangeness is conserved, but the weak interaction can change the strangeness of an individual quark.AQA
Conservation of total energy: Adding the two curves at every displacement gives a horizontal straight line at the constant total energy ½mω²A².OCR
Conservative field: A field where the work done depends only on the initial and final potentials, not on the path taken.AQA
Consistency of sign conventions in circuit analysis: Circuit analysis, Kirchhoff's laws and most equations such as I = ΔQ/Δt assume positive charge flow, so signs must be handled consistently.OCR
Consistency of units and signs: Velocities opposite to the chosen positive direction are negative, and masses and velocities are substituted with consistent units.AQA
Consistency of units in thermodynamic calculations: Units are handled consistently, for example pressure in Pa and volume in m³ giving work in J.AQA
Constant acceleration: Constant force produces constant acceleration, so the suvat equations apply.OCR
Constant acceleration equations: Four equations link initial velocity u, final velocity v, acceleration a, displacement s and time t for constant acceleration in a straight line, and each…OCR
Constant acceleration perpendicular to that direction: The vertical velocity component changes at a steady rate because the acceleration is constant and perpendicular to the horizontal direction.OCR
Constant acceleration under gravity: Vertically the only force is weight, giving constant downward acceleration g, so vertical velocity changes steadily.OCR
Constant and changing velocity: A straight line shows constant velocity, a horizontal line shows zero velocity, and a curve shows changing velocity.OCR
Constant mass: The condition under which Newton's second law becomes F = ma, so the momentum form is needed when mass changes.AQA
Constant nuclear density: Nuclear volume is proportional to A, so nuclear density is roughly constant for all nuclei.AQA
Constant physical conditions: The requirement that conditions such as temperature stay unchanged for Ohm's law to apply.AQA
Constant speed in circular motion: The object travels in a circular path at constant speed because the force does no work on it.OCR
Constant speed under a perpendicular force: Speed remains constant in the circular orbit because the magnetic force is perpendicular to velocity, so it changes direction only and does no work.OCR
Constant temperature during phase change: The temperature of the substance remains constant because the average random kinetic energy of the particles does not change.OCR
Constant velocity: Motion that is equally valid as equilibrium, because zero resultant force means zero acceleration.AQA
Constant velocity in one direction: The horizontal component of velocity is unchanged because there is no horizontal resultant force.OCR
Constant velocity when resultant force is zero: Horizontally there is no resultant force, so horizontal velocity stays constant and horizontal displacement is velocity × time.OCR
Constant-mass reduction to F = ma: For constant mass, Δp = mΔv, so F = Δp/Δt reduces to F = ma.OCR
Constant-ratio property: For equal time intervals, the ratio of the quantity at the end of the interval to that at the start is constant.OCR
Constructive and destructive superposition: Constructive superposition occurs when displacements are in the same direction, producing a larger resultant amplitude, while destructive superposition occurs…OCR
Constructive interference: Gives maximum amplitude when the waves arrive in phase, with phase difference zero or a whole number of cycles and path difference zero or a whole number of…AQA, OCR
Constructive interference condition for multiple slits: Maxima are produced when the path difference between light from adjacent slits equals a whole number of wavelengths.OCR
Constructive superposition: A path difference of a whole number of wavelengths gives this, producing bright fringes.AQA, OCR
Contact time: Increasing it decreases the average force for a fixed momentum change, and decreasing it increases the average force.AQA
Continuous spectrum: Contains all wavelengths of visible light and appears as an unbroken band of colour, produced by a hot dense source.AQA, OCR
Contrast: A star can be 10⁹ times brighter than a reflected-light planet, so the planet's faint signal is swamped.AQA
Contrast enhancement: Introducing a material with much higher attenuation than the surrounding tissue where neighbouring soft tissues attenuate almost equally.AQA
Contrast from differences in tissue attenuation: Dense tissue such as bone attenuates more than soft tissue, so contrast appears in the image.AQA
Contrast medium: A substance introduced into the body to increase absorption in the region of interest.OCR
Control of beam intensity and photon energy: Tube current controls beam intensity, while tube voltage controls photon energy, since increasing V increases hf_max = eV.AQA
Control of variables: Keep temperature constant in the Boyle's law experiment and pressure constant in the Charles's law experiment; use a fixed mass of gas and avoid leaks.AQA
Control of variables in a practical investigation: One quantity is varied at a time, keeping the wire perpendicular to the field, in the same region of field, and avoiding heating effects by limiting current.AQA
Control rods: Components that absorb neutrons strongly to regulate the rate of fission, needing a high neutron absorption cross-section and the ability to be inserted and…AQA, OCR
Convenience: Patient experience and clinical practicality, covering scan duration, need for stillness, portability, invasiveness and availability.AQA
Conventional current: The direction in which positive charge would flow, from the positive terminal of a source, through the external circuit, to the negative terminal.OCR
Conventional current versus electron flow: The second finger must point along conventional current, which is opposite to electron flow.OCR
Convergence: Several rods connect onto one neurone, so their signals are pooled and acuity is lower.AQA
Conversion between degrees and radians: Degrees are changed to radians by multiplying by π ÷ 180, and radians to degrees by multiplying by 180 ÷ π.AQA
Conversion between joules and electronvolts: Multiply eV by 1.60 × 10⁻¹⁹ to get joules, or divide joules by 1.60 × 10⁻¹⁹ to get eV.OCR
conversion between kW h and joules: Since 1 kW = 1000 W and 1 h = 3600 s, 1 kW h = 1000 × 3600 = 3.6 × 10⁶ J.OCR
Conversion between moles and number of particles: The number of particles N in a sample is N = nN_A, and the number of moles is n = N/N_A.OCR
Conversion between temperature scales: To convert from Celsius to kelvin, add 273 (approximately); to convert from kelvin to Celsius, subtract 273 (approximately).OCR
conversion factor: The value 1 u = 931.5 MeV, which links the atomic mass unit to energy via Einstein's mass–energy equivalence, E = mc².AQA
Convex secondary mirror: Placed before the primary focus, it reflects the converging beam back through a hole in the centre of the primary mirror.AQA
Coolant: A substance that transfers thermal energy efficiently from the core to a heat exchanger, remains stable at high temperature and ideally has low neutron…AQA
Coplanar forces at a point: For two coplanar forces, equilibrium requires equal magnitudes acting in exactly opposite directions along the same line.AQA
Cornea: Provides most of the fixed refraction, contributing the greater fixed convergence, while the lens provides variable fine adjustment.AQA
Coronagraphs: Techniques that reduce these problems, though direct imaging remains hardest for small, close-in planets.AQA
Corpuscles: Tiny particles that Newton pictured light as, travelling in straight lines.AQA
Correct connection: Connecting a multimeter in series to measure current and in parallel to measure potential difference.OCR
Correct measurement technique: Using instruments correctly, including zeroing a balance, checking for zero error, reading a meniscus at eye level to avoid parallax, and choosing a suitable…OCR
Corrected count rate: The count rate after background count rate has been subtracted, plotted against absorber thickness to show absorption.AQA, OCR
Cosine and sine rules for non-perpendicular vectors: For non-right-angled triangles, the cosine rule gives the resultant magnitude and the sine rule gives an angle.OCR
Cosine form: The solution x = A cos(ωt), which starts at maximum displacement when t = 0.OCR
Cosine variation with angle: The amplitude of the induced AC voltage is directly proportional to the maximum flux linkage at that angle, demonstrating the cosine relationship.AQA
Cosmic microwave background: Radiation released at recombination, when electrons and nuclei combined into neutral atoms so radiation could travel freely, now cooled to about 2.7 K.OCR
Cosmic microwave background radiation: A faint, almost perfectly uniform microwave signal coming from every direction in space, with a black-body spectrum at about 2.7 K.OCR
Cosmological microwave background radiation: Thermal radiation at about 2.7 K filling all space, relic radiation from when the early Universe became transparent.AQA
Cosmological principle: On sufficiently large scales, the universe is homogeneous and isotropic, and the laws of physics are universal.OCR
Cosmological redshift: Large redshifts indicate recession and therefore distance in an expanding Universe, and are used to infer quasar distances.AQA, OCR
Cost: Equipment purchase, installation shielding, running and maintenance, and staff training, not just the scanner price.AQA
cost calculation: Total cost is found by multiplying energy in kW h by the unit price.OCR
Cost of installation and maintenance: The expense of setting up and upkeeping a medium, which must be weighed against its other advantages.AQA
Coulomb potential energy expression: E = kQq/r, with k = 1/(4πε₀), equated to ½mv² to estimate the distance of closest approach.AQA
Coulomb's law for point charges: For a point charge Q, the force on q is F = kQq/r², where k = 1/(4πε₀).AQA
Couple: A pair of equal forces acting in opposite directions along different lines of action, so their resultant force is zero but they still rotate an object.AQA, OCR
Coupling gel: Needed in scanning because at a soft-tissue–air boundary Z₂ is much smaller than Z₁, so the fraction reflected is close to 1; the gel improves transmission.AQA, OCR
Coupling gel in ultrasound scanning: A special gel applied between the transducer and the skin that has an acoustic impedance similar to that of soft tissue, reducing the impedance mismatch at…OCR
Critical: The reactor is kept in this state when on average one neutron per fission causes another fission.OCR
Critical angle: Related to the refractive indices by sin C = n₂ / n₁, where n₁ is the core index and n₂ is the cladding index, and n₁ must exceed n₂ for guidance to be…AQA, OCR
Critical angle and refractive index: The critical angle C is given by sin C = n_cladding / n_core, where n_core > n_cladding.AQA
Critical damping: Returns the system to its equilibrium position in the shortest time without overshoot, and is the boundary between oscillatory and non-oscillatory behaviour.AQA, OCR
Critical temperature: A temperature which depends on the material, at and below which certain materials have zero resistivity.AQA
Cross-sectional imaging and display: The result is a two-dimensional map of attenuation, displayed on a monitor for interpretation by a radiologist.OCR
Crossed electric and magnetic fields: A region occupied by both electric and magnetic fields, through which a charged particle moves in a velocity selector.OCR
Crumple zones: Safety features that extend collision time, reducing the average force for the same momentum change.AQA
Cube-root dependence of radius on nucleon number: The radius grows as the cube root of the nucleon number A, following R ≈ 1.2 × 10⁻¹⁵ m × A^(1/3).AQA
Current: The rate of flow of charged particles, given by I = ΔQ/Δt and measured in amperes where 1 A = 1 C s⁻¹.OCR
Current balance: Because negligible current enters the ideal op-amp input, the sum of the input currents equals the current through the feedback resistor.AQA
Current direction in capacitor circuits: The discharging current direction is opposite to the charging current direction.AQA
Current limiting: The role of a series resistor, which drops the difference between the input voltage and the Zener voltage and limits the Zener current.AQA
Current limiting and power dissipation: The resistor value must limit current so the Zener power dissipation does not exceed its rating at minimum load current, when Zener current is highest.AQA
Current–voltage characteristics: The current–voltage graph of a component, which is a straight line through the origin for an ohmic conductor but curves for a filament lamp.OCR
Current, charge and voltage graphs: Graphs of charge, current and potential difference against time are exponential curves, and the area under a current-time graph gives charge.OCR
Cyclic process: A process in which ∆U = 0 over a full cycle, so net Q = net W.AQA
Cyclotron resonance and constant period: The period T = 2πm/(Bq) is independent of speed and radius, so the alternating voltage stays in step as the radius grows.AQA
Cylindrical lens: A lens whose refractive power acts perpendicular to its axis, providing the difference in power between the two principal meridians.AQA

D

Damping: The removal of energy from an oscillatory system, usually transferred to thermal stores, so the amplitude of free oscillations decreases with time.AQA, OCR
Damping and energy dissipation: Damping in a mechanical system removes energy from the oscillations, often as thermal energy, so amplitudes decay or steady-state amplitudes are reduced.AQA
Damping and resonance amplitude: Reducing damping raises and narrows the resonant peak, while increasing damping lowers and broadens it and shifts the peak slightly below the natural frequency.OCR
dark energy: A controversial component making up about 68% of the energy density of the Universe, to which accelerated expansion is attributed.AQA
Dark energy and accelerated expansion: The component associated with the accelerating expansion of the universe.OCR
Dark matter and its gravitational evidence: Matter that does not emit or absorb light but exerts gravity, with evidence including galaxy rotation curves and gravitational lensing.OCR
Data handling and convenience: Images are recorded digitally, stored, processed and shared, while the eye gives a real-time but non-permanent view.AQA
Data logger: A device connected to sensors such as temperature, light, force or motion sensors to collect readings automatically over time.OCR
Data logging and graphing: Record current and p.d.OCR
Data transmission rate: The actual transmission speed of a medium, which is related to but distinct from bandwidth.AQA
de Broglie wavelength: Given by the relation λ = h/p, which links an electron's momentum to its wavelength and supports the wave description of particles.AQA, OCR
de Broglie wavelength of electrons: Given by λ = h/p, where p is momentum, it relates to the electron momentum for electrons accelerated through an anode voltage.AQA
De-sensing: The strong local signal overloading or desensitising the receiver, reducing its ability to detect the weak incoming signal.AQA
Decay constant: A quantity, sometimes written m, related to half-life by the equation λ t₁/₂ = ln 2, allowing conversion between the two.OCR
Decay constant and its unit: λ is the decay constant with unit s⁻¹, and t is the elapsed time.OCR
Decibel scale: A scale relating intensity to loudness, where a tenfold intensity increase corresponds to about 10 dB.AQA
Decimal prefixes: The prefix kilo (symbol k) means one thousand, so it multiplies a unit by 10³.OCR
Decoding gate: A logic gate that combines the appropriate Q outputs so its output goes high only at the unwanted state.AQA
Deep geological disposal: A final destination for high-level waste that isolates it from the biosphere.AQA
Defining equation: The equation for a quantity that is taken and has base units substituted for each symbol.AQA
Defining equation of simple harmonic motion: Acceleration is proportional to displacement and directed towards the equilibrium position, written a = −ω²x, with the minus sign showing opposition to…OCR
Definition of the coulomb: The amount of charge transferred by a current of 1 ampere in 1 second, so that 1 C = 1 A × 1 s.OCR
Definition of the parsec: One parsec is defined as the distance at which one astronomical unit subtends an angle of one second of arc, and 1 pc ≈ 3.09 × 10¹⁶ m, about 3.26 light-years.OCR
Definition of the tesla: The magnetic flux density producing a force of one newton on a straight conductor of length one metre carrying a current of one ampere perpendicular to the…AQA
Degrees to radians: To convert degrees to radians, multiply by π/180.OCR
Demodulation: Recovers the original information signal from the modulated carrier at the receiver.AQA
Demultiplexer: Uses the same timing at the far end to route each sample to the correct output.AQA
Density: Measures how much mass is packed into each unit of volume, given by ρ = m/V, where ρ is in kg m⁻³, m in kg and V in m³.AQA, OCR
Density and speed of ultrasound: The two quantities multiplied together to give acoustic impedance, where ρ is the density of the medium and c is the speed of ultrasound in that medium.OCR
Density as a material property: Density is a property of the material, so it can identify a substance, but it changes with temperature for gases and slightly for liquids.OCR
Dependence of internal energy: For an ideal gas it depends only on its temperature, not on its volume or pressure.AQA
Depth: The fraction of starlight blocked, approximately equal to the square of the ratio of planet radius to star radius.AQA
Depth calculation from echo time: Depth is found from the echo time using distance = speed × time ÷ 2, since the pulse travels to the boundary and back.AQA
Derivation of Kepler's third law: Combining v = 2πr/T with v² = GM/r leads to T² = 4π²r³/(GM), which is Kepler's third law.OCR
Derived unit: A unit formed by combining base units, for example speed in m s⁻¹ or force in kg m s⁻².AQA
Derived units: Units such as the newton (N) or joule (J) that are combinations of base units and are equally valid in calculations.OCR
Derived units with time: Time appears in derived quantities such as speed (m s⁻¹), acceleration (m s⁻²) and frequency (s⁻¹ or Hz).OCR
Destructive interference: Gives minimum amplitude when the waves arrive in antiphase, with phase difference an odd number of π and path difference an odd number of half-wavelengths.AQA, OCR
Destructive superposition: A path difference of a half-number of wavelengths gives this, producing dark fringes.AQA, OCR
Detection and conversion of waves to measurable signals: A microphone detects sound and converts it to an electrical signal for display or measurement.OCR
Detectors: Devices such as the Geiger-Müller tube, cloud chamber or spark counter that are used to detect ionising radiation.OCR
Determining the time constant from a graph: Found by reading the time at which the value is 1/e of its starting value, or from the gradient of a graph of ln V against time, whose magnitude equals 1/RC.AQA
Development of scientific ideas: Scientific ideas can be accepted slowly when evidence is incomplete or a dominant theory exists.AQA
Development of scientific models: Ideas about the nucleus have developed as new evidence and techniques appeared, with each change following new experimental evidence that the previous model…AQA
Diagnostic imaging and therapeutic treatment: Imaging of fractures and tumours gains accurate information at low dose, while therapy uses higher doses to kill tumour cells.AQA
Diesel cycle: Air alone is drawn in and compressed to a high pressure and temperature, fuel is injected and self-ignites, then expansion and exhaust follow.AQA
Difference between speed and velocity: In uniform circular motion the speed is constant but the velocity changes direction continuously, so the velocity is not constant.OCR
Differential absorption: Dense tissue such as bone absorbs more X-rays than soft tissue, so it casts a clearer shadow on the detector.OCR
Differential amplification: The output voltage is proportional to the difference between the input voltages, multiplied by the open-loop gain.AQA
Differential equation for discharging a capacitor: The equation ΔQ/Δt = −Q/CR describes the rate of change of charge Q on a discharging capacitor, where C is capacitance, R is resistance and CR is the time…OCR
Diffraction: The spreading of waves passing through an aperture or around an obstacle, producing a central maximum with subsidiary maxima for a single slit.AQA, OCR
Diffraction and gap-to-wavelength ratio: The spreading of a wave as it passes through a gap or around an obstacle, most pronounced when the gap width is comparable to the wavelength.OCR
Diffraction and resolution: Both types of telescope are diffraction-limited, so angular resolution improves as the aperture diameter increases and as the observed wavelength decreases.AQA
Diffraction grating equation: Constructive interference occurs when d sin θ = nλ, where d is the slit spacing, θ is the angle from the normal and n is an integer.AQA
digital image matrix: Numerical values for every pixel, produced by readout electronics, forming the digital image displayed on a monitor.AQA
Digital pulse: Produced from a peak Hall voltage, and counting these over a fixed time yields the rotation rate.AQA
Digital versus analogue imaging: A flat panel detector produces an electrical signal that is digitised immediately, so the image is available within seconds, whereas photographic film must be…AQA
Dilated time: The longer interval measured by any observer moving relative to the clock, because the moving clock runs slow relative to them.AQA
Dimensional analysis: Checking whether an equation is consistent by replacing each quantity with its base units and seeing whether both sides match.AQA, OCR
Dimensional consistency: Both sides of an equation have the same base units when validating a relationship.AQA
Dimensional consistency and unit conversion: Keeping units consistent by converting every quantity to SI base units, such as centimetres to metres or grams to kilograms, before substituting into the…AQA
Dioptre: The unit (D) of lens power, found from P = 1/f with focal length in metres.AQA
Direct addition of capacitance: The formula is applied by adding the capacitance values directly, without taking reciprocals.OCR
Direct and alternating potential difference: For a dc input the steady vertical displacement from the zero line is read, while for an ac input the amplitude is measured from the centre line to a peak.AQA
Direct proportionality: The special case in which current is directly proportional to potential difference, shown by a straight line through the origin.AQA
Direct proportionality and graphical analysis: A straight line through the origin on a graph of force against the independent variable supports direct proportionality, and the gradient yields B, l or I as…AQA
Direction and magnitude: Arrows on field lines show the direction of force on a positive test charge, and the spacing of lines indicates field strength.AQA
Direction and strength from field lines: The direction of a field line at any point gives the direction of the force on a positive test charge there, and the spacing of the lines indicates field…OCR
Direction from sign: A negative gradient indicates motion in the negative direction.OCR
Direction in one dimension: Represented by a positive or negative sign.AQA
Direction of electric field: The field is directed from the positive plate to the negative plate.OCR
Direction of field relative to charge sign: The field points radially away from a positive charge and radially towards a negative charge.OCR
Direction of force on a conductor: Given by the thumb of the left hand when the first finger points along the magnetic field and the second finger along conventional current.OCR
Direction of gravitational force: The direction of the field line at a point gives the direction of the gravitational force on a small test mass placed there.AQA, OCR
Direction of gravitational, electric and magnetic fields: Determined by inspection: towards the attracting mass for gravitational, the force on a positive test charge for electric, and the way a compass needle's…AQA
Direction of induced current: This is determined by Lenz’s law or by the right-hand rule for a moving conductor.AQA
Direction of magnetic field: The direction of the force on a small free north pole placed at that point.OCR
Discharging: The supply is removed and the plates reconnect through a component, so electrons flow from the negative plate, through the external circuit, back to the…OCR
Discharging circuit: In a discharging circuit, τ is the time for the charge, potential difference or current to fall to 1/e (about 37%) of its initial value.OCR
Discrete energy levels: Only certain energy changes occur, shown by sharp lines rather than the continuous spread a continuous spectrum would imply.AQA, OCR
Discrete nature of charge: Charge is not continuous; it arises because charge is carried by discrete particles like electrons and protons, so an object gains or loses whole electrons.OCR
Dispersion of white light: White light gives a continuous spectrum in each non-zero order, with violet diffracted least and red most.AQA
Displacement: The straight-line distance in a stated direction from start to finish, so it is a vector and can be zero after a round trip.AQA, OCR
Displacement nodes and antinodes in air columns: In air columns, closed ends form displacement nodes and open ends form antinodes.OCR
Displacement–distance graph: Shows the displacement of particles from their rest positions along the direction of travel at one instant, with one full cycle spanning one wavelength λ.OCR
Displacement–position envelope: A graph at a fixed instant showing a sine or cosine curve with regularly spaced nodes and antinodes, crossing the axis at nodes and reaching maxima at…OCR
Displacement–time graph: Shows how the displacement of one particle varies with time, with one full cycle spanning one period T.OCR
Displacement–time oscillation: A graph for a single point showing simple harmonic oscillation whose amplitude depends on the point's position: zero at a node, maximum at an antinode.OCR
Dissipated or wasted energy: Energy transferred to the internal energy store of the surroundings when friction or drag acts, though the total energy is still conserved.AQA
Dissipation and efficiency: In real transfers, some energy is often dissipated to the surroundings as thermal energy, but the total remains constant.OCR
Dissipation of energy during plastic deformation: Some transferred energy is dissipated by internal friction and becomes thermal energy, so the energy recovered on unloading is less than the energy put in.AQA
distance modulus: The relation m − M = 5 log₁₀(d / 10 pc), used to determine distance from apparent and absolute magnitudes.AQA
Distance modulus and parsecs: The relationship m − M = 5 log₁₀(d/10) links apparent magnitude, absolute magnitude and distance in parsecs.AQA
distance of closest approach: The distance d at which a head-on alpha particle stops, where Coulomb potential energy equals its initial kinetic energy Ek.AQA
Doppler effect: Occurs when ultrasound is reflected from moving blood cells, so the frequency of the reflected wave is shifted by an amount Δf compared with the transmitted…AQA, OCR
Doppler shift: The change in observed wavelength of the star's spectral lines caused by the star's line-of-sight motion as it orbits the common centre of mass.AQA
dose reduction: Both methods increase the signal produced per unit patient dose, so they reduce the dose required for a diagnostic image.AQA
Dose reduction and safe practice: Minimise exposure time, handle sources with tongs at arm's length, store them in lead-lined containers, and monitor dose with a film badge.AQA
Dose shaping and beam geometry: A multi-leaf collimator shapes the X-ray beam to match the tumour outline, and several beams from different directions or a rotating beam concentrate dose…AQA
Dose, half-life and penetrating power: The balance between risk and benefit depends on dose, type of radiation and alternative options.AQA
Double-slit arrangement: Uses two coherent sources, or a single source with double slits, to produce an interference pattern observed on a screen as equally spaced bright and dark…AQA
Double-slit interference: Light from the laser passes through two narrow slits and the overlapping waves superpose on a screen, producing evenly spaced bright and dark fringes.AQA
Down-link: The link from satellite to Earth, which uses a different carrier frequency from the up-link.AQA
Drag: A contact force exerted by a fluid on a body moving through it, acting opposite to the relative motion of the fluid.AQA, OCR
Drag dependence on speed: As speed rises, drag rises, so the acceleration of a falling object decreases gradually rather than switching off instantly.AQA
Drag in gases: The drag force on an object moving through air depends on several factors, including speed, cross-sectional area, shape, air density and surface roughness.OCR
Drain: The terminal that collects the carriers and connects to the load or supply.AQA
Ductile: A material that shows an initial straight elastic region obeying Hooke's law, then yields and undergoes large plastic strain before breaking.OCR
Duty cycle: The high time expressed as a fraction or percentage of the period, for example 0.25 or 25 %.AQA
Dynodes: Each is at a successively higher positive potential than the previous one, so electrons are accelerated and each impact releases several secondary electrons,…AQA

E

E = mc²: Einstein's mass–energy equivalence, through which the conversion 1 u = 931.5 MeV links the atomic mass unit to energy.AQA
E.m.f.: The energy transferred from other forms, such as chemical energy in a cell, to electrical energy per unit charge as charge passes through the source.OCR
E.m.f. as a property of the source: A property of the source itself, not of the external circuit, representing the energy supplied per unit charge.OCR
Eddy currents: Currents induced in the iron core by changing magnetic flux, which circulate and heat the core.AQA
Effect of drag on projectile trajectories: Drag reduces both the horizontal and vertical velocity components, so the path becomes asymmetric and the range and maximum height fall.AQA
Effect of larger R or C: A larger resistance or a larger capacitance increases the time constant and therefore slows the charging or discharging process.AQA
Effect of loading on a divider output: The simple divider equations assume negligible current is drawn from the output, so a connected load changes the output voltage.OCR
Effect of loading on output p.d.: If a load is connected across the output, the effective resistance of that part of the circuit changes and the output p.d.OCR
Effect of time step on accuracy of numerical methods: The accuracy improves as Δt is made smaller.OCR
Effect of tube current and voltage on the spectrum: Increasing tube current increases the number of electrons and hence the intensity of X-rays, while increasing tube voltage increases photon energy and reduces…OCR
Efficiency: The ratio of useful output energy to total input energy, expressed as a percentage using efficiency = (useful output energy / total input energy) × 100%.AQA, OCR
Efficient equation form: The form matching the data, chosen to require the fewest rearrangements and reduce arithmetic slips.OCR
Elastic and plastic behaviour: The deformation is elastic if the material returns to its original shape when the forces are removed, and plastic if it does not.OCR
Elastic collision: A collision in which the velocity component perpendicular to the wall reverses, so the molecule's momentum change is −2mu and the wall gains +2mu.AQA
Elastic deformation: Reversible change in which the material returns to its original shape and size when the deforming force is removed.OCR
Elastic limit: The point beyond which the material no longer returns to its original length.AQA, OCR
Elastic potential energy: The energy stored in a material when it is stretched or compressed elastically, recovered when the deforming force is removed.AQA, OCR
Elastic potential energy store: The initial energy source of the stretched or compressed spring, given by ½k(ΔL)².AQA
Elastic potential energy stored in deformed materials: Work done by the stretching force transfers energy to the elastic potential store, equal to the area under a force–extension graph.AQA
Elastic strain energy: The energy stored in a stretched or compressed material when it is deformed elastically, which is recovered when the deforming force is removed.AQA
Elastic versus inelastic collisions: In an elastic collision both momentum and kinetic energy are conserved; in an inelastic collision momentum is still conserved but kinetic energy is…AQA
Electric current: The rate of flow of charge, given by I = ΔQ/Δt, so one ampere is one coulomb per second.AQA, OCR
Electric field: One of a number of forms of field giving rise to a force, existing around any charged object and exerting a force on any other charge placed in it.OCR
Electric field between the plates: Directed from the positive plate to the negative plate and is uniform between parallel plates.AQA
Electric field strength: The force per unit positive charge at a point in a field, E = F/Q, measured in N C⁻¹ or equivalently V m⁻¹.AQA, OCR
Electric force and acceleration: The constant electric force F = qE causes acceleration a = F/m = qE/m.OCR
Electric force on a charge: A force of magnitude QE on a charge Q in a uniform field E, whose direction depends on the sign of Q and the direction of E.AQA
Electric force on a charged droplet in a uniform field: With the electric field applied, the force on the droplet is QV/d, where V is the potential difference across plates separated by d.AQA
Electric potential: The work done per unit positive charge in bringing a small positive test charge from infinity to that point.AQA, OCR
Electric potential difference: The work done per unit positive charge in moving a positive charge between two points, ΔV = W/Q.AQA
Electric potential energy: The work done bringing a charge q from infinity to a distance r from a point charge Q.OCR
Electrical activity of the heart: Depolarisation and repolarisation of cardiac muscle produce small potential differences detected by electrodes on the skin.AQA
Electrical and mechanical analogues: Mass corresponds to inductance, the spring constant corresponds to the reciprocal of capacitance, voltage maps to force, charge to displacement and current to…AQA
Electrical conductivity of samples: The surface must be electrically conducting or semiconducting, so insulators are unsuitable.AQA
Electrical meter connections: Connecting an ammeter in series and a voltmeter in parallel with the component, checking meter positions and polarity before describing the procedure.OCR
Electrical power: The rate of energy transfer in a component, given by P = VI.OCR
Electrical power transmission: The transfer of electrical power through cables that have resistance, so some energy is dissipated as heat.AQA
Electrode polarity: The cathode is the negative electrode and the anode is the positive electrode.OCR
Electromagnetic and kinetic energy: About 10⁴⁴ J released by a supernova, roughly the same as the Sun does over its entire lifetime.AQA
Electromagnetic force: Acts between particles that have charge and is responsible for electric attraction and repulsion, and for magnetic forces.AQA
Electromagnetic induction: A changing flux linkage induces an electromotive force (emf).AQA
Electromagnetic spectrum: A continuous range ordered by frequency or wavelength, with radio waves at low frequency and gamma rays at high frequency.AQA
Electromagnetic wave: Fizeau's value for c closely matching Maxwell's predicted speed provided strong evidence that light is an electromagnetic wave.AQA
Electromagnetic waves: Transverse oscillations of electric and magnetic fields that propagate through a vacuum at the speed of light and can be polarised.AQA
Electromotive force: The energy transferred per unit charge by the source.OCR
Electromotive force (e.m.f.): The energy transferred from other forms to electrical energy per unit charge passing through a source such as a cell or power supply.OCR
Electromotive force and internal resistance: A real source of e.m.f.AQA, OCR
Electron acceleration: The role of the high voltage between cathode and anode, which accelerates the electrons towards the anode.OCR
Electron capture: An inner-shell electron combines with a proton to form a neutron, so Z decreases by 1 and N increases by 1 while A stays constant; a neutrino is emitted.AQA
Electron degeneracy pressure: Pressure arising because electrons cannot occupy the same quantum state, which supports a white dwarf against gravitational collapse instead of fusion.OCR
Electron diffraction: The spreading of electrons after passing through a narrow gap or around an obstacle, producing a pattern of maxima and minima.OCR
Electron flow and conventional current: The flow of electrons constitutes a current in the external circuit, opposite in direction to conventional current.OCR
Electron flow in metallic conductors: In metallic conductors the actual moving charges are electrons, which are negatively charged, so electron flow is in the opposite direction to conventional…OCR
Electron lepton number: A quantum number, Lₑ, that is conserved separately in particle interactions.AQA
Electron optics and magnetic focusing: Magnetic lenses focus and magnify the electron beam because glass would absorb or scatter electrons.AQA
Electron volt: A unit of energy defined as the work done when an electron, carrying elementary charge e = 1.60 × 10⁻¹⁹ C, is accelerated through a potential difference of 1 V.AQA
electronic scanning: Readout electronics address pixels sequentially, typically row by row, transferring each pixel's charge to an amplifier and analogue-to-digital converter.AQA
Electronvolt: A unit of energy equal to the kinetic energy gained by an electron accelerated through a potential difference of one volt.OCR
Electronvolt (eV): The kinetic energy gained by an electron when it is accelerated through a potential difference of 1 volt ($1 \text{ eV} = 1.6 \times 10^{-19} \text{ J}$).Pearson Edexcel
Electrostatic force between point charges: The force is proportional to the product of the charges Q and q and acts along the line joining them, being repulsive for like charges and attractive for…OCR
Electrostatic forces: The repulsion between the positive alpha particle and the positive nucleus, which causes the deflection.AQA
Elementary charge: A fundamental unit of charge, with magnitude e = 1.60 × 10⁻¹⁹ C, equal to the magnitude of the charge on an electron or proton.AQA, OCR
Elliptical planetary orbits: Each planet moves in an ellipse with the Sun at one focus, not at the geometric centre.OCR
Emergency shut-down: Rapidly inserts control rods or injects a neutron absorber such as boron to reduce the neutron population and stop the chain reaction.AQA
Emission: An alternating potential difference is applied across the crystal, causing it to vibrate at the driving frequency and produce ultrasound.OCR
Emission line spectrum: Shows only bright lines at specific wavelengths on a dark background, produced when excited atoms in a low-pressure gas emit photons as electrons drop to…OCR
Empirical versus theoretical knowledge: Empirical knowledge is based on experimental measurements and generalised patterns, whereas theoretical knowledge begins with stated assumptions and derives…AQA
Encapsulation: A storage method in which intermediate-level waste, such as reactor components, is enclosed in cement.AQA
Enclosed loop area: Represents net work done per cycle, while the loop direction shows whether work is done by or on the gas.AQA
Encoding: Each quantisation level is represented by a binary code, and the sequence of codes is transmitted as pulses.AQA
End correction in real tubes: An end correction must be added to the measured tube length to account for the antinode lying just outside the open end.OCR
Energy (voltage) response curve: A curve showing how the output of an LC filter varies with driving frequency, often plotted as output voltage against frequency.AQA
Energy and motion at equilibrium and maximum displacement: At maximum displacement, velocity is zero and acceleration is maximum; at equilibrium, velocity is maximum and acceleration is zero.OCR
Energy changes between orbits: The energy required to move a satellite between circular orbits is calculated using the difference in total energy.AQA
Energy condition for escape: At the surface, radius R, the total energy is ½mv² − GMm/R, and setting this equal to zero at infinity gives ½mv² = GMm/R.OCR
Energy conservation in atomic transitions: ΔE is the difference between two energy levels, so it is positive for an emitted photon.OCR
Energy conservation in electron acceleration: Electrons accelerated through an anode voltage V gain kinetic energy eV, which is combined with the momentum relation to derive the voltage.AQA
Energy conservation in mechanics: A powerful tool for solving mechanics problems without knowing the details of the forces involved.AQA
Energy conservation in orbital motion: No energy is transferred when a satellite moves in a circular orbit because its path lies along an equipotential.AQA
Energy dissipation in resistive cables: Energy is dissipated as heat in cables that have resistance when electrical power is transmitted through them.AQA
Energy efficiency in power transmission: Zero-resistance cables would remove I²R heating losses, raising efficiency, though refrigeration still consumes energy.AQA
Energy equations for capacitors: The energy stored is W = ½QV, and substituting Q = CV gives W = ½CV² or W = ½Q²/C.OCR
Energy equivalent: A value of 1 u = 931.5 MeV used when working in atomic mass units.AQA
Energy exchange in oscillating systems: Both an LC circuit and a mass–spring system oscillate by exchanging energy between two stores.AQA
Energy gain in an alternating electric field: Particles cross a gap repeatedly, accelerated by an alternating potential difference whose frequency matches the cyclotron frequency f = Bq/(2πm).AQA
energy in kW h: Found by multiplying power in kilowatts by time in hours.OCR
Energy in stationary waves: Energy is not transferred along the wave but is stored and exchanged between kinetic and potential forms.AQA
Energy levels: In an isolated gas atom, electrons can occupy only certain allowed levels, each with a definite energy.AQA, OCR
Energy losses and laminated cores: Laminations reduce eddy currents and energy loss.OCR
Energy release in fission and fusion: Fission and fusion release energy because the products have higher binding energy per nucleon, so they are more tightly bound.OCR
Energy release in nuclear reactions: The large value of c² means even a tiny mass defect corresponds to a very large energy release, which is why nuclear reactions release far more energy per…AQA, OCR
Energy release in particle decays: The total rest mass of the pions is less than the kaon rest mass, so the energy difference appears as kinetic energy of the pions.AQA
Energy released: The result of applying ΔE = Δmc², appearing mainly as kinetic energy of the product nuclei and neutrons.AQA
Energy released per kilogram of fuel: A quantity, found using the number of nuclei in the sample, used to compare nuclear energy with fossil fuels in a decision-making context.AQA
Energy storage: A flywheel absorbs energy when the driving torque exceeds demand and releases it when demand exceeds supply.AQA
Energy storage and dissipation: Energy stored elastically is recoverable, whereas energy used in plastic deformation is dissipated, often as heat.OCR
Energy storage in a capacitor: The energy stored by a capacitor can be written in three equivalent forms using Q = CV.OCR
Energy storage in electric fields: A capacitor stores energy in its electric field, analogous to a spring storing elastic potential energy.AQA
Energy storage in magnetic fields: The magnetic field of an inductor stores energy as ½LI², which plays the role of kinetic energy ½mv².AQA
Energy storage rather than transfer: No net energy is transferred along a stationary wave; instead energy is stored, oscillating between kinetic and potential forms.OCR
Energy stored in an electric field: A capacitor stores energy in the electric field between its plates.OCR
Energy stores and forms: Energy exists in different forms or stores, including kinetic, gravitational potential, elastic potential, thermal, chemical, nuclear, electrostatic and…OCR
Energy transfer and charge: The equations W = VQ and W = EQ give the energy transferred when charge moves through a potential difference or an electromotive force.OCR
Energy transfer and temperature change: The equation E = mcΔθ relates energy transferred, mass, specific heat capacity and temperature change.OCR
Energy transfer and the equation E = m l: The energy transferred is E = m l, where m is the mass and l is the specific latent heat.OCR
Energy transfer at resonance: At resonance, energy is transferred most efficiently from the driving source to the oscillating system.OCR
Energy transfer by heating and work: Heating is energy transfer due to a temperature difference; work is energy transfer by mechanical, electrical or other non-heating means.AQA
Energy transfer in circuits: When charge Q moves through a component with potential difference V, the work done is W = QV.AQA
Energy transfer in electric fields: Electrical work done on a charged particle accelerated through a potential difference V equals qV, or eV when the charge magnitude is the elementary charge e.OCR
Energy transfer in electrical circuits: Energy transferred by an electrical component equals power multiplied by time, so W = V I t.OCR
Energy transfer in melting and boiling: Melting and boiling absorb energy, while freezing and condensing release energy, transferred to or from the substance during the phase change.OCR
Energy transfer pathways: Energy can be transferred between stores by mechanical work, electrical work, heating or radiation.OCR
Energy transfer per unit charge in a source: E.m.f.OCR
Energy transfer versus energy storage: A progressive wave transfers energy through the medium, while a stationary wave stores energy without net transfer.OCR
Energy unit conversion: Changing an energy between electron volts and joules using the definition 1 eV = 1.60 × 10⁻¹⁹ J, multiplying or dividing as required.AQA
Energy unit equivalences: 1 eV = 1.60 × 10⁻¹⁹ J and 1 kW h = 3.6 × 10⁶ J.AQA
Energy-mass equivalence: Rest mass can be converted to energy where necessary using E = mc², so a decrease in rest mass appears as an increase in kinetic energy or as photon energy.AQA
Energy–displacement relationships: At any displacement x, kinetic energy is ½mω²(A² − x²) and potential energy is ½mω²x², so their sum is constant.OCR
Energy–frequency relationship: The energy of one photon is directly proportional to the frequency of the radiation, giving E = hf.AQA
Energy–mass equivalence: Binding energy relates to the mass defect through E = mc², where m is the difference between the mass of separate nucleons and the nuclear mass.AQA
Engineering applications: Flywheels are used in machines such as a vehicle, a press or a wind turbine.AQA
Enhancement MOSFET: A device that requires a gate-source voltage above threshold to form its channel, so no channel exists at zero gate-source voltage.AQA
EOR: Exclusive-OR gives 1 when inputs differ, and differs from OR only for the 1,1 input.AQA
Ep = mgh: The equation giving the change in gravitational potential energy when an object of mass m is raised by a height h in a uniform gravitational field.OCR
Equal and opposite charges on plates: The two plates carry charges of equal magnitude and opposite sign.OCR
Equal areas in equal times: The line joining a planet to the Sun sweeps out equal areas in equal times, so a planet moves faster when nearer the Sun.OCR
Equal loudness curve: A curve joining points of equal perceived loudness, produced by adjusting a test tone until it sounds equally loud as a reference tone at 1 kHz.AQA
Equal loudness curves: Curves that join points of different frequency and intensity that sound equally loud.AQA
Equal potential difference in parallel: Each capacitor in a parallel arrangement has the same potential difference across it.OCR
Equation balancing as a validity test: Adding the charges on each side of a quark transformation equation and showing the totals are equal, which also helps identify whether a proposed…OCR
Equations of motion: The suvat equations describe motion under constant force: v = u + at, s = ut + ½at², v² = u² + 2as, s = ½(u + v)t.OCR
Equations of uniform acceleration: Problems will be solvable using these, applying them separately to the horizontal and vertical directions.AQA
Equatorial plane: The plane must contain the equator; any inclined orbit would drift north and south.AQA
Equilibrium: Zero resultant force and constant velocity, distinguished from a body with a non-zero resultant force, which accelerates.AQA
Equilibrium and opposing moments: Clockwise and anticlockwise moments oppose each other and are treated as opposing when applying equilibrium.OCR
Equilibrium conditions: The resultant force acting on the object is zero and the resultant torque about any point is zero, so there is no linear acceleration and no angular…AQA, OCR
Equilibrium of forces: The condition in which the resultant force on the droplet is zero, so the upward electric force balances the downward weight.AQA
Equilibrium state: Each point on a p–V diagram fixes a unique equilibrium state of a fixed mass of gas.AQA
Equipotential surface: A surface on which the gravitational potential V is the same at every point, so the potential difference between any two points on it is zero.AQA
Equipotential surfaces: A surface on which the electric potential is constant, so the potential difference between any two points on it is zero.AQA
Equipotential surfaces and field lines: Surfaces on which potential is constant are always perpendicular to electric field lines; in a uniform field they are parallel planes and around a point…AQA
Equivalence of all inertial frames: Physical laws have the same form in all inertial frames, so no inertial observer can perform an experiment that reveals absolute rest or absolute motion.AQA
Equivalent energy formulae: W = ½QV, W = ½CV² and W = ½Q²/C are the three equivalent expressions for energy stored by a capacitor.OCR
Equivalent forms of the energy equation: E = ½QV, E = ½CV² and E = ½Q²/C are equivalent, each useful depending on which quantities are known.AQA
Error bar: Drawn through a point to show the absolute uncertainty associated with a plotted measurement.AQA
Error bars: Bars drawn on a graph to show the uncertainty attached to each plotted value, centred on the plotted point.AQA
Error reduction: Repeating for several currents and plotting a graph, which averages random errors and improves reliability.OCR
Escape speed: Given by v = √(2GM/r), it is independent of the escaping mass and is a speed, not a fixed direction.AQA
Escape velocity: The minimum speed a projectile needs at the surface of a mass M to escape its gravitational field and reach infinity with zero kinetic energy.AQA, OCR
Escape velocity formula: The expression v = √(2GM/R) obtained by equating total energy at the surface to zero at infinity.OCR
Estimating the age of the universe: If expansion has been roughly uniform, the time since galaxies were coincident is the distance divided by the speed, giving t ≈ H₀⁻¹.OCR
Estimating the age of the universe from 1/H₀: The reciprocal 1/H₀ has units of seconds and gives a rough age of the universe, about 4.4 × 10¹⁷ s or 1.4 × 10¹⁰ years.OCR
estimation: Typically involves reading a spectrum to find z, using v = zc and d = v / H₀ to estimate distance, then comparing apparent flux with a known standard to…AQA
Estimation of stellar surface temperature: Surface temperature is estimated using T = 2.9 × 10⁻³ / λmax, after measuring the wavelength at which the continuous spectrum is most intense.OCR
Ethical and environmental trade-offs in transport design: Ethical design weighs safety benefits against cost, mass, fuel consumption and environmental impact.AQA
Eustachian tube: Connects the middle ear to the throat and equalises pressure across the tympanic membrane.AQA
eV = hc/λ: The equation stating that the energy lost by an electron crossing the LED junction equals the emitted photon energy.OCR
Event horizon: The boundary inside which nothing can escape, including light.OCR
Evidence and anomaly: Each model survived only while it matched observation, and anomalies forced revision.AQA
Evidence and theory: Acceptance was delayed by scientific authority, limited evidence and the difficulty of detecting interference with early equipment.AQA
Evidence for Maxwell’s electromagnetic theory of light: The measured speed agreed with the speed of light, supporting Maxwell’s prediction that light is an electromagnetic wave.AQA
Evidence for the photon model: This independence supports the photon model over a continuous wave model of light.OCR
Evidence for transverse nature: If a wave can be polarised it must be transverse, because only transverse waves have perpendicular oscillation directions that can be selected.AQA
Evidence from scattering: Most alpha particles pass straight through the atom in scattering experiments, while only those passing very close to the nucleus are strongly deflected.OCR
Evolution of the universe: After the Big Bang the universe expanded and cooled, passing through a sequence of stages up to the present.OCR
Exchange particles: Forces between elementary particles are explained by particles that transmit the force between them.AQA
Exchange particles and Feynman diagrams: The W⁻ boson is the exchange particle mediating the weak interaction, and it then decays into an electron and an antineutrino.AQA
Exchange particles as carriers of forces: A wavy line represents the exchange particle, which is a virtual photon for electromagnetic interactions and a W⁺ or W⁻ boson for weak interactions.AQA
Excitation: When an electron gains a specific amount of energy and moves to a higher energy level within the atom.AQA
Expansion and cooling of the universe: As space-time expanded, the radiation was stretched to microwave wavelengths and cooled to its present 2.7 K.OCR
Expansion of space-time: Space-time began to expand immediately after the Big Bang and continues to expand today, so the space between galaxies stretches.OCR
Expansion of the Universe: The Big Bang model describes an expanding Universe from a hot, dense early state.AQA
Experimental accuracy in radioactivity: Improved by subtracting background, removing a constant offset from half-life or absorption measurements.AQA
Experimental control of diffraction variables: Shining a laser through a single slit and gradually narrowing the slit shows the central maximum spreading out as the slit becomes narrower.AQA
Experimental design and control of variables: Procedures must control variables, repeat readings and reduce random and systematic errors.OCR
Experimental design and evaluation: Identifying and controlling sources of error such as parallax, delay in manual timing and air resistance, then comparing the experimental value of g with the…AQA
Experimental design and uncertainty: Controlling variables such as source activity, detector settings and measurement time, and repeating readings to reduce random uncertainty.AQA
Experimental determination of resistivity: The resistivity of a metal wire is measured using a micrometer, ammeter and voltmeter, with resistance calculated using R = V/I and area using A = πr².AQA, OCR
Experimental evidence: Firing electrons through a thin graphite film or narrow slit and observing a diffraction pattern on a fluorescent screen.AQA, OCR
Experimental evidence for cosmological models: Penzias and Wilson's 1965 detection provided key experimental support for the Big Bang, and it is now mapped in detail by satellites such as COBE and Planck.OCR
Experimental generation and detection of radio waves: A high-voltage spark across a gap produced oscillating charges that radiated electromagnetic waves, and a resonant loop receiver tuned to the same frequency…AQA
Experimental measurement and uncertainty: Repeating measurements and calculating a mean, and considering uncertainties in length, time and mass, improves reliability.AQA
Experimental measurement of charge and p.d.: Current and p.d.OCR
Experimental measurement of half-life: A sample is placed near a Geiger-Muller tube connected to a counter or ratemeter, and the count rate is recorded at regular time intervals.OCR
Experimental observation through a microscope: In the demonstration, smoke particles suspended in air are observed through a microscope and are seen to move in irregular zigzag paths.OCR
Experimental precision: Reflected directly by the size of the error bar for that specific data point.AQA
Experimental technique: Mounting a rigid straight wire horizontally between the poles so it is perpendicular to the field, with the magnet supported on a digital balance.OCR
Experimental uncertainty and evaluation: Using several colours gives a range of frequencies and reduces the effect of random uncertainty, and the value obtained is approximate because the threshold…OCR
Experimental uncertainty and improvement: Repeat readings and calculate means, or use a graphical method, to reduce the effect of random errors.AQA
Explaining shape, volume and pressure with the model: A solid keeps a fixed shape and volume, a liquid has a fixed volume but takes the shape of its container, and gas pressure arises from collisions of particles…OCR
Explosions as momentum-conserving separations: An initially stationary or moving body separates, and the total momentum afterwards equals the total momentum before, often zero if the system started at rest.AQA
Exponential attenuation: The decrease in beam intensity is exponential rather than linear because each thin layer removes a constant fraction of the photons still present.AQA
Exponential attenuation of X-rays: The attenuation of a collimated X-ray beam follows I = I₀e^(−μx), where I₀ is the incident intensity, I is the transmitted intensity, x is the absorber…OCR
Exponential change: The shape of the charging and discharging curves, which have a changing gradient rather than being straight lines.AQA
Exponential charging and discharging: Charging current is largest at the start and falls exponentially as the capacitor voltage rises, while discharging reverses the process so charge, current and…OCR
Exponential decay: On discharge, Q, V and I all fall exponentially from their initial values towards zero.AQA, OCR
Exponential decay and growth: Describing quantities that fall according to x = x₀e^(−t/CR) or rise towards a maximum steady value according to x = x₀(1 − e^(−t/CR)).OCR
Exponential decay graph: A curve that decreases asymptotically towards zero, showing a quantity decreasing at a rate proportional to its current value.OCR
Exponential decay in context: The age can be estimated using N = N₀e^(−λt) or by counting how many half-lives have passed.OCR
Exponential decay law: The rate of decay is proportional to the number of undecayed nuclei, leading to N = N₀e^(−λt).OCR
Exponential decay of activity: The equation A = A₀e^(−λt) relates activity at time t to initial activity, decay constant and elapsed time.OCR
Exponential decay of undecayed nuclei: The equation N = N₀e^(−λt) relates the number of undecayed nuclei at time t to the initial number.OCR
Exponential fall during discharge: During discharge, the charge, current and potential difference all fall exponentially, and after one time constant each quantity has fallen to 1/e of its…AQA
Extension: The change in length, not the total length, so always subtract the original length.OCR
Extension and compression: The increase in length from the natural length and the decrease in length from the natural length respectively, both measured in metres.OCR
Extragalactic: Lying beyond our Galaxy, as quasars do, being extremely remote objects rather than nearby stars.AQA
Eye safety with electromagnetic radiation: Even a brief exposure to a direct or reflected laser beam can cause permanent retinal injury because the lens focuses the beam onto a small area of the retina.AQA
Eyepiece: A converging lens of short focal length placed so the intermediate image lies at its principal focus, so emergent rays are parallel.AQA

F

Factors affecting capacitance: Capacitance increases with plate area and permittivity, and decreases with separation.OCR
Factors that change flux linkage: Flux linkage changes if B changes, if the area A changes, or if the angle θ between field and normal changes.OCR
Fail-safe: Safety systems are designed to be fail-safe and may include multiple independent shut-down mechanisms.AQA
Familiar physical anchors: Known everyday values, such as a person's mass of about 70 kg, that are scaled to the target quantity.OCR
Farad: The unit of capacitance, where one farad is one coulomb per volt, so 1 F = 1 C V⁻¹.OCR
Faraday's law: The magnitude of an induced e.m.f.AQA, OCR
Faraday's law of induction: The observed induced e.m.f.AQA
Feedback control: A control system compares the count rate with a set value and adjusts rollers to keep thickness constant.AQA
Feedback control systems: A feedback system adjusts the tip height to keep the tunnelling current constant as the tip is scanned across the surface.AQA
Fiducial marker: A clear reference point for judging when a moving object passes a position, improving consistency.OCR
Field: A region of space in which a suitable object experiences a force.AQA, OCR
Field direction: This depends on the direction of conventional current or on the orientation of the magnet.OCR
Field emission: Process distinct from thermionic emission in which a strong electric field pulls electrons from a surface.AQA
Field line conventions: A visual convention for mapping a magnetic field, in which lines emerge from a north pole and enter a south pole outside a magnet.OCR
Field lines and charge sign: Field lines start on positive charges and end on negative charges, and they never cross.OCR
Field lines and potential: Both fields can be represented by field lines, and both have a potential concept.AQA
Field patterns of current-carrying conductors: Each current-carrying shape produces a characteristic field pattern, allowing prediction of field direction and relative strength in each case.OCR
Field representation: Electric field lines represent the electric field in a region, with the tangent at any point giving the direction of the force on a positive test charge.AQA
Field strength: This decreases with distance from the current or magnet.OCR
Field strength and line spacing: The spacing of lines indicates field strength: closer lines mean a stronger field, and line density decreases with distance from the centre.AQA, OCR
Field strength definition: Electric field strength is defined as the force per unit positive charge.OCR
Field strength from line spacing: Closely spaced lines mean a strong field, while widely spaced lines mean a weak field.OCR
Field-line diagrams: Show these vectors, with arrows indicating direction and spacing indicating relative magnitude.AQA
Filament lamp: Its resistance increases as current increases because the filament temperature rises, so the I–V graph curves and flattens.AQA
Finite speed: The result showed light has a finite speed, contradicting instantaneous propagation.AQA
First law of thermodynamics: A statement of conservation of energy for a system, linking internal energy change to heating and work.AQA
first minimum: Occurs at an angle θ where sin θ ≈ λ/D, with D the nuclear diameter and λ the de Broglie wavelength of the electrons.AQA
Fission: A heavy nucleus absorbs a neutron and splits into two smaller nuclei, releasing two or three neutrons and energy.AQA
Fixed mass of gas: A condition of the gas laws, since the mass of gas must stay fixed for each relationship to hold.AQA
Fixed versus moving patterns: A progressive wave's waveform moves through space, while a stationary wave's nodes and antinodes remain in fixed positions.OCR
Fleming's left-hand rule: Gives the direction of the force: first finger field, second finger current, thumb motion or force.AQA, OCR
Fluid: Any liquid or gas, so air counts as a fluid.OCR
Fluorescent tube: A tube in which accelerated electrons collide with mercury atoms, and a fluorescent coating absorbs ultraviolet photons and re-emits visible light.AQA
Fluorine–18: A tracer used in positron emission tomography because it emits positrons; when a positron meets an electron, two gamma photons are produced in opposite…OCR
fluoroscopic image intensification: Converts the X-ray pattern into visible light, then into electrons, accelerates and focuses them, and produces a small bright output image.AQA
Flux leakage: Where some magnetic field lines do not link both coils.AQA
Flux linkage: For a coil of N turns, NΦ, derived as NΦ = BANcosθ, identifying B, A, N and θ with correct units: T, m², dimensionless and degrees or radians.AQA
Flywheel: A rotating mass that stores rotational kinetic energy, with angular momentum that resists rapid change so it smooths torque and speed.AQA
Folded optical path: The convex secondary increases the effective focal length and allows a long focal length to be folded into a compact tube.AQA
Force and motion: If the force and velocity are not in the same direction, only the component of force along the velocity contributes, so P = Fv cos θ.OCR
Force as a rate of change: The net force equals the change in momentum divided by the time interval over which it occurs.OCR
Force as rate of change of momentum: The force on occupants depends on how quickly momentum changes, since force is the rate of change of momentum.AQA
Force balance on a moving charge: The electric force QE and magnetic force BQv act in opposite directions, and when they balance QE = BQv.OCR
Force constant: The force constant k measures stiffness: the force needed per unit extension, in N m⁻¹.OCR
Force on a charge in a field: Rearranging gives F = Eq, so a charge q in a field E experiences a force Eq.OCR
Force on a current-carrying conductor: Has magnitude F = BIl sin θ, where θ is the angle between the conductor and the field, and is maximum when perpendicular and zero when parallel.AQA
Force on a current-carrying wire: F = BIl when the field is perpendicular to the current, and the force is perpendicular to both the field and the current.AQA
Force representation: Forces are represented by arrows, where length indicates magnitude and direction indicates the force's direction.OCR
Force-separation relationship: Between about 0.5 fm and 3 fm the force is attractive, with a maximum attraction around 1 fm, and below about 0.5 fm it becomes repulsive, preventing nucleons…OCR
Force–extension graphs: A graph that plots applied force F on the y-axis against extension x on the x-axis for springs and wires.OCR
Force–extension relationships: A graph plotting the applied force on the vertical axis against the extension (or compression) on the horizontal axis.OCR
Force–time graphs: A plot of force on the y-axis against time on the x-axis, where the area between the graph line and the time axis equals the impulse.OCR
Force–time relationships: The impulse of a force is the product of the force and the time for which it acts: impulse = FΔt.OCR
Forced oscillation: Occurs when a periodic external driving force is applied, so the system oscillates at the driving frequency rather than its natural frequency.OCR
Forced vibration: Occurs when a periodic driving force makes the object oscillate at the driving frequency.AQA
Formation of a real intermediate image: The secondary mirror redirects the rays to cross and form a real intermediate image at the focal plane, strictly up to the eyepiece.AQA
Formation of stationary waves: Two progressive waves of the same frequency and similar amplitude travel in opposite directions through the same medium and superpose.AQA
Forward and reverse bias: A diode conducts only above a forward threshold and blocks reverse current, and an LED is a diode that emits light when forward biased.OCR
Forward bias: The condition where the anode is positive relative to the cathode, giving conduction at about 0.7 V for a silicon Zener.AQA
Fovea: The region where cones are densely packed, each connecting to its own bipolar and ganglion cell, giving high acuity in good light.AQA
Fractional quark charge: Quark charges are given as fractions of the elementary charge e, such as +2/3 e for the up quark and −1/3 e for the down and strange quarks.OCR
Fractional uncertainty: Equals the absolute uncertainty divided by the measured value, so 0.001 ÷ 0.842 ≈ 0.00119.AQA
Fracture and breaking point: The point at which the material breaks, shown where the graph ends and the force falls to zero.AQA
Free fall and uniform acceleration: Motion in which an object falls with uniform acceleration, determined here by timing a falling object over a measured distance.AQA
Free fall in a uniform gravitational field: For a body falling in a uniform gravitational field without air resistance, acceleration is g, approximately 9.81 m s⁻² downwards, and a consistent positive…OCR
Free fall with drag: Motion of an object falling in a uniform gravitational field in the presence of drag, where acceleration is not constant.OCR
Free oscillation: Occurs when a system oscillates at its natural frequency with no external periodic driving force, after being displaced and released.OCR
Free vibration: Occurs when an object oscillates at its natural frequency after being displaced, with no periodic external force.AQA
Free-body diagrams: A diagram of the point showing each force with its correct direction and labelling unknown magnitudes clearly.AQA, OCR
Frequency: The number of complete revolutions per second, measured in hertz (Hz), where 1 Hz = 1 s⁻¹.AQA, OCR
Frequency and period relationship: Frequency is the reciprocal of the time for one complete cycle, so carrier frequency is found from f_c = 1/T_c and information frequency from f_m = 1/T_m.AQA
Frequency as a source-determined property: When a wave crosses into a new medium its speed and wavelength change, but frequency stays constant because it is set by the source.OCR
Frequency control: The running frequency is set by the external resistor and capacitor values in the timing network.AQA
Frequency modulation: The carrier's amplitude stays constant while its frequency shifts above and below the unmodulated carrier value at the modulating rate, so the trace looks…AQA
Frequency modulation (FM): A technique in which the carrier frequency is varied while its amplitude stays constant.AQA
Frequency offset: A duplex spacing that allows filtering so the receiver is not de-sensed.AQA
Frequency response: The bandwidth is the range up to the 3 dB point, and typical units are hertz since gain is a dimensionless ratio and bandwidth is a frequency.AQA
Frequency response curve: A plot of the gain of an operational amplifier against the frequency of the input signal, usually on logarithmic axes.AQA
Friction: A force that opposes relative motion or the tendency of relative motion between surfaces in contact.AQA, OCR
Friction power: The indicated power minus the brake power.AQA
Frictional torque: Opposes motion in rotating machinery, so the net torque is the applied torque minus the frictional torque.AQA
Fringe spacing: The spacing w between equally spaced fringes formed on a screen in Young's slit experiment, measured by recording the distance across several fringes and…AQA, OCR
Fuel rods: These hold fissile material such as uranium-235, often as uranium dioxide, and provide the nuclei that undergo induced fission.OCR
Full turn in radians: Because the circumference is 2πr, a full turn is 2π radians, so 360° = 2π rad.OCR
Functional medical imaging: A technique that gives functional information about physiology rather than sharp anatomical detail, showing the distribution of tracer uptake.OCR
Fundamental (base) units: The small set of independently defined SI units from which every other unit is built.AQA
Fundamental forces and their ranges: The strong force is short-range, acting only over a few femtometres, with attraction up to approximately 3 fm and repulsion closer than approximately 0.5 fm.AQA
Fundamental frequency and harmonics: The patterns determine the resonant frequencies, and the fundamental frequency of a closed tube is half that of an open tube of the same length.OCR
Fundamental interactions: Gravity, electromagnetic, weak nuclear and strong nuclear are the four interactions into which all forces between particles are grouped.AQA
Fundamental mode: The lowest frequency stationary wave pattern, also called the first harmonic.OCR
Fundamental particles: Particles that have no internal quark structure, unlike protons and neutrons.AQA
Fundamental thermodynamic principles: The basis of the scale, such as the efficiency of a Carnot engine, rather than the expansion of a specific material.OCR
Fusion: Two light nuclei combine to form a heavier nucleus, releasing energy.AQA
Fusion and fission: Fusion of light nuclei moves them up the steep left-hand side towards the peak, and fission of heavy nuclei splits them into mid-mass fragments nearer the…AQA

G

Gain and bandwidth: For a given device, the product of voltage gain and bandwidth is approximately constant.AQA
Gain-bandwidth product: A finite value meaning the available gain falls as signal frequency increases, limiting performance at high frequencies.AQA
Galaxy: A gravitationally bound system containing stars, gas, dust and dark matter, of which the Milky Way is one example.AQA
Gamma emission: The emission of a high-frequency electromagnetic quantum with no mass or charge when a nucleus de-excites to a lower energy state, leaving proton and nucleon…AQA
Gamma photon energy and detector efficiency: The energy must be sufficient to penetrate the body yet suitable for efficient detection, so an intermediate energy such as about 100 keV is chosen.AQA
Gamma ray burst: A brief, intense emission of gamma rays, extremely energetic flashes of gamma radiation.AQA
Gas laws: The equation combines Boyle's law, Charles's law and Avogadro's law, and for a fixed mass of gas pV/T is constant.AQA, OCR
Gas laws from experiment: Boyle's law, Charles's law and the pressure law were established by making measurements of p, V and T and generalising patterns from the data.AQA
Gate: The control terminal, separated from the channel by a thin insulating oxide layer, whose voltage sets the channel conductivity.AQA
Gate-source voltage: The controlling input, while the drain-source path carries the output current.AQA
Gates as functional building blocks: Each gate is treated as a single functional block with defined inputs and outputs, and its internal structure or circuit is not required.AQA
Geiger-Müller tube: A detector used with a counter to measure count rate when investigating absorption of α-particles, β-particles and γ-rays by appropriate materials.OCR
Geometric centre: The point at which the whole weight can be placed in moment calculations for a uniform regular solid.AQA
Geometry determining capacitance: For an isolated sphere the capacitance depends only on the radius of the sphere and the permittivity of free space, not on the charge stored.OCR
Geostationary orbit: A circular orbit around Earth's equator with a period of one sidereal day, moving west to east at a specific altitude above Earth's surface.AQA, OCR
Geostationary satellites: These orbit above the equator with a period of 24 hours, so a dish can be fixed; they are used for television and telephony but have a long path and…AQA
Geosynchronous orbit: An orbit with a 24-hour period that may be inclined rather than equatorial, of which the geostationary orbit is a special case.OCR
Giants and supergiants: Stars in the upper right region with relatively cool surfaces but very high luminosity, which requires very large radii.AQA
Glare: Starlight scattered by telescope optics and dust adds background that can hide the planet's faint signal.AQA
Gluons: Exchange particles that carry the strong interaction and act between quarks, holding the quarks together inside protons and neutrons.AQA
Gold-leaf electroscope demonstration: A clean zinc plate on the cap of a gold-leaf electroscope can demonstrate the photoelectric effect.OCR
Gradient: The rate of change of the vertical quantity with respect to the horizontal quantity, found by drawing a tangent and dividing the vertical change by the…AQA
Gradient and force constant: The gradient of the linear region equals the force constant k, since F = kx.OCR
Gradient and intercept: Values found from a line of best fit, whose uncertainties are obtained by halving the difference between the steepest and shallowest values.AQA
Gradient as a graphical derivative: The gradient of the displacement–time graph gives velocity, and the gradient of the velocity–time graph gives acceleration.OCR
Gradient as a rate of change: On a displacement–time graph the gradient gives velocity, and on a velocity–time graph it gives acceleration, found using a tangent where the graph is curved.AQA
Gradient as rate of change: The gradient of the graph equals velocity, calculated as change in displacement divided by change in time, with units.OCR
Gradient interpretation of a v–d graph: H₀ is the gradient of a velocity–distance graph, relating recession velocity to distance by v = H₀d.OCR
Graph extrapolation: Extending the straight line to p = 0 gives the intercept on the T-axis, which is absolute zero.OCR
Graph gradients: On a displacement–time graph the gradient gives velocity, and on a velocity–time graph the gradient gives acceleration.OCR
Graph gradients, intercepts and linearisation: Plotting y against x gives gradient m and intercept c for y = mx + c, while logarithms linearise power and exponential relationships.OCR
Graph interpretation: A component is identified from the shape, symmetry and threshold of its graph, using the gradient or the ratio V/I to explain resistance changes.AQA, OCR
Graph plotting and analysis: A data-logger gives smooth exponential graphs and allows the time constant to be found from the curve.OCR
Graphical analysis: Plotting fₙ against 1/L, against √T and against 1/√μ to obtain straight-line relationships through the origin if the theory holds.AQA, OCR
Graphical analysis of electrical circuits: Plotting the variable against time, identifying the initial value and the asymptote, then checking the curve falls to about 37% after one time constant.AQA
Graphical analysis of exponential decay: Corrected count rate is plotted against time on a graph, and the half-life is read as the time for the count rate to halve, or found from a suitable…OCR
Graphical analysis of Hooke's law data: Plotting force against extension gives a straight line through the origin within the elastic limit, and its gradient k is used with E = kL/A.OCR
Graphical analysis of linear relationships: Plotting V against I gives a line of best fit whose V-axis intercept is ε and whose gradient magnitude is r.AQA
Graphical analysis of motion: Plotting s against t² gives a straight line through the origin with gradient ½g, so g is twice the gradient.AQA
Graphical analysis of V and I: Plot V against I and use the gradient to give a more reliable R than a single pair of readings.OCR
Graphical analysis using gradients: A graph of resistance R against length L is plotted, and since the gradient equals ρ/A, resistivity is found from gradient × A.AQA
Graphical analysis using natural logarithms: Taking natural logarithms gives ln I = ln I₀ − μx, so a graph of ln I against x is a straight line of gradient −μ.AQA
Graphical determination: Plotting ln V against time for discharge, where the gradient is −1/RC, so τ is the negative reciprocal of the gradient.AQA
Graphical determination of rate of change and time constant: The gradient of the Q–t graph at any instant equals ΔQ/Δt, and plotting ln Q against t gives a straight line of gradient −1/CR.OCR
Graphical determination of stored energy: Energy stored equals the area under the force–extension graph; for a straight-line region this is ½FΔL, and for curved regions the area is found by counting…AQA
Graphical integration and estimation techniques: The area is estimated by splitting the region between the path and the volume axis into strips or simple shapes, counting grid squares, or using the trapezium…AQA
Graphical interpretation of area and gradient: The area under a graph of charge against potential difference represents the energy stored, while the gradient is the capacitance C.AQA
Graphical interpretation of force–extension behaviour: A hysteresis loop encloses an area representing energy dissipated per cycle, and beyond the elastic limit the material does not return all the stored energy.AQA
Graphical interpretation of gradients: The gradient of the displacement–time graph at a given instant gives the instantaneous velocity, and the gradient of the velocity–time graph at a given…AQA
Graphical interpretation of ρ against T: A graph of ρ against T has a positive gradient for a metal and a negative gradient for a semiconductor, with the semiconductor fall being non-linear and…OCR
Graphical methods: Plotting the modelled values of N against t to visualise the exponential decay curve and determine the half-life.OCR
Graphical representation of energy: The energy stored is the work done in transferring charge against the increasing p.d., which equals the area under the V–Q graph.OCR
Graphical representation of force against distance: A curve decreasing with distance, asymptotic to the distance axis, that falls steeply as r increases and never crosses the axis for like charges.OCR
Graphical representation of uncertainty: Plotting data with error bars, then drawing a line of best fit through the points so that the trend is represented fairly.OCR
Graphing and interpreting experimental data: Plot a graph of output voltage against temperature or light intensity and compare with the predicted divider equation.OCR
Graphs of x, v and a against time: They share the same period, with v maximum at equilibrium and zero at maximum displacement, and a maximum at maximum displacement and zero at equilibrium.AQA
Grating equation: The relation d sin θ = nλ giving the angles θ at which sharp maxima occur when monochromatic light passes through the grating.OCR
Grating spacing: The spacing d of a diffraction grating, determined from the number of lines per metre using d = 1 / N.AQA, OCR
Gravitational collapse: A region of interstellar dust and gas, mainly hydrogen, becomes dense enough for gravity to overcome internal pressure, pulling matter inward.OCR
Gravitational constant G: A universal constant, approximately 6.67 × 10⁻¹¹ N m² kg⁻², appearing in F = −GMm/r².OCR
Gravitational field: One of a number of forms of field giving rise to a force, acting on mass and always producing an attractive force.OCR
Gravitational field lines: A visual tool to map the gravitational field around a mass, used to represent the field without calculations.OCR
Gravitational field outside and inside a sphere: Outside the sphere, g = GM/r² with r measured from the centre; inside a uniform sphere the field is not given by GM/r² and depends on the mass enclosed within…OCR
Gravitational field strength: The gravitational force per unit mass on a small test mass placed at a point, g = F/m, measured in N kg⁻¹.AQA, OCR
Gravitational fields: A region produced by an object with mass in which another mass experiences an attractive gravitational force.OCR
Gravitational force as centripetal force: For a satellite in a circular orbit, gravitational attraction provides the centripetal force, leading to mv²/r = GMm/r² and hence v² = GM/r.AQA
Gravitational force as the centripetal force: The gravitational attraction between the planet and the Sun acts towards the Sun and provides the required centripetal force.OCR
Gravitational potential: The work done per unit mass in bringing a small test mass from infinity to that point.AQA, OCR
Gravitational potential difference: The difference in gravitational potential between two points, equal to the work done per unit mass in moving a small test mass between them.AQA
Gravitational potential energy: The product of the mass and the gravitational potential at that point, given by E = mV = −GMm/r at a distance r from a point mass M.AQA, OCR
Gravitational potential energy conversion: Infalling matter forms an accretion disc, and gravitational potential energy transfers to thermal energy, heating the disc to millions of kelvin.AQA

H

h = eVλ/c: The rearrangement of eV = hc/λ used to estimate the value of Planck constant h from measured values.OCR
Hadron: A particle composed of quarks that experiences the strong nuclear force (e.g., protons, neutrons, mesons).Pearson Edexcel
Hadron classification: Hadrons are particles that feel the strong interaction and are not fundamental, and the simple quark model explains them as composites of quarks.OCR
Hadrons: Particles made of quarks and antiquarks, divided into baryons, which contain three quarks, and mesons, which contain a quark and an antiquark.AQA, OCR
Half-life: The average time taken for half the nuclei in a sample to decay, or for the activity to halve, and is a constant for a given isotope.AQA, OCR
Half-life and its meaning: The average time for half the nuclei in a sample to decay, and it is constant for a given isotope.OCR
Half-value thickness: The half-value thickness x½ satisfies x½ = ln2/μ, the thickness that reduces intensity to half its initial value.AQA, OCR
Hall effect: A steady transverse voltage is produced across a current-carrying conductor placed in a magnetic field due to charge separation.AQA
Hall effect sensor: Acts as the sensing element by detecting a small magnet attached to the rotating object.AQA
Hall voltage: Given by V_H = B I / (n t e), where B is magnetic flux density, I is current, n is charge carrier density, t is thickness and e is elementary charge.AQA
Harmonic number n: Equals the number of half-wavelengths on the string and also the number of antinodes present.AQA
Harmonics: Distinct patterns in which a stationary wave on a stretched string or in a pipe can vibrate, with frequencies that are integer multiples of the fundamental…AQA, OCR
Hazard identification: Showing awareness of safety issues associated with using lasers, whose narrow, intense, monochromatic beams can damage eyes.AQA
Heat engine: It converts thermal energy into useful work, but only if thermal energy flows from a hot source to a cold sink.AQA
Heat engine efficiency: Useful work divided by heat absorbed, given as W/Q_h, and always less than 1 when Q_c is positive.AQA
Heater: A filament that is heated by a low-voltage supply and controls the number of electrons released and hence the tube current.OCR
Heavy damping: Causes the system to return to equilibrium slowly, without completing oscillations.AQA, OCR
High voltage and electron kinetic energy: The high voltage supply maintains a large potential difference between cathode and anode, giving the electrons their kinetic energy.OCR
homogeneity: A property of physical equations checked by base-unit analysis, such as breaking the newton down into kilograms, metres and seconds.OCR
Homogeneous: The distribution of matter and energy is the same everywhere, so no location is special.OCR
Hooke's law: The extension of a spring or wire is directly proportional to the applied force, provided the limit of proportionality is not exceeded.AQA, OCR
Hooke's law behaviour: For a material obeying Hooke's law the line is straight through the origin, so force is proportional to extension.AQA, OCR
Hot and cold reservoirs: A hot source and a cold sink, both required by a real engine, which rejects some heat Q_c to the cold sink.AQA
Hot spots and pathological uptake: Areas of high uptake appear as hot spots and may indicate tumours, active inflammation or specific brain activity.OCR
HR diagram: A diagram plotting absolute magnitude against decreasing temperature, on which the Sun's position must align with the 5700 K and +5 marks.AQA, OCR
Hubble constant: Describes how fast the universe expands, relating recession speed to distance as v = H₀d, so it has units of s⁻¹.OCR
Hubble's law: The recessional speed v of a galaxy is approximately proportional to its distance d from the observer: v ≈ H₀ d.AQA, OCR
Hubble's law and recession of galaxies: Distant galaxies recede faster because the space between them stretches, so more distant galaxies have greater recession speeds.OCR
Huygens’ construction: Every point on a wavefront acts as a source of secondary wavelets whose envelope gives the next wavefront.AQA
Hydrostatic equilibrium: A stable main-sequence star exists when inward gravitational force balances outward pressure.OCR
Hypermetropia: Long sight, where the eye is too weak or too short, so parallel rays would focus behind the retina and near objects blur.AQA
Hypothesis testing in physics: Conservation laws drive the discovery of new particles, and experimental detection of the neutrino later supported the hypothesis.AQA
Hysteresis: A feature of polymeric materials where the loading and unloading curves differ, and the area between them represents energy dissipated per unit volume.AQA, OCR

I

I = ΔQ/Δt: The equation relating current I to the charge ΔQ that passes a point in time Δt.OCR
Ideal ammeter: Has zero resistance and is connected in series with the component, so no potential difference is dropped across it and it does not change the current.AQA
Ideal assumption: Unless a question specifies non-ideal meters, treat the readings as the true values in calculations and circuit analysis.AQA
Ideal gas: A gas for which potential energy is negligible, so internal energy depends only on total kinetic energy and therefore on temperature and number of particles.AQA
Ideal gas assumptions: A large number of identical particles in random motion, with negligible volume and no intermolecular forces except during elastic collisions.OCR
Ideal gas equation: Written as pV = nRT for n moles and pV = NkT for N molecules, relating pressure, volume, amount of gas and absolute temperature.AQA
Ideal model: Assumes infinite open-loop gain, infinite input impedance, zero output impedance and infinite bandwidth.AQA
Ideal op-amp assumptions: The open-loop gain is infinite and the input resistance is also infinite, so no current flows into either input.AQA
Ideal voltmeter: Has infinite resistance and is connected in parallel with the component, so no current flows through it and it does not change the potential difference.AQA
Image quality and contrast mechanisms: Spatial resolution and contrast, and why one technique shows particular tissues or functional information better than another.AQA
Image quality and contrast resolution: Because the pixel response is linear over a wide range of X-ray intensities, subtle differences in attenuation remain visible, improving contrast resolution.AQA
Image quality: sharpness and contrast: Sharpness is improved by a small focal spot and reduced object-to-detector distance, while contrast depends on differential attenuation.AQA
Image resolution: How small a separation of features can be distinguished, so a technique producing sharp, high-contrast images of fine structure scores well.AQA
Impedance matching: The middle ear acts as an impedance-matching system, improving energy transfer from air to the fluid-filled inner ear.AQA
Impedance mismatch and reflection: The difference in impedance between two media, on which the fraction of ultrasound reflected at a boundary depends.OCR
Impulse: The integral of force with respect to time, equal to the area under a force–time graph and to the change in momentum.AQA
Impulse and change in momentum: Impulse equals the change in momentum, so impulse = Δp = mΔv.OCR
Impulse and momentum change: Impulse equals the change in momentum, so the area under a force–time graph has units N s or kg m s⁻¹.OCR
Independence from projectile mass: The mass m cancels, so escape velocity depends only on the mass and radius of the body, not on the projectile.OCR
Independence of KEmax from intensity: KEmax = hf − φ depends only on the photon frequency and the work function of the metal, so it is independent of intensity.OCR
Independence of mass in free fall: In the absence of air resistance, all bodies fall with the same acceleration g regardless of mass.OCR
Independence of perpendicular components of motion: The motion splits into two perpendicular components that do not affect each other, so the horizontal and vertical motions of a projectile are independent.OCR
Independence of perpendicular directions: Because the components are perpendicular, they can be treated independently, and they can be recombined using Pythagoras' theorem and trigonometry.OCR
Independence of perpendicular motions: The horizontal velocity stays constant while the vertical velocity changes uniformly, so the two components of motion are independent.AQA
Independence of selected speed from charge and mass: The selected speed is independent of charge and mass, which is why the device can pick a chosen velocity from a mixed beam.OCR
Indicated power: The area of the p−V loop multiplied by the number of cycles per second and by the number of cylinders.AQA
Induced fission: Begins when a slow neutron is absorbed by a heavy nucleus such as uranium-235, forming an unstable uranium-236 nucleus that splits into two smaller nuclei…AQA
Induced n-channel: A conducting channel formed between source and drain when a positive gate-source voltage above threshold attracts electrons to the surface.AQA
Industrial measurement: Thickness measurements of aluminium foil, paper and steel, with a source and detector placed on opposite sides of the material.AQA
Inelastic collision: A collision in which momentum is conserved but kinetic energy is not; some is transferred to thermal energy, sound or deformation.OCR
Inertia and opposition to change: An inductor opposes changes in current, just as mass opposes changes in velocity, and its back e.m.f.AQA
Inertial frame of reference: A coordinate system in which a body with no resultant force acting on it remains at rest or moves with constant velocity in a straight line.AQA
Information signal: The slower envelope, or the repeating pattern of frequency change in FM, representing the information signal.AQA
Ingress and egress: The sloping parts of the curve where the planet's disc partially overlaps the star's disc.AQA
Input and output connections: There are two inputs, an output, and power supply connections; the inverting input is marked with a minus sign and the non-inverting input with a plus sign.AQA
Input offset voltage: A cause of a small unwanted output voltage even when the input is zero.AQA
Input transducer: Converts the original information, such as sound or light, into an electrical signal.AQA
Instantaneous resultant displacement: The principle applies at every instant, so the resultant displacement changes as the waves move.OCR
Instantaneous speed: The magnitude of velocity at one instant, found from the gradient of a distance–time graph or a tangent.OCR
Instantaneous versus average velocity: Average velocity is the overall gradient between two points, while instantaneous velocity uses the tangent at one point.OCR
Instrument selection: Choosing a digital instrument appropriate to the quantity being measured, such as a multimeter for electrical measurements.OCR
Insulating oxide layer: A thin layer between gate and channel that insulates the gate, giving very high input resistance and negligible steady gate current.AQA
intensifying screen: Contains a phosphor that absorbs X-ray photons and emits many visible photons, which expose the film far more efficiently.AQA
Intensity: Increasing this increases the number of photons per second, so more electrons are released, but it does not change each photon's energy.AQA, OCR
Intensity and photoelectric current: Increasing intensity increases the number of electrons per second, not their maximum kinetic energy.AQA
Intensity as photons per second: The energy transferred per unit area per unit time, so for fixed frequency it is proportional to the number of photons arriving per second.OCR
Intensity of radiation received: The measured quantity on which apparent magnitude is based, rather than human perception, giving the same intensity ratio every time with a light meter or CCD…AQA
Intensity proportional to amplitude squared: Intensity is proportional to the square of amplitude, so doubling the amplitude quadruples the intensity.OCR
Intensity ratio: The ratio of received intensities of two stars, related to their magnitude difference by m₂ − m₁ = 2.5 log₁₀(I₁/I₂).AQA
Intensity reflection coefficient: The fraction of incident intensity reflected at a boundary, given by Iᵣ/I₀ = (Z₂ − Z₁)²/(Z₂ + Z₁)², where I₀ is the incident intensity and Iᵣ is the reflected…AQA, OCR
Intensity–angle graph: A graph showing a central maximum at zero angle and a series of minima and weaker maxima at increasing angles.AQA
Intensity–wavelength curve: Rises steeply to a single peak then falls away at long wavelengths, with hotter bodies peaking at shorter wavelengths and curves never crossing.AQA
Intercept: The value of the dependent variable when the independent variable is zero.OCR
Interference: The superposition of waves from two or more sources, producing regions of maximum and minimum resultant displacement.AQA, OCR
Interference and diffraction: Phenomena the wave model handles naturally but the particle model does not.AQA
Interference as evidence for waves: Young’s double slits experiment provided clear evidence for interference, which only waves can produce, and helped establish the wave theory.AQA
Interference fringes: Bright and dark fringes produced when the two recombining beams have a phase difference because the two paths differ slightly in length.AQA
Interference of microwaves: A single transmitter feeds two horn aerials to produce coherent waves, and a receiver moved along a line detects maxima and minima.OCR
Interference of sound: Two loudspeakers driven by the same signal act as coherent sources, and a microphone moved along a line parallel to the speakers detects alternating loud and…OCR
Interferometer: Apparatus that splits a light beam into two perpendicular arms and recombines the beams to form fringes, comparing light travel times.AQA
Intermediate image: A real image formed at the objective's focal plane by non-axial parallel rays, sitting exactly at the shared focal plane of both lenses.AQA
Internal energy: The sum of the random distribution of kinetic and potential energies associated with the molecules of a system.AQA, OCR
Internal energy and particle potential energy: The internal energy changes during a phase change because the potential energy of the particles changes as bonds are broken or formed.OCR
Internal energy and the ideal-gas model: The total kinetic energy of the molecules of an ideal gas, since the ideal-gas model treats intermolecular potential energy as negligible.OCR
Internal energy changes in gases: Compressing a gas quickly does work on it, increasing internal energy and raising temperature; a gas expanding and pushing a piston does work on the…AQA
Internal energy of an ideal gas: Entirely the kinetic energy of its atoms or molecules, because there is no potential energy associated with their separation.AQA
Internal imaging: Allows imaging of organs such as the stomach or colon, with the advantages of flexibility, smaller incisions and real-time visualisation.AQA
Internal resistance: The resistance to current flow within the source itself, which causes a potential drop when current flows.OCR
Internal resistance and terminal p.d.: In a real cell some energy is dissipated inside the source because of its internal resistance, so the terminal p.d.OCR
International cooperation and large-scale facilities: Progress depends on sharing costs, expertise and data across countries and institutions, as in experiments such as those at the Large Hadron Collider.AQA
Interparticle forces and their strength in each state: Strong forces hold particles in a regular lattice in a solid, weaker forces allow particles to slide in a liquid, and forces are negligible in a gas.OCR
Interpretation of hot spots and cold spots: Areas of abnormally high or low uptake are interpreted as hot spots or cold spots that may indicate disease.OCR
Interstellar distance scales: Both units are used for interstellar distances, with parsecs common in professional astronomy and light-years common in popular science.OCR
Invariance of the speed of light: The speed of light in a vacuum, c, is constant for all inertial observers, regardless of the motion of the light source or the observer.AQA
Invariance of the speed of light in free space: Every inertial observer measures the same value c, regardless of the motion of the source or the observer.AQA
Inverse relationship between potential and distance: For a point charge Q, V = Q/(4πε₀r), where r is the distance from the centre of Q and ε₀ = 8.85 × 10⁻¹² F m⁻¹.OCR
Inverse-distance dependence: Because E is inversely proportional to r, doubling the separation from r to 2r changes E from −GMm/r to −GMm/2r, an increase of GMm/2r.OCR
Inverse-distance law for gravitational potential: V = −GM/r, a negative curve proportional to −1/r that approaches zero from below as r tends to infinity.AQA
Inverse-square force laws: Both gravitational and electrostatic forces have inverse-square force laws that have many characteristics in common.AQA
Inverse-square law: Apparent brightness decreases with distance squared, so a quasar visible at large distance must have a very large intrinsic power output.AQA, OCR
Inverse-square law for a point source: For a point source radiating equally in all directions, the power spreads over a sphere of area 4πr², so I = P/(4πr²) and intensity falls with the square of…OCR
Inverse-square law for electrostatic force: The electrostatic force between point charges obeys F = Qq/(4πε₀r²), an inverse-square relationship.OCR
Inverse-square law for gravitational field strength: The magnitude of g is GM/r², a positive curve proportional to 1/r² that tends to zero as r tends to infinity.AQA
Inverse-square law for point masses: For a point mass or uniform sphere of mass M, g = GM/r², where r is the distance from the centre.OCR
Inverse-square law for point sources: The relationship tested by plotting corrected count rate against 1/d², where a straight line through the origin supports it.AQA
Inverse-square laws: Both the electrostatic force and the gravitational force obey this, so the ratio of electrostatic to gravitational force is independent of separation.AQA
Inverse-square relationship: For a point source radiating uniformly in all directions, the same power spreads over a sphere of area 4πr², giving I = P/(4πr²).AQA
Inverted magnitude ordering: A smaller magnitude number means a brighter object, so magnitude 1 is brighter than magnitude 6.AQA
Inverted magnitude scale: Brighter stars have smaller magnitudes, including zero and negative values, so magnitude 6 is fainter than magnitude 1.AQA
Iodine compounds: Injected or infused into the bloodstream, these are used for the urinary tract and blood vessels because iodine has a high atomic number and absorbs X-rays…OCR
Ion formation and charge: The total charge is found from the number of protons minus the number of electrons, and the sign of the charge must be included for an ion.AQA
Ion movement in electrolytes: Positive ions move in the conventional current direction while negative ions move the other way, so both contribute to the same conventional current.OCR
Ionisation: Collisions between charged particles and gas atoms cause this, creating positive gas ions and free electrons.AQA
Ionising power: Alpha is strongly ionising, beta is less ionising than alpha, and gamma is weakly ionising.AQA
Ionising power and its relationship to penetration: Penetration and range are inversely related to ionising power: alpha is most ionising but least penetrating, while gamma is least ionising but most penetrating.OCR
Ionising radiation: Radiation for which alpha, beta and gamma types have different penetrating and ionising powers, and which is handled safely and measured using detectors.AQA, OCR
Ionising radiation and biological damage: High-energy X-rays are ionising and damage DNA, and because cancer cells divide rapidly they are less able to repair this damage than most healthy cells.AQA
Ionising radiation and biological harm: Ionising radiation can damage DNA, raising cancer risk, so its medical use trades this harm against benefit.AQA
Ionising radiation and penetration: Alpha particles are strongly ionising but travel only a few centimetres in air and are stopped by paper, while gamma rays are highly penetrating.AQA
Ionosphere and signal variability: The ionosphere's varying ionisation means sky-wave reception changes with frequency, time of day and solar activity.AQA
Irradiance: The quantity received from an object that apparent magnitude is a logarithmic measure of, and which depends on both luminosity and distance.AQA
Isobaric: A constant pressure process in which heating lets the gas expand, doing work pΔV while temperature rises.AQA
Isochoric: A constant volume process in which heating does no work because ΔV is zero, so all heat raises internal energy and temperature.AQA
Isochoric process: A constant-volume process in which W = 0, so Q = ∆U.AQA
Isochronous oscillator: An oscillator whose period does not depend on its amplitude.OCR
Isolating objects: A single object is shown isolated from its surroundings, drawn as a simple shape or dot, with all external forces acting on it.OCR
Isothermal: A process in which temperature is held constant, so the internal energy of an ideal gas stays fixed and any heat added equals work done by the gas.AQA
Isothermal change: T is fixed so pV is constant, giving a smooth p–V curve.AQA
Isotopes: Atoms of the same element with the same proton number Z but different nucleon numbers A, so they have different numbers of neutrons.AQA, OCR
Isotopic data: Relative abundances and masses, which are used to calculate relative atomic mass.AQA
Isotropic: The universe looks the same in every direction from any observation point.OCR
Isotropic-emission assumption: Power is radiated uniformly in all directions, so the same total power passes through every sphere of radius r.AQA
Iterative numerical modelling: A step-by-step numerical method in which the change in nuclei is calculated as ΔN = -λNΔt and the new number of nuclei is N_new = N_old + ΔN, repeated across…OCR

J

Johnson counter: Uses a shift register with the inverted last output fed back to the first input, producing a sequence of 2n states for n flip-flops.AQA
Joule (J): The unit of energy, equivalent to kg m² s⁻².OCR
Junction and loop rules: Kirchhoff's first law concerns currents at a junction, and his second law concerns voltages around a closed loop.OCR
Justified comparison: Naming the technique, stating the clinical task, then weighing resolution against convenience and safety to justify a choice.AQA

K

Kelvin: The unit of the absolute scale, with 0 K being absolute zero, the temperature at which particles have minimum possible kinetic energy.OCR
Kelvin scale: An absolute thermodynamic temperature scale, and temperatures in kelvin are used in the ideal gas equation because T is directly proportional to the average…AQA, OCR
Kelvin–Celsius conversion: Convert between kelvin and degrees Celsius using T/K = θ/°C + 273.15, so 0 K is about −273.15 °C.OCR
Kelvin–Planck statement: No cyclic device converts all absorbed heat into work.AQA
kilowatt-hour: A unit of energy equal to the energy transferred by a 1 kW device operating for 1 hour.OCR
Kinetic and potential energy interchange: The interchange is continuous, with kinetic energy converting to potential energy and back each cycle.OCR
Kinetic energy: The energy an object possesses due to its motion, calculated using Ek = 1/2 mv² where m is mass in kg and v is speed in m s⁻¹.OCR
Kinetic energy contribution: Arises from the random motion of the molecules, and the kinetic energies of the molecules are randomly distributed.OCR
Kinetic energy of charged particles: The energy gained by a particle starting from rest becomes kinetic energy, given by ½mv², where m is the mass and v is the final speed.OCR
Kinetic energy of molecules: Each molecule has a mean translational kinetic energy of 3/2 kT, so for N molecules the total is U = 3/2 NkT.OCR
Kinetic theory and observation: Brownian motion provides direct evidence that matter is made of atoms or molecules in continuous random motion, supporting the molecular kinetic theory model.AQA
Kinetic theory assumptions and derivation: The model starts from assumptions about molecules, including random motion, elastic collisions and negligible molecular volume, and derives the same…AQA
Kinetic theory model of gases: A simple molecular model treats a gas as many identical molecules in random motion, colliding elastically with the container walls.AQA
Kinetic, potential and total energy in circular orbits: Kinetic energy is Eₖ = GMm/(2r), gravitational potential energy is Eₚ = −GMm/r, and total energy is E = Eₖ + Eₚ = −GMm/(2r).AQA
Kirchhoff's laws: At any junction the sum of currents entering equals the sum leaving, and around any closed loop the algebraic sum of potential differences is zero.OCR
Kirchhoff’s second law: In any closed loop of a circuit, the sum of the electromotive forces equals the sum of the potential differences.OCR

L

Laser safety: Laser light must never be directed towards eyes, and beams should be kept below eye level.OCR
Latent heat: The energy transferred at constant temperature during a change of state, which does not appear as a temperature rise.AQA
Latent heat of fusion: The latent heat used for melting, the change from solid to liquid.AQA
Latent heat of vaporisation: The latent heat used for boiling, the change from liquid to gas.AQA
Lattice vibrations: These also increase with temperature, but the dominant effect is the rise in carrier number, so resistance decreases.AQA
LDR: A component whose resistance falls as light intensity rises, so its output p.d.AQA, OCR
Left-hand rule and conventional current: First finger field, second finger conventional current, thumb force; for a negative particle the conventional current is opposite to the particle's velocity,…AQA
Length and the metre: Length is an S.I.OCR
Length–wavelength relationships for strings and tubes: A stretched string fixed at both ends and an open tube both give L = nλ/2, while a closed tube gives L = (2n − 1)λ/4.OCR
Lens power: The reciprocal of focal length in metres, P = 1/f, measured in dioptres (D).AQA
Lens separation: In normal adjustment the intermediate image lies at the principal focus of both the objective and the eyepiece, so the separation equals f₀ + fₑ.AQA
Lenz's law: The induced current flows in a direction that opposes the change in flux linkage that produced it.AQA, OCR
Lenz's law and opposition to change: The direction of the induced effect opposes the change producing it, for example the coil face becomes a like pole as a magnet approaches.AQA
Lenz's law and the negative sign: The negative sign represents Lenz's law: the induced e.m.f.OCR
Lepton: A fundamental particle that does not experience the strong nuclear force (e.g., electron, muon, neutrino).Pearson Edexcel
Lepton charge: The electron has charge −1, the positron +1, and neutrinos zero.OCR
Lepton families: The named charged leptons are the electron and the muon, and the named neutral leptons are the electron neutrino and the muon neutrino.AQA
lepton number: A quantity conserved in particle interactions, with the electron antineutrino balancing the electron lepton number and the muon neutrino conserving the muon…AQA
Lepton number conservation: Lepton number is conserved within each lepton family, with 1 for leptons, −1 for antileptons and 0 for hadrons.AQA, OCR
Lift: A contact force exerted by a fluid on a body moving through it, acting perpendicular to the relative flow and arising when a shape deflects fluid.AQA
Light curve: A plot of apparent magnitude against time showing a rapid rise, a broad peak, then an exponential decline with a half-life of about 77 days.AQA
Light damping: A gradual exponential-like decrease in amplitude with the oscillator still completing many oscillations and the period remaining approximately constant.AQA, OCR
Light intensity as a variable: It can be varied by changing the distance from a lamp or by using different light intensities when investigating an LDR.OCR
Light year: The distance electromagnetic radiation travels through a vacuum in one year, found from speed × time with c = 3.00 × 10⁸ m s⁻¹.AQA
Light-year (ly): The distance light travels in a vacuum in one year, about 9.46 × 10¹⁵ m, convenient for distances to stars and galaxies.OCR
Light-year as a distance unit: The distance light travels in a vacuum in one year, about 9.46 × 10¹⁵ m.OCR
Limit of proportionality: The point up to which the expressions for stored energy apply; beyond it the stored energy is not given by these equations.OCR
Limitations of classical wave theory: It predicts that light of any frequency can eject electrons if intense enough or shining long enough, because energy is absorbed continuously and can…AQA
Limitations of dating methods: Carbon dating applies only to once-living material and to ages up to roughly 50 000 years, because the remaining activity becomes too small to measure…OCR
Limitations of simple cosmological models: The reciprocal is an estimate, not an exact age, because gravity and dark energy change the expansion rate over time.OCR
Limiting angle: An angle measured in radians that depends on the wavelength λ and the diameter D of the objective aperture.AQA
Limiting speed c for massive particles: As v approached c the speed levelled off below c, so no massive particle reaches the speed of light.AQA
Limits of the uniform-field approximation: Uniformity fails over large heights, where g decreases as 1/r².OCR
Limits on observable orders: The maximum possible order is limited because sin θ cannot exceed 1, so n ≤ d / λ.OCR
Line of action: The infinite straight line along which the force acts.AQA
Line of best fit: A straight line that must be checked to see if it passes through the drawn error bars.AQA
Line of best fit and worst line: The line of best fit balances points above and below it, while a worst line is the steepest or shallowest line that still passes through every error bar.OCR
line of stability: A curved band on a graph of neutron number N against proton number Z on which stable nuclei lie.AQA
Line spectra: Because the level spacings are fixed, the photons absorbed or emitted have specific energies and therefore specific frequencies and wavelengths, producing…OCR
Line spectrum: A series of sharp, separate bright lines on a dark background, each at a precise wavelength.AQA
Linearisation using natural logarithms: Taking natural logarithms gives ln(I₀/I) = μx, so a plot of ln(I₀/I) against x is linear with gradient μ.OCR
Linearising data using 1/λ: Since f = c/λ, equating eV = hc/λ gives V = (hc/e)(1/λ), so a graph of V against 1/λ is a straight line through the origin.OCR
Lines of best fit: A line drawn through the data taking account of error bars, from which the gradient and intercept are found.AQA
Logarithmic brightness ratio: A difference of 5 magnitudes corresponds to a brightness ratio of 100, so a magnitude 1 star is 100 times brighter than a magnitude 6 star.AQA
Logarithmic brightness scale: A difference of five magnitudes corresponds to a brightness ratio of exactly 100, so each step is about 2.512 times in brightness.AQA
Logarithmic plot: A graph of ln Q against t, which is a straight line with gradient −1/CR.OCR
Logarithmic scale: The nature of the magnitude scale, used because human perception is not a reliable linear measure, replacing subjective judgement with a calibrated numerical…AQA
Long-duration GRB: A burst that can occur when a rapidly rotating supergiant star collapses at the end of its life, associated with supernovae in distant galaxies.AQA
Longitudinal: Describes a wave in which the oscillations of the particles are parallel to the direction of energy transfer, as with sound in air.AQA
Longitudinal wave: A wave in which the oscillations are parallel to the direction of travel, producing compressions and rarefactions, for example sound in air.AQA, OCR
Lorentz factor: Given by γ = 1 / √(1 − v²/c²), it links the proper time to the dilated time through t = γt₀.AQA
Lorentz force on charge carriers: Increased by the increasing magnetic flux density perpendicular to the Hall probe as the magnet passes the sensor.AQA
Lost volts: The potential difference across the internal resistance of the source, equal to Ir, where I is the current and r is the internal resistance.OCR
Loudness perception: The way loudness is perceived, in which a sound must increase by roughly a factor of ten in intensity to be heard as about twice as loud.AQA
Low orbit: A small radius orbit, so it is fast and has a short period, used for Earth observation, imaging, and weather monitoring.AQA
Low-Earth-orbit satellites: These have shorter delays but move relative to the ground, so tracking or constellations are needed.AQA
Luminosity: How much light an object emits, which apparent magnitude does not describe, so apparent magnitude is not an intrinsic property.AQA, OCR
Luminosity and detectability: Very high luminosity allows quasars to be observed at much greater distances than ordinary galaxies.AQA
Luminosity and radius: For a star of known luminosity and temperature, Stefan’s law gives its radius.AQA

M

Magnetic field generation: A superconductor can carry large currents with no resistive heating, so electromagnets reach fields ordinary copper coils cannot sustain.AQA
Magnetic flux: The product of flux density and the area perpendicular to the field, Φ = BAcosθ, with θ measured between the field direction and the normal to the coil.AQA, OCR
Magnetic flux and webers: Measured in webers, where Φ = BA for a uniform field perpendicular to area A, and is distinguished from magnetic flux density B.AQA
Magnetic flux density: Measures the strength of a magnetic field in terms of the force it exerts on a current, and is a vector quantity.AQA, OCR
Magnetic flux density measurement: Using the change in balance reading when current flows to give the force F on the wire, then B = F / (IL) because the wire is perpendicular to the field.OCR
Magnetic flux linkage: Defined as NΦ, the product of the number of turns N and the magnetic flux Φ through one turn.OCR
Magnetic force: A force on a current-carrying wire given by F = BIL sin θ, or on a single charge by F = Bqv sin θ, perpendicular to both velocity and field.AQA
Magnetic force on a moving charge: A charged particle moving in a magnetic field experiences a force only when its velocity has a component perpendicular to the field.AQA
Magnetic force on moving charges: A charged particle moving through a magnetic field experiences a force only when its velocity has a component perpendicular to the field.AQA
Magnitude and direction: The two features a vector quantity has, where direction matters because vectors combine by direction as well as size.OCR
Magnitude and direction of a resultant: Both must always be stated for a resultant, and the answer checked to see that it is sensible relative to the original vectors.OCR
Magnitude difference: A difference of 1 on the magnitude scale is equal to an intensity ratio of 2.51, because 2.51 is the fifth root of 100.AQA
Main sequence: A diagonal band from top left (high luminosity, high temperature) to bottom right (low luminosity, low temperature), containing stars fusing hydrogen in their…AQA, OCR
Malus's law: The transmitted intensity I is given by I = I₀ cos²θ, where θ is the angle between the transmission axes.OCR
Margin of error: The range within which the true value is expected, such as a reading of 25.0 cm³ ± 0.1 cm³.OCR
Mass and the kilogram: Mass measures the quantity of matter, and its S.I.OCR
Mass as the source of gravity: Every object with mass has a gravitational field, and the field belongs to the mass that produces it.OCR
Mass defect: In nuclear fission and fusion, the total mass of the products is slightly less than that of the reactants, and this difference appears as energy.AQA, OCR
Mass dependence of stellar properties: The mass of the collapsing cloud determines the star's eventual luminosity, surface temperature and lifetime.OCR
Mass difference: The total mass before the reaction minus the total mass after the reaction.AQA
Mass distribution: Mass placed further from the axis contributes more to the moment of inertia, so a hollow hoop has a larger moment of inertia than a solid disc of equal mass…AQA
Mass flow rate: The mass passing a point each second, given as mass divided by time or as density × cross-sectional area × speed for a fluid in a pipe.AQA
Mass of the central body determining orbital speed: In v² = GM/r the planet's mass cancels, leaving the Sun's mass M.OCR
Mass per unit length: A quantity determined by weighing a measured length of string, used when strings of different material or thickness are investigated.AQA
Mass-volume relationship: Density is calculated as mass divided by volume, with mass in kg and volume in m³.OCR
Mass–energy conservation: The increase in binding energy per nucleon corresponds to a decrease in total mass, converted by E = mc².AQA
Mass–energy equivalence: The relation ΔE = Δmc², which explains the energy released when a nucleus undergoes fission or fusion.AQA
Massive stars: Stars with much more mass than the Sun, which evolve off the main sequence to become a red super giant.OCR
Material dispersion: Arises because the refractive index of the core depends on wavelength, so different wavelengths in a source travel at different speeds and arrive at different…AQA
Material properties in vehicle design: Material choice links to properties such as density, stiffness, toughness and ultimate tensile strength, balancing lower mass against crash protection.AQA
Materials and critical limits: Critical temperature, critical current and critical field limit applications; above any critical value the material reverts to normal resistance.AQA
Maxima: Regions where path difference equals a whole number of wavelengths, giving constructive superposition.AQA
Maximum height: Found by equating elastic potential energy to total gravitational potential energy, ½k(ΔL)² = mg(h + ΔL).AQA
Maximum kinetic energy: The kinetic energy of the fastest emitted electrons, where slower electrons have lost more energy before escaping.AQA, OCR
Maximum photon energy: Given by hf_max = eV, so the tube voltage sets a hard limit and increasing V increases it.AQA
Maximum speed at equilibrium: Speed is greatest at the equilibrium position, where x = 0, giving vmax = ωA.OCR
Maxwell's theory: Predicts the speed of electromagnetic waves, and Fizeau's value for c was very close to this predicted speed.AQA
Mean drift velocity: The average velocity of charge carriers along a conductor due to an applied electric field.OCR
Mean molecular kinetic energy: For a monatomic ideal gas, (3/2)kT per atom, where k is the Boltzmann constant and T is thermodynamic temperature.AQA
Mean square speed: A quantity that pressure depends on, related to pressure by p = (1/3)ρ⟨c²⟩, where ρ is density.OCR
Measurement accuracy: Dependence on a sharp pencil, a sensible scale and careful measurement, with a scale chosen to make the drawing large enough to measure accurately.AQA
Measurement and uncertainty: The fringe spacing x is measured between corresponding points on adjacent bright fringes, ideally by measuring across several fringes and dividing by the…OCR
Measurement and unit conversion using scale settings: Every measured division is converted into volts or seconds using the relevant control value, with units converted consistently.AQA
Measurement of elementary charge: Millikan’s method, in which the droplet is held stationary by adjusting V and the measured charges are found to be multiples of e.AQA
Measurement of motion: Techniques such as light gates, ticker timers or video analysis are used to measure time, displacement and velocity.OCR
Measurement of small charges and quantisation of charge: Repeating the measurement for many droplets gives values of Q that are integer multiples of the elementary charge e, demonstrating charge quantisation.AQA
Measurement of speed using v = fλ: Wavelength was determined from standing-wave patterns or interference and frequency from the transmitter circuit, then speed was calculated using v = fλ.AQA
Measurement of the elementary charge: Repeating measurements showed every droplet charge was an integer multiple of a smallest value, the elementary charge e, giving a value for e.AQA
Measurement of wavelength: Using the grating equation d sinθ = nλ, since the angle depends on wavelength, to measure wavelength accurately.AQA
Measurement of wavelength, frequency, period and speed: Quantities such as wavelength, frequency, period or speed are measured using an appropriate method for the chosen source.OCR
Measurement uncertainty: May be smaller than the size of the plotted point itself, so no error bar is drawn.AQA
Measurement uncertainty and control of variables: The diameter is measured at several positions and orientations and averaged to reduce random error, and the current is kept small and switched off between…AQA
Measurement uncertainty and error reduction: Repeating each height and calculating a mean time reduces random error compared with a single measurement.AQA
Measurement uncertainty and experimental design: Readings are repeated and averaged to reduce random uncertainty, and systematic effects such as the balance zero and the mass of connecting leads are…AQA
Mechanical power: When a constant force F acts on an object moving at constant velocity v in the direction of the force, the mechanical power delivered is P = Fv.OCR
Mechanical power in rotational motion: Given by P = Tω, where ω is angular velocity, mirroring linear P = Fv.AQA
Medical imaging: The use of display modes such as A-scan and B-scan, for example in measuring the length of the eye or foetal scanning.OCR
Medical imaging and endoscopy: Fibre bundles carry light into the body and return images or signals, enabling endoscopy, illumination and some sensing applications without large incisions.AQA
Medical tracer: A radioactive substance introduced into the body so that its emitted radiation can be detected outside and used to image or monitor an organ.OCR
Meson classification: Kaons are mesons containing a strange quark or antiquark, and pions are lighter mesons with baryon number zero.AQA
Mesons: Particles containing one quark and one antiquark, such as the pion and the kaon.AQA
Metabolic imaging in diagnosis: PET scanning is used to diagnose and monitor conditions by imaging metabolic activity, with diagnosis relying on comparing the pattern and intensity of uptake…OCR
Metal wire: Guides an electrical signal along conductors, is cheap and easy to connect, but suffers attenuation and can pick up interference.AQA
Meter placement: An ammeter is placed in series with the component whose current is being measured, while a voltmeter is placed in parallel across the component whose…OCR
Meter placement in circuits: An ammeter must be in series with the branch, and a voltmeter in parallel with the component being measured.OCR
MeV: A convenient unit for nuclear processes, to which nuclear masses expressed in u can be converted directly.AQA
Michelson-Morley experiment: An experiment whose null result supports the invariance of the speed of light, a fundamental postulate of special relativity.AQA
micro (μ): A prefix, symbol μ, that represents 10⁻⁶, so a micrometre (1 μm) is 1 × 10⁻⁶ m.OCR
Micrometer: An instrument used to measure external dimensions to 0.01 mm, read using the sleeve and thimble scales.OCR
Microscopic interpretation of pressure: Pressure depends on the number of molecules, their mass, the mean square speed and the volume, linking microscopic motion to measurable pressure.OCR
Microscopic interpretation of temperature: Temperature alone fixes mean kinetic energy, so at a given T all gas molecules have the same average translational energy regardless of mass.OCR
Microscopic model of conduction in metals: Increased lattice vibration increases the rate of collisions between conduction electrons and ions, while the number density of charge carriers is essentially…OCR
Microscopic model of current: Since I = Anev, at a fixed current a smaller n requires a larger drift velocity.OCR
Microscopic origin of pressure: Pressure is caused by the rate of change of momentum of molecules colliding with the walls.OCR
Microwave reflection and detection: A transmitter directs waves at a metal plate; the reflected wave superposes with the incident wave, and a probe detects nodes and antinodes spaced λ/2 apart.OCR
Microwave stationary waves: A transmitter directs waves at a metal reflector and the reflected wave superposes with the incident wave, producing nodes and antinodes that can be detected…AQA
Microwave transmission: Frequencies in the microwave region, roughly 1–30 GHz, pass through the ionosphere and atmosphere with little absorption or reflection.AQA
Microwaves: Free-space electromagnetic radiation that needs line of sight but supports high bandwidth.AQA
milli (m): A prefix, symbol m, that represents 10⁻³, so a millimetre (1 mm) is 1 × 10⁻³ m.OCR
Millikan’s oil-drop experiment: Tiny charged oil droplets were balanced or moved by an electric field, and their charges were calculated from force balances, clustering around integer…AQA
Minimum angular resolution: The smallest angular separation between two point sources that can still be distinguished as separate.AQA
Minimum operating current: A typical current, labelled I_Z(min), marked on the reverse branch as the lower limit below which regulation is unreliable.AQA
Modal dispersion: Arises because rays taking different paths through a step-index core have different path lengths, so they arrive at different times even at the same wavelength.AQA
Model limitations and evidence: Agreement between theoretical predictions and experimental data supports the model, while disagreement reveals its limits, for example at high pressure or…AQA
Modelling with probability: Each die models one unstable nucleus, and a chosen face represents that nucleus decaying in that time interval.OCR
Moderator: Surrounds the fuel and slows fast fission neutrons to thermal energies through elastic collisions with light nuclei, increasing the probability of further…AQA, OCR
Modes and boundary conditions: The number of half-wavelengths fitting the boundary conditions identifies the mode of vibration, such as the fundamental mode on a string fixed at both ends.OCR
Modulation: Imposes the information signal onto a carrier wave, matching the signal to the transmission medium and allowing efficient radiation or sharing of a channel.AQA
Modulo-n counter: Counts from 0 up to n − 1 and then resets to 0, so it has n distinct states.AQA
Modulus: Identified by analysing each circuit through the next-state table and the sequence of output states.AQA
Molar calculations: Finding the number of moles by dividing mass in grams by molar mass in g mol⁻¹, or by dividing number of particles by the Avogadro constant.OCR
Molar gas constant: R = 8.31 J mol⁻¹ K⁻¹, which applies to a mole of particles.AQA, OCR
Molar mass: The mass per mole of a substance, usually in g mol⁻¹, on which the mass of one mole depends.OCR
Molecular model of gases: A gas is treated as a very large number of identical point molecules in continuous random motion with a range of speeds.OCR
Moles and molar gas constant: n is the number of moles of gas and R is the molar gas constant, 8.31 J mol⁻¹ K⁻¹.OCR
Moment: The turning effect of a force, calculated as force × perpendicular distance from the pivot to the line of action of the force.AQA
Moment of a couple: The magnitude of one force multiplied by the perpendicular distance between the lines of action of the two forces.AQA
Moment of a force: The turning effect of that force about a pivot, calculated as the product of the force and the perpendicular distance from the pivot to the line of action of…OCR
Moment of inertia: A quantity that depends on mass and how that mass is distributed about the chosen axis, so the same body can have different values for different axes.AQA
Momentum: The product of mass and velocity, p = mv, for a particle of mass m moving at velocity v.AQA, OCR
Momentum and diffraction: Increasing momentum decreases λ, so the wave spreads less and the pattern becomes narrower with fringes closer together; decreasing momentum does the reverse.AQA
Momentum change in collisions: Each collision involves a change in momentum, and the rate of change of momentum gives the force on the wall.OCR
Momentum conservation in road collisions: In a crash, total momentum is conserved, though the force on occupants depends on how quickly momentum changes.AQA
Monochromatic light: Light of a single wavelength or colour, which through a single narrow slit gives a broad central maximum with narrower, fainter secondary maxima on each side.AQA
Motion: How the atoms or molecules move, vibrating about fixed mean positions in solids, sliding past one another in liquids and moving rapidly and randomly in gases.OCR
Motion representation: Graphs show how displacement, speed, velocity or acceleration vary with time, so axis labels and units must be read first.OCR
Motional emf: For a straight conductor of length L moving at speed v perpendicular to a uniform field B, the induced emf is ε = BLv.AQA
Motor effect: The force whose direction is given by the thumb when using Fleming's left-hand rule on a current-carrying conductor in an external magnetic field.OCR
Moving charges: A steady current is a flow of charge and so creates a field around the conductor, and a single charge moving with velocity v produces a magnetic field.OCR
Multi-stage heating: In mixed problems, calculate the energy for each stage separately and sum the values.AQA
Multimodality imaging: Because PET shows function rather than fine anatomy, it is often combined with CT or MRI so that metabolic hot spots can be located precisely.OCR
Multiplexer: Samples each input in turn so the channel carries a rapid sequence of samples.AQA
Multiplication and division rule: For multiplication and division, combine the percentage uncertainties by adding them instead of the absolute uncertainties.AQA
muon: A fundamental lepton, identical to the electron in charge and weak interaction but about 207 times more massive, which is unstable and decays by the weak…AQA
Muon lepton number: A quantum number, L_μ, that is conserved separately in particle interactions.AQA
Myopia: Short sight, where the eye converges too strongly or is too long, so parallel rays focus in front of the retina and distant objects blur.AQA

N

n-type inversion channel: A conducting channel formed at the surface when the gate-source voltage exceeds the threshold voltage, connecting the two n-type regions and allowing current…AQA
Naked-eye visibility limit: The faint limit of naked-eye visibility is apparent magnitude 6, the dimmest stars visible to the unaided eye under good dark-sky conditions.AQA
Natural frequency: Set by the system's inertia and stiffness, while damping mainly changes peak height and width rather than this frequency.AQA
Natural frequency of an oscillatory system: The frequency at which a system oscillates freely once displaced.OCR
Natural frequency versus driving frequency: The peak is at the driving frequency nearest the natural frequency, not at an arbitrary point, and increased damping shifts the peak slightly below the…OCR
Negative potential near a mass: Because the gravitational force is attractive, the work done by the field is positive when moving from infinity, so the potential is negative near a mass.AQA
Negative potential near masses: Because the gravitational force is attractive, the work done by the gravitational field is positive when moving from infinity to a point, so the potential is…OCR
Negative sign: It means the potential energy is zero at infinity and decreases as the mass moves closer, so energy is released when a mass falls towards M.OCR
Negative temperature coefficient of resistance: The effect in which resistance falls as temperature rises, as shown by an NTC thermistor.OCR
Negative total energy and bound orbits: The total energy is negative, showing the satellite is bound, and becomes less negative as r increases.AQA
Net impulse: The total impulse over a complete interaction, found by treating areas below the time axis as negative so they reduce it.AQA
Neutrality of the atom: The atom is neutral because proton number equals electron number, so a carbon-12 atom with six protons, six neutrons and six electrons has total charge zero.AQA
Neutrinos: Electrically neutral particles with very small mass that interact only via the weak interaction, so they are hard to detect.AQA
Neutron absorption cross-section: A property that must be high in control-rod materials and low in moderators and ideally coolants, relating material choice to each component's function.AQA
Neutron moderation and chain reactions: Fission neutrons are released fast and must be slowed to sustain further fission, since moderating them increases the probability of further induced fission.AQA
Neutron star: The collapsed core left after a supernova, extremely dense, with a mass similar to the Sun packed into a radius of about 10 km, and supported against further…AQA, OCR
Neutron-induced fission: A nucleus absorbs a neutron and becomes unstable, splitting into two smaller nuclei plus two or three further neutrons and releasing energy.OCR
newton: The SI unit of force, defined as the force that gives a mass of one kilogram an acceleration of one metre per second squared.OCR
Newton's first law and inertia: If the force were removed, the object would continue in a straight line tangent to the circle, as Newton's first law predicts.OCR
Newton’s law of gravitation: The gravitational force F between two point masses is directly proportional to the product of their masses and inversely proportional to the square of the…OCR
Newton's laws apply along each axis independently: Newton's second law is applied along each axis separately, using the parallel component to find acceleration or the balance of forces for equilibrium.AQA
Newton's second law: The net force acting on an object equals its mass multiplied by its acceleration, written F = ma.OCR
Newton's second law applied to falling bodies: A freely falling object experiences only the gravitational force, so Newton's second law gives a = F/m = g.OCR
Newton's second law for rotation: Written as T = ∆(Iω)/∆t, and rearranged to T ∆t = ∆(Iω).AQA
Newton's second law in circular motion: F = ma, and substituting the centripetal acceleration gives F = mv²/r.OCR
Newton's second law in momentum form: The net (resultant) force on a body equals the rate of change of its momentum: F = Δp/Δt.OCR
Newton's third law applied to magnetic forces: The field exerts a force on the wire, and the wire exerts an equal and opposite force on the magnet, so the balance reading changes.AQA
Newton's third law pair: The force is mutual: each mass pulls the other equally, so the force on each mass is equal in magnitude and opposite in direction.AQA
Newtonian derivation of Kepler's laws: These laws are empirical but follow from Newton's law of gravitation and centripetal force.OCR
Next-state logic: The logic analysed by tracing how each input affects it, for example showing that a reset pulse overrides the clock.AQA
No emission below threshold at any intensity: Radiation below f₀ cannot release electrons no matter how intense it is, because each photon then carries less than φ.OCR
Node: A point of permanently zero displacement, where the two waves always cancel because they arrive in antiphase.AQA, OCR
Node and antinode spacing: The separation between adjacent nodes, or between adjacent antinodes, is equal to half a wavelength.OCR
Node and antinode spacing in terms of wavelength: Adjacent nodes are separated by half a wavelength, and a node and the next antinode are separated by one quarter of a wavelength.OCR
Nodes and antinodes: Nodes are points of permanently zero displacement, while antinodes are points where displacement varies with maximum amplitude.AQA, OCR
Noise-induced loss: Typically produces a dip around 4 kHz and reduces sensitivity at high frequencies.AQA
Non-coherent fibre bundle: Has fibres in random relative positions, so it transmits illumination but not a clear image.AQA
Non-inertial frame: A frame that is accelerating or rotating, producing apparent forces that are not caused by physical interactions.AQA
Non-ionising radiation safety: Ultrasound is used because it is non-ionising and therefore safer than X-rays for repeated or prenatal imaging.AQA
Non-ionising safety: Ultrasound avoids the radiation risk associated with X-rays and CT scans, making it safer for fetuses and repeated examinations.AQA
Non-linear resistance–temperature graphs: Curves that are steep at low temperatures and flatten at higher temperatures, so the sensitivity of the sensor changes with temperature.AQA
Non-ohmic behaviour: Shown by components whose I–V graph is not a straight line through the origin, such as the diode and the filament lamp.AQA, OCR
Non-uniform acceleration: Shown by a curve on a velocity–time graph, where the acceleration is not constant.AQA
Non-uniform or irregular solid: A body whose centre of mass may lie elsewhere than the geometric centre and must be found experimentally, for example by suspending it and using a plumb-line.AQA
Normal adjustment: The eyepiece is positioned so the intermediate image lies at its principal focus, and rays leave the eyepiece parallel, allowing viewing without accommodation.AQA
Normal contact force: A force that acts perpendicular to a surface where two objects touch.OCR
Normal contact force is perpendicular to a surface: It is balanced by the perpendicular component of weight when there is no acceleration perpendicular to the surface.AQA
Normal incidence: Monochromatic light arrives perpendicular to the grating surface.AQA
NOT: A bar over a variable means NOT, so the output is the inverse of the input.AQA
ntc thermistor: A semiconductor device whose resistance falls as temperature rises, giving a negative temperature coefficient of resistance.AQA, OCR
Nuclear density: Mass divided by volume, giving about 2.3 × 10¹⁷ kg m⁻³ and approximately constant for all nuclei because the A terms cancel.AQA
Nuclear equations: Balanced decay equations showing the parent nuclide producing a daughter plus the emitted particle, with A and Z balanced on both sides.AQA
Nuclear equations and nuclide notation: A missing particle can be found by subtracting the known totals from the totals on the other side, remembering that electrons have nucleon number 0 and charge…OCR
Nuclear excited state: A state in which a daughter nucleus is left with excess energy after alpha or beta decay, shown as a higher line on an energy level diagram.AQA
Nuclear fusion in stellar cores: When the core reaches roughly 10⁷ K, hydrogen nuclei fuse into helium, releasing energy.OCR
Nuclear mass: Approximately A × 1.66 × 10⁻²⁷ kg, using the atomic mass unit.AQA
Nuclear notation: A representation of nuclei that places the nucleon number as a superscript and the proton number as a subscript before the symbol X.OCR
Nuclear radius: The graph provides evidence for a value of the order of 10⁻¹⁵ m and supports the model of the nucleus as a small, dense, roughly spherical object.AQA, OCR
Nuclear stability: The short-range attraction explains why large nuclei need more neutrons for stability, because the strong force only acts between neighbouring nucleons while…AQA, OCR
Nuclear volume: Found by treating the nucleus as a sphere of uniform matter, using V = (4/3)πR³ = (4/3)πr₀³A.AQA
Nucleon interactions: Attractive between nucleons separated by up to about 3 fm, becoming repulsive below about 0.5 fm to prevent nucleons collapsing into each other.AQA
Nucleon number: The total number of nucleons (protons plus neutrons), not the proton number.AQA, OCR
Nucleon quark composition: The proton has composition uud, two up quarks and one down quark, and the neutron has composition udd, one up quark and two down quarks.OCR
Nuclide notation: Representation of a parent nuclide and daughter with A and Z, as used in decay equations.AQA
null result: No significant fringe shift was observed, so the expected aether effect was not detected beyond experimental uncertainty.AQA
Number density of charge carriers: The number density n of charge carriers available to move distinguishes the three classes of material.OCR
Number of bits per sample: Determines how many discrete levels are available; increasing it reduces the voltage step between adjacent levels, lowering quantisation error and improving…AQA
Number of undecayed nuclei: The actual count of nuclei present at that instant, to which activity is proportional.OCR
Numerical modelling using spreadsheets: A small time step Δt is used to update Q iteratively as Q_new = Q_old + (ΔQ/Δt) × Δt, with ΔQ/Δt calculated from the current Q.OCR
Numerical precision and significant figures: Presenting the converted value with an appropriate unit and a sensible number of significant figures, keeping powers of ten consistent.AQA

O

Objective: A converging lens of longer focal length than the eyepiece, on a common principal axis, on which non-axial parallel rays from a distant object are incident.AQA
Objective lens: Forms an image on the distal face of the coherent bundle, while a viewing lens or camera at the proximal end reconstructs the image.AQA
Observational cosmology and current models: These proportions come from combining observations such as the CMB, supernova distances and galaxy surveys, and they remain an active area of research.OCR
Observational evidence for cosmological models: Hubble's law, the cosmic microwave background radiation at 2.7 K, and the observed abundances of light elements such as hydrogen and helium.OCR
Ohm: The unit of resistance, symbol Ω, where one ohm is one volt per ampere.OCR
Ohm’s law: The current through a metallic conductor is directly proportional to the potential difference across it, provided physical conditions such as temperature…OCR
Ohmic and non-ohmic behaviour: A fixed resistor at constant temperature has current directly proportional to p.d., giving a straight line through the origin, while a filament lamp is…AQA, OCR
Ohmic and non-ohmic conductors: An ohmic conductor at constant temperature gives a straight-line I–V graph through the origin, while a filament lamp is non-ohmic because heating raises its…AQA
Ohmic conductor: At constant temperature, current is directly proportional to potential difference, so the I–V graph is a straight line through the origin and resistance is…AQA, OCR
On-site supply of short-lived tracers: The generator gives hospitals a reliable, on-site source of a versatile tracer without needing a reactor for each scan.AQA
One-dimensional vector resolution: All velocities lie along a single straight line, so momentum conservation becomes a scalar equation once a positive direction is chosen.AQA
One-to-one interaction: A single photon is absorbed by a single electron at or near the metal surface, rather than the interaction being spread over many electrons.OCR
Operational amplifier: A high-gain differential voltage amplifier supplied as an integrated circuit, treated as an important system building block.AQA
Operational amplifier design trade-offs: Increasing the closed-loop gain by reducing negative feedback makes the bandwidth over which that gain is maintained proportionally smaller, and vice versa.AQA
Opposite-travelling progressive waves: Two progressive waves of the same frequency and similar amplitude travel in opposite directions through the same medium.AQA
Optic fibre: Guides infrared or visible light by total internal reflection, with very low attenuation and immunity to electromagnetic interference, but needs…AQA
Optical fibres: Light entering within the acceptance angle strikes the core–cladding interface at an angle greater than the critical angle, so it reflects repeatedly and…AQA
Optical ray construction and labelling: Ray directions are marked with arrows, and the path is drawn strictly up to the formation of the intermediate image, without including the eyepiece.AQA
Orbital period: The interval between repeated dips, distinguished from the duration of one dip.AQA
Orbital radius: Fixed once the period T is chosen, given by r³ = GMT²/(4π²) for a satellite in a circular orbit around a planet of mass M.AQA
Order: The number n of a sharp maximum produced by a diffraction grating, with angles θ given by d sin θ = nλ.AQA, OCR
Order of magnitude: A scale based on powers of ten, where two quantities differ by one order of magnitude if one is about ten times the other.AQA
Order of the maximum: An integer n in d sin θ = nλ, with the zeroth-order maximum at θ = 0° and higher orders appearing symmetrically on both sides.AQA, OCR
Order-of-magnitude estimation in physics: Taking λ ≈ 10⁻¹⁰ m and substituting values gives V ≈ 150 V, showing that around 10² V is needed to reach atomic-scale wavelengths.AQA
Order-of-magnitude reasoning: Producing a sensible approximate value for a quantity by combining known benchmarks with simple reasoning, then rounding to one significant figure and…AQA
Ordering: How regularly the atoms or molecules are arranged, being a regular lattice in solids, random in liquids and negligible in gases.OCR
Orders of magnitude across the spectrum: Approximate wavelength ranges for each principal region, not sharp boundaries, with adjacent regions overlapping slightly.OCR
Origin and evolution of the universe: The Big Bang marks the origin of space-time itself, not an explosion of matter inside a pre-existing space.OCR
Origin of upthrust from a pressure difference: A submerged or partly submerged object experiences upthrust because the pressure on its lower surface exceeds the pressure on its upper surface.OCR
Oscillating quantity: The quantity that must be identified, then compared with the direction of energy propagation, to classify an example.AQA
Oscillation: Continuous switching between two output levels with no stable state and no external trigger needed.AQA
Ossicles: The malleus, incus and stapes transmit and amplify vibrations from the tympanic membrane to the oval window.AQA
Output transducer: Converts the recovered electrical signal back into a form the user can perceive, such as sound from a loudspeaker.AQA
Output voltage as a function of sensor resistance: Relate the measured output voltage to the changing resistance of the sensor using the divider equation.OCR
Overall efficiency: Brake power divided by input power, combining thermal and mechanical efficiencies.AQA
Overlapping region boundaries: The boundaries between regions are orders of magnitude rather than exact, and adjacent regions overlap slightly.OCR

P

p = F/A: The equation applying to solids, liquids and gases, in which F is the force perpendicular to the relevant surface and A is the area.OCR
P = I²R: Obtained by substituting V = IR into P = VI, and useful when current and resistance are known.OCR
P = V²/R: Obtained by substituting I = V/R into P = VI, and useful when voltage and resistance are known.OCR
p–V diagram: Plots pressure on the vertical axis against volume on the horizontal axis, with a process drawn as a continuous path joining an initial point to a final point.AQA
Pair annihilation: A particle and its antiparticle combine and their mass is converted into photons, commonly two gamma-ray photons each of energy m₀c² emitted in opposite…OCR
Pair creation: The conversion of energy into a particle and its antiparticle, for example a photon producing an electron and a positron.OCR
Pair production: The strong interaction creates a strange quark and a strange antiquark together, so the total strangeness of the products starts at zero.AQA, OCR
Parabolic concave primary mirror: Collects parallel light from a distant object and removes spherical aberration for on-axis parallel rays.AQA
Parabolic energy curves: The potential energy curve is a parabola, Ep = ½mω²x², and the kinetic energy curve is an inverted parabola, Ek = ½mω²(A² − x²).OCR
Parabolic mirror: Reduces spherical aberration, which arises when a spherical mirror brings marginal rays to a different focus from paraxial rays.AQA
Parallax: A star shifts against the background as Earth orbits, and d = 1/p with d in parsecs when p is in arcseconds.AQA
Parallax and distance measurement: Because the definition is built from a parallax angle, the parsec links directly to stellar parallax and to the equation d = 1/p.OCR
Parallax and the parsec: The parsec is a cosmological distance unit defined using parallax: the distance at which one astronomical unit subtends an angle of one arcsecond.OCR
Parallax angle p: Half the total angular shift measured over six months.OCR
Parallax avoidance: Reading the scale perpendicular to it so that parallax error is avoided.OCR
Parallel combination of capacitors: A connection in which the total capacitance is the sum of the individual capacitances, C = C₁ + C₂ + ...OCR
Parallel plate capacitor: Consists of two conducting plates separated by a distance d, with area A overlap.OCR
Parsec: The distance at which 1 astronomical unit subtends an angle of 1 arcsecond, defined geometrically and linked to the parallax method.AQA, OCR
Part-silvered mirror: Splits a beam of monochromatic light into two perpendicular paths and later recombines the returning beams so they fall on a detector or eyepiece.AQA
Particle classification: Leptons are classified as fundamental particles that do not feel the strong force, with the electron and neutrino as examples.OCR
Particle emission and the neutrino hypothesis: The neutrino or antineutrino carries away energy and momentum, explaining the continuous beta energy spectrum, so it must be emitted in beta decay.AQA
Particle physics and the neutrino: A neutral, nearly massless particle hypothesised to account for the missing energy, with the antineutrino emitted in beta-minus decay and the neutrino in…AQA
Particle reaction feasibility: A reaction with unequal total strangeness on the two sides is forbidden as a strong interaction.AQA
Particle stability and decay chains: Baryons heavier than the proton are unstable and decay through a chain of processes until a proton is produced.AQA
Particles in continuous random motion: The simple kinetic model treats a substance as a large number of particles in continuous random motion.OCR
Pascal: The unit of pressure, where one pascal equals one newton per square metre.AQA, OCR
Path difference: The extra distance one wave travels compared with another from the same source to a point, determining whether they arrive in step or out of step.AQA, OCR
Path difference and fringe formation: Bright fringes occur where the path difference from the two slits is a whole number of wavelengths, and dark fringes where it is an odd number of half…OCR
Path difference and order of maxima: The condition for maxima is d sin θ = nλ, where n is the order and takes integer values 0, 1, 2, ...OCR
Path difference conditions: Maxima occur where path difference is a whole number of wavelengths, and minima where path difference is an odd number of half-wavelengths.OCR
Patient dose and radiation risk: Techniques using ionising radiation deliver a dose whereas ultrasound and MRI do not.AQA
Patient dose optimisation: Digital data can be windowed and enhanced after exposure without repeating the scan, and lower retake rates and reduced repeat exposures follow from immediate…AQA
Patient safety and treatment planning: Accurate patient positioning, immobilisation and imaging-based treatment planning ensure the beam targets the tumour and avoids critical organs.AQA
Peak emf: ε₀ = BANω, reached when the coil plane is parallel to B, where flux linkage is momentarily zero but changing fastest.AQA
Peak power: A core-collapse supernova reaches a peak electromagnetic luminosity of 10³⁶ W, roughly 2.6 × 10⁹ times the solar luminosity.AQA
Peak wavelength: The wavelength of peak intensity, read from the curve or spectrum and converted to metres to estimate a source's temperature.AQA, OCR
Peak-to-peak value: The difference between the maximum positive and maximum negative values, so V_pp = 2V₀.AQA
Peer review: Independent experts scrutinise methods, data and conclusions before publication, checking whether conclusions follow from evidence and whether analysis is…AQA
Penetrating ability: Paper stops alpha, a few millimetres of aluminium stops beta, but gamma needs thick lead to reduce intensity appreciably.AQA
Penetrating power and external detection: Gamma photons penetrate tissue and reach an external detector, whereas alpha and beta radiation is largely absorbed within the body.AQA
Perceived loudness: What each curve represents, being constant along a curve and labelled by the 1 kHz level in phons.AQA
Percentage difference: A comparison of an experimental value with an accepted value: difference ÷ accepted value × 100.OCR
Percentage error: Compares the uncertainty with the measured value, calculated as (uncertainty ÷ measured value) × 100%.OCR
Percentage form: Useful for comparing precision across different quantities, because a small absolute uncertainty on a large value gives a tiny percentage uncertainty.AQA
Percentage uncertainty: Equals the fractional uncertainty multiplied by 100, giving about 0.119% in this example.AQA
Perfectly elastic collision: A collision in which both momentum and kinetic energy are conserved.OCR
Perfectly inelastic collision: A special case in which the bodies stick together and move with a common velocity, giving maximum kinetic energy loss.OCR
Period: The time in seconds for one complete revolution, related to angular velocity by ω = 2π/T.AQA, OCR
Period and frequency relationships: Since f = 1/T, the forms ω = 2π/T and ω = 2πf are equivalent, with T the period in seconds and f the frequency in hertz.OCR
Period equations: For a mass on a spring T = 2π√(m/k), and for a simple pendulum T = 2π√(l/g); neither expression contains the amplitude.OCR
Period–semi-major-axis relationship: The square of the orbital period T is proportional to the cube of the semi-major axis a, so T²/a³ is constant for all planets orbiting the same central body.OCR
Permanent magnets: These produce a magnetic field because atomic magnetic moments within domains are aligned, giving a net magnetic field in materials such as iron, cobalt and…OCR
Permitted changes in strangeness: In weak interactions the permitted changes are 0, +1 or −1; changes of magnitude greater than 1 are not observed in a single weak vertex.AQA
Permittivity: Related to the permittivity of free space ε₀ by ε = εrε₀, where εr is the relative permittivity of the material.OCR
Permittivity of free space: A quantity with the value 8.85 × 10⁻¹² F m⁻¹ that, together with the radius, fixes the capacitance of an isolated sphere in free space.OCR
Perpendicular components: The two parts along chosen perpendicular axes that replace a single vector and can be treated independently.AQA, OCR
Perpendicular distance: Measured perpendicular to the force direction, between the two parallel lines along which the forces act, not between their points of application.AQA
Perpendicular distance between lines of action: The perpendicular separation d used in torque = Fd, taken between the lines of action of the two forces rather than between the points of application.OCR
Perpendicular field and current: The thumb, first finger and second finger are held mutually perpendicular; if current and field are parallel, the force is zero.OCR
Perpendicular force and change of direction: A constant net force perpendicular to the velocity changes the direction of motion but not the speed.OCR
Perpendicularity of field lines and equipotentials: The gravitational force acts perpendicular to the equipotential surface, so equipotential surfaces are always perpendicular to field lines.AQA
Perpendicularity of force and displacement: The electric force is always perpendicular to the equipotential surface, so the force has no component along the displacement.AQA
Petrol cycle: Fuel–air mixture is drawn in, compressed, ignited by a spark, hot gas expands doing work, and burnt gas is expelled.AQA
Phase difference: An angle in radians, linked to path difference by Δφ = 2πΔx/λ, where Δx is path difference and λ is wavelength.AQA, OCR
Phase relationship: Displacement and acceleration are in antiphase, while velocity leads displacement by a quarter cycle.AQA
Phase relationships between points: Adjacent points between nodes oscillate in phase, while points on opposite sides of a node are in antiphase.OCR
Phase relationships in simple harmonic motion: Where displacement is maximum the gradient is zero so velocity is zero, and where displacement crosses zero the gradient is steepest so velocity is maximum in…AQA
Phon: The label of each curve, given by the sound intensity level of the 1 kHz reference tone for that curve.AQA
Photo-conductive mode: Uses reverse bias and an external supply, giving a wide depletion region and a strong electric field across the junction.AQA
Photocathode: Releases electrons when visible photons strike it by the photoelectric effect, the number of photoelectrons being proportional to the light intensity.AQA
Photocurrent: Produced when absorbed photons create electron-hole pairs, which are separated by the field; approximately proportional to irradiance over the intended…AQA
Photocurrent against voltage at fixed irradiance: A characteristic curve plots current against voltage for several fixed irradiances, and in reverse bias each irradiance gives a near-horizontal line whose…AQA
Photocurrent and emission rate: Increasing intensity increases the number of photoelectrons emitted per second and hence the photocurrent.OCR
Photodiode: A p-n junction operated in reverse bias that converts light into an electrical signal.AQA
photodiode pixel: A tiny light-sensitive semiconductor element in a two-dimensional array that converts received light into a stored electrical charge.AQA
Photoelectric current: A current proportional to the rate of emission of photoelectrons, so proportional to intensity at constant frequency above threshold.OCR
Photoelectric effect: A photon is fully absorbed and ejects a tightly bound inner-shell electron, with probability rising strongly with Z and falling rapidly as photon energy…AQA, OCR
Photoelectric emission: The emission of electrons from a metal surface when electromagnetic radiation of sufficiently high frequency is incident on it.OCR
Photoelectric equation: The expression hf = ϕ + Ek(max), which expresses conservation of energy for one photon absorbed by one electron.AQA
Photomultiplier tubes: An array of tubes in which each tube's photocathode releases electrons by the photoelectric effect and dynodes multiply the signal by secondary emission.OCR
Photon: A quantum of electromagnetic radiation carrying energy E = hf, where h is the Planck constant and f is the frequency.AQA, OCR
Photon and particle energies: The energy of any particle or photon can be expressed in eV, a convenient unit for atomic and photon scales.OCR
Photon emission: Each line arises when an electron drops from a higher discrete level to a lower one, emitting one photon of energy hf equal to the level difference.AQA, OCR
Photon energy: The relationship E = hf links photon energy to frequency, while intensity relates to the rate of photons rather than to the energy of each photon.AQA, OCR
Photon energy and frequency: Photon energy is directly proportional to frequency, E = hf, where h is the Planck constant (6.63 × 10⁻³⁴ J s) and f is frequency in hertz.AQA, OCR
Photon energy and the Planck constant: The minimum energy needed equals the photon energy hf, and photon energy is related to frequency by E = hf and to wavelength by E = hc/λ.OCR
Photon model: Electromagnetic radiation can be modelled as a stream of discrete packets of energy called photons, treating radiation as localised quanta rather than a…AQA, OCR
Photon model and one-to-one interaction: One photon interacts with one electron, transferring energy hf; if hf ≥ φ the electron is emitted.AQA
Photon model of light: A model in which one photon interacts with one electron, explaining the observations on photoelectricity.AQA
Physical interpretation: Relating the gradient and intercept to the physical quantities in the experiment, such as acceleration and initial velocity.OCR
Physical principles of medical imaging: The physical interaction each technique uses, such as differential attenuation for X-rays, annihilation photon detection in PET, or reflection of ultrasound…AQA
Physical quantities: What each unit measures, for example the pascal is the unit of pressure and the coulomb is the unit of charge.OCR
physical quantity: Something that is expressed as a numerical value multiplied by a unit, and without a unit the numerical value is meaningless in physics.OCR
Piezoelectric effect: The production of a potential difference across certain crystals when they are mechanically deformed, and the reverse effect where an applied potential…AQA, OCR
Pion: A meson made of a quark and an antiquark, acting as the exchange particle of the strong nuclear force.AQA
Planck constant: The constant of proportionality between the energy of one photon and the frequency of the radiation, denoted h.AQA, OCR
Planck's quantum hypothesis: Energy is emitted or absorbed in discrete quanta of energy E = hf, limiting high-frequency emission because fewer oscillators can acquire large energy quanta,…AQA
Plane polarised wave: A wave in which the oscillations are confined to a single plane.OCR
Planet: Orbits a star, is massive enough for gravity to make it approximately spherical, and has cleared other bodies from its orbital neighbourhood.OCR
Planetary nebula: An expanding shell of gas ejected from the star and ionised by the exposed hot core; the name is historical and has nothing to do with planets.OCR
Plastic deformation: Permanent change in which the material does not return to its original shape when unloaded, because atomic planes slip past one another.OCR
Plastic deformation and permanent extension: Occurs beyond the elastic limit, where the material does not return to its original length when the force is removed, leaving a permanent extension.AQA
Plumb line: A true vertical reference, useful for checking verticality or centring.OCR
Point charge field: Given by E = kQ/r² where k = 1/(4πε₀) and r is the distance from the centre.AQA
Point-by-point addition of ordinates: Adding the displacements of the individual waves at each point to obtain the resultant displacement.OCR
Point-mass model: A uniform spherical mass can be modelled as a point mass located at its centre.OCR
Point-source assumption: The source must be small compared with the distance at which it is observed, so that rays are effectively radial.AQA
Polar molecule: Has a permanent dipole because its positive and negative charge centres do not coincide.AQA
Polarisation: Shows that transverse waves oscillate in one plane, and the procedures demonstrate that microwaves and light are transverse waves because their oscillations…AQA, OCR
Polarisation of a dielectric: Caused by the alignment of many polar molecules, which reduces the resultant field between the plates.AQA
Polarisation of transverse waves: Restricting oscillations to a single plane, a property exclusively of transverse waves, so longitudinal waves cannot be polarised.OCR
Polarising filter: A device that transmits only the component of oscillation parallel to its transmission axis, reducing intensity and producing plane-polarised light.AQA
Polarity and DC supply connections: Connecting DC power supplies and cells with the polarity and terminal arrangement indicated, keeping polarity consistent with the diagram.OCR
Polaroid material: Contains long-chain molecules that transmit light oscillations parallel to one axis while absorbing components in other planes, so unpolarised light becomes…AQA
Polaroid sunglasses: Reduce glare from horizontal surfaces, because reflected glare is partly polarised horizontally and a vertical transmission axis blocks much of it.AQA
Polycrystalline graphite: A thin slice of this material contains a regular arrangement of carbon atoms that diffract the electrons passing through it.OCR
Positive ion bombardment: Positive ions accelerated towards the cathode strike it, causing the emission of electrons from its surface.AQA
Positron: Antiparticle of the electron, with charge +1 rather than -1.AQA
Positron-emitting radionuclide: A substance, often fluorine-18 in fluorodeoxyglucose, that is administered and taken up by metabolically active cells, emitting a positron that travels a…OCR
Potential difference: The difference in potential between two points, where the work done W in moving a charge q between them is W = qΔV.AQA, OCR
Potential difference and energy transfer: The eV is an energy unit, not a unit of potential difference or charge; the volt is potential difference.OCR
Potential difference division in series: The potential differences across capacitors in series add to give the total potential difference.OCR
Potential difference measurement and amplification: A differential amplifier amplifies the voltage between pairs of electrodes while rejecting common-mode interference such as mains hum.AQA
Potential divider: A circuit of two or more components in series across a power supply, used to supply a constant or variable potential difference from that supply.AQA, OCR
Potential divider circuits: Arrangements in which a thermistor produces a potential difference that varies with temperature, changing the share of the supply potential difference across…AQA
Potential divider ratio: For two resistors R₁ and R₂ in series across a supply of p.d.OCR
Potential energy contribution: Arises from the forces between the molecules and depends on their separation.OCR
Power: The rate at which work is done or energy is transferred, defined as the work done W divided by the time taken t, so P = W/t.AQA, OCR
Power as rate of energy transfer: Power is the rate of transfer in watts, while energy is the total transferred in joules.OCR
Power conservation in ideal transformers: For an ideal transformer, power in equals power out, so Ns/Np = Vs/Vp = Ip/Is.OCR
Power supply rails and saturation: The power supply connections provide the positive and negative rails that limit the output voltage.AQA
Powers of ten: The prefix can be written as a power of ten before doing any arithmetic, so 1 km becomes 1 × 10³ m.OCR
Powers of ten and unit conversion: Calculations must be done correctly with powers of ten, keeping units consistent and converting all wavelengths to metres in standard form.OCR
Powers of ten and unit prefixes: The prefixes kilo- and mega- make kpc and Mpc larger than a parsec by 10³ and 10⁶ respectively, so the prefix must be checked when converting.OCR
Practical applications of thermodynamics: Uses such as domestic refrigerators, air conditioners and heat pumps for space heating.AQA
Practical control systems: Circuits in which a thermistor can switch on a heater when it is cold or trigger a cooling fan when it is hot.AQA
Practical measurement and graphing: Apply a small sinusoidal input of fixed amplitude, vary its frequency across a wide range, and measure the output amplitude with an oscilloscope or meter,…AQA
Practical measurement of resistance: Place an LDR in a circuit with an ammeter and voltmeter and calculate resistance from V/I.OCR
Practical observation of induced e.m.f. and current: These experiments let students observe induced voltage on a galvanometer or oscilloscope and relate observations to Faraday's law.AQA
Precession in a magnetic field: Protons in the strong uniform field of a superconducting magnet initially align parallel to the field and precess about the magnetic field lines.AQA
Precision: Describes how close repeated measurements are to each other, often indicated by the spread or uncertainty.AQA, OCR
Precision, units and safety: Recording values with the correct unit and a precision consistent with the instrument used, and following safe and secure technique such as clamping apparatus…OCR
Prefixes and unit conversion: Prefixes scale the metre, for example 1 km = 10³ m and 1 nm = 10⁻⁹ m, and centimetres or millimetres are converted to metres before substituting.OCR
Pressure: Obtained by summing over N molecules and dividing by wall area A, giving p = (1/3)ρc̄², where ρ is density and c̄² is the mean square speed.AQA, OCR
Pressure from molecular collisions: Pressure arises from the rate of change of momentum as molecules strike unit area of wall.AQA
Pressure variation with depth in a fluid: In a fluid at rest, pressure increases with depth according to p = ρgh, where ρ is the fluid density, g is the gravitational field strength and h is the depth…OCR
Pressure–temperature relationship: Measuring the pressure of a fixed mass of gas at constant volume as its temperature is varied gives a straight line when pressure is plotted against…OCR
Principle of conservation of energy: Energy cannot be created or destroyed; it can only be transferred from one store to another or transformed from one form to another.AQA
Principle of moments: For an object in rotational equilibrium, the sum of the clockwise moments about any point equals the sum of the anticlockwise moments about that same point.AQA
Principle of superposition: Displacements add algebraically at every point.AQA
Product term: A term that is true only for one row of a truth table, written using AND gates and NOT gates for inputs that are 0.AQA
Progressive wave: A disturbance that transfers energy through a medium or space without transferring matter, as the disturbance travels away from the source.OCR
Projectile motion: Horizontal acceleration is zero (constant velocity) and vertical acceleration is g downwards.OCR
Projectile motion analogy: When a particle enters perpendicular to the field, its motion is analogous to projectile motion: constant velocity parallel to the plates and constant…OCR
Projectile motion with drag: Air resistance opposes the velocity of a projectile at every point, reducing both horizontal and vertical velocity components and making the trajectory…AQA
Propagation through vacuum at c: All regions travel through a vacuum at c = 3.00 × 10⁸ m s⁻¹.OCR
Proper time: The time interval measured by an observer who sees two events occur at the same place, recorded by a single clock at rest relative to those events.AQA
Proportional reasoning: The period is approximately proportional to RC, so doubling R or C roughly halves the frequency.AQA
Proportional reasoning about length and area: Doubling L doubles R, and doubling A halves R, for a fixed material.OCR
Proportional reasoning in nuclear physics: Combining the radius equation with the volume of a sphere to show that volume is directly proportional to A, the number of nucleons.AQA
Proportionality of charge and p.d.: For a capacitor, the p.d.AQA, OCR
Proton and nucleon number: The proton number gives the number of protons and identifies the element, while the nucleon number is the total number of protons and neutrons.OCR
Proton number: The number of protons in a nucleus, which defines the element.AQA, OCR
Provisional nature of science: A model survives while it explains the evidence, and is modified or replaced when new evidence conflicts with it.AQA
Pulsar: A rapidly spinning neutron star that emits beams of radiation, detected as pulses when the beam sweeps past Earth.OCR
Pulse broadening: The temporal spreading of a signal pulse as it propagates, so the output pulse has a longer duration than the input pulse.AQA
Pulse width: The time the output stays high in one cycle; the remainder is the low time.AQA
Pulse-echo principle: The principle, used by both A-scan and B-scan, of detecting reflections of pulsed ultrasound at boundaries between media.OCR
Pulse-echo timing: The time between emission and echo gives depth using d = vt/2.AQA
Pulse–echo depth measurement: A transducer emits high-frequency sound pulses into the body, and the time delay of the echoes provides depth information.AQA
Pulse–echo principle: A pulse is emitted and echoes from boundaries are detected, used by both A-scans and B-scans with the same speed of sound in tissue.AQA
Pure astigmatism: A condition that can be corrected using only a cylindrical lens, meaning the spherical component is 0.00 DS.AQA
pV = NkT: Relates pressure p, volume V, number of molecules N, Boltzmann constant k and thermodynamic temperature T, and is the per-particle form of pV = nRT.OCR
Pythagoras check: Confirms resolved components using Fx² + Fy² = F², catching a swapped sine and cosine.AQA

Q

Q = ml: The equation used instead of Q = mcΔθ during a change of state, where Q is the energy needed to change the state of mass m.AQA
Qualitative modelling: With air resistance the trajectory becomes asymmetric, with reduced range, lower maximum height, and a descent that is steeper than the ascent.AQA
Qualitative reasoning in wave optics: The relationship is qualitative: no calculation is required, but the direction of change must be stated correctly.AQA
Qualitative treatment of friction: Describing the effects of friction in words and identifying the direction of the frictional force without calculating coefficients.AQA
Quality factor Q: A quantity relating to the width of the peak: a higher value gives a narrower, sharper resonance.AQA
Quanta: Discrete packets in which the energy exchanged between matter and electromagnetic radiation comes, rather than continuously.AQA
Quantisation: Assigns each sample to the nearest available discrete level, where the number of levels is 2ⁿ and n is the number of bits per sample.AQA
Quantisation of charge: The net electric charge on any particle or object can only take discrete values that are integer multiples of the elementary charge e.OCR
Quantisation of charge and the elementary charge: The elementary charge e = 1.60 × 10⁻¹⁹ C is the charge carried by an electron, used with the potential difference in W = QV to derive 1 eV = 1.60 × 10⁻¹⁹ J.AQA
Quantisation of electric charge: Every droplet charge was an integer multiple of a smallest value, demonstrating that charge exists in discrete units carried by electrons.AQA
Quantisation of electromagnetic radiation: Electromagnetic radiation is emitted, absorbed or transferred in discrete quanta called photons, each of energy E = hf.AQA
Quantisation of energy: Energy is emitted or absorbed in discrete quanta, with energy E = hf, which suppresses high-frequency emission and matches the observed black-body spectrum.AQA, OCR
Quantised energy packets: Black-body radiation and the photoelectric effect could not be explained by classical wave theory alone, leading to quantised energy packets or photons.AQA
Quantitative analysis of power loss: For a given power P transmitted at voltage V, the current is I = P ÷ V, and the power lost in the transmission line of resistance R is P_loss = I²R.AQA
Quantitative form of Faraday's law: The induced e.m.f.OCR
Quantitative prediction: The relationship allows the bandwidth to be predicted for a chosen gain, or the gain to be predicted for a required bandwidth.AQA
Quantum efficiency: The fraction of incident photons that produce a detectable signal, which is low for the eye but can exceed 80 per cent for a CCD.AQA
Quantum tunnelling: Electrons can cross the gap between a very sharp metal tip and a conducting surface when a small potential difference is applied.AQA
Quark and anti-quark composition: Baryons contain three quarks (qqq), while mesons contain a quark and an anti-quark (qq̄), using up (u), down (d) and strange (s) quarks and their respective…OCR
Quark model of hadrons: Hadrons are particles that experience the strong interaction and are built from quarks.AQA
Quark model of nucleons: In this model the neutron is udd and the proton is uud, so in β⁻ decay one down quark must change into an up quark.OCR
Quark properties: Charge, baryon number and strangeness, which are combined arithmetically when identifying particles or checking whether a reaction is allowed.AQA
Quark structure of mesons: Mesons are composed of a quark and an antiquark, for example π⁺ is u d̄ and π⁻ is ū d.AQA
Quark transformation and hadron change: One down quark changes into an up quark, so the neutron (udd) becomes a proton (uud).AQA
Quarks: Fundamental particles that combine to form hadrons such as protons and neutrons, with flavours up (u), down (d) and strange (s).AQA
Quasar: An extremely luminous active galactic nucleus powered by accretion onto a supermassive black hole, often outshining its host galaxy.AQA
Quasars: Compact, intense radio sources with faint star-like optical images, later shown to lie at vast distances with exceptionally high intrinsic luminosity.AQA
Quasi-stellar radio sources: The original name for quasars, given because optical follow-up showed faint star-like points rather than extended galaxies.AQA

R

Radial field of a point charge: A field that points away from a positive charge and towards a negative charge.OCR
Radial field patterns: For a point mass or a uniform sphere, the field is radial, with lines pointing towards the centre of the mass.OCR
Radial fields: Around a spherical mass the lines are radial and evenly spaced at a given distance.AQA
Radial velocity method: A detection technique in which the periodic Doppler shift of the star's spectral lines is measured as the star moves towards and away from the observer.AQA
Radian: The SI unit of angle, defined so that one radian is the angle subtended at the centre of a circle by an arc equal in length to the radius.AQA, OCR
Radians and phase: One complete oscillation corresponds to a phase change of 2π radians.OCR
Radians to degrees: To convert radians to degrees, multiply by 180/π.OCR
Radiation protection and patient dose: Dose is reduced by collimating the beam, minimising exposure time, using appropriate filtration and avoiding unnecessary repeat exposures.AQA
Radiation safety and source handling: The source must be handled with tongs and kept away from the body, as gamma radiation is highly penetrating.AQA
Radio astronomy surveys: Surveys in the 1950s and 1960s that detected strong, compact radio sources before their optical counterparts were recognised.AQA
Radio waves: Free-space electromagnetic radiation that diffracts around obstacles and passes through walls.AQA
Radio-quiet valley: A site where a radio dish is often placed to reduce interference.AQA
Radioactive dating: A method using the ratio of parent to daughter nuclei and the known half-life to calculate an age.AQA, OCR
Radioactive decay: The fading remnant is powered by the decay of nickel-56 and cobalt-56, giving a characteristic timescale of roughly 77 days.AQA
Radiopharmaceutical uptake and distribution: A gamma-emitting radiopharmaceutical is administered and designed to accumulate in the target organ or tissue, and the computer forms a two-dimensional image…OCR
Radiotherapy and the therapeutic ratio: Fractionation divides the total dose into many smaller treatments, allowing healthy cells time to repair sub-lethal damage between fractions while tumour…AQA
Radius of a charged particle orbit: Equating BQv to mv²/r gives r = mv/(BQ), so the radius depends on mass, speed, flux density and charge.OCR
Radius of geostationary orbit: Substituting T = 8.64 × 10⁴ s and GM = 3.99 × 10¹⁴ m³ s⁻² gives r ≈ 4.22 × 10⁷ m from Earth's centre.AQA
Raising to a power: Raising a quantity to a power multiplies its percentage uncertainty by that power.AQA
Random and spontaneous decay: Radioactive decay is random and spontaneous, so a single nucleus's fate cannot be predicted, yet large samples show exponential decay.OCR
Random errors: Unpredictable variations in repeated measurements about a mean value, caused by factors such as fluctuating temperature, vibration or inconsistent reading of…AQA, OCR
Random molecular motion: Fluid molecules move randomly and collide unevenly from different directions, so a suspended particle experiences a fluctuating resultant force.AQA
Random motion: Motion of particles with no net movement of the body as a whole, with individual particles having a distribution of speeds.AQA
Random thermal motion versus drift motion: Carriers have much larger random thermal speeds, but these average to zero and do not contribute to the current, whereas the field gives each carrier a small…OCR
Random variation: Fluctuation reduced by repeating the background measurement and averaging.AQA
Randomness of radioactive decay: For any given nucleus, we cannot know when it will decay, and in a sample, different nuclei decay at different times.OCR
Range and mass: The infinite range of the electromagnetic force is consistent with the zero rest mass of the photon.AQA
Range and resolution: Choosing a suitable range so that the reading is within the display limits and gives appropriate precision.OCR
Range and strength: The range and strength of each interaction relate to the mass of its exchange particle, with heavier exchange particles giving shorter range.AQA
Range and strength of the strong nuclear force: Attractive around 1 fm and negligible beyond roughly 3 fm, with a repulsive core below 0.5 fm that is not due to pion exchange.AQA
Rapid discharge applications: Uses exploit this: camera flashes charge a capacitor then release energy rapidly through a tube, and defibrillators deliver a controlled pulse.OCR
Ratchet: A part of the micrometer used to apply consistent pressure and avoid over-tightening, compressing the object or damaging the instrument.OCR
Rate of change of flux linkage: The magnitude of the induced emf equals this quantity, written ε = N ΔΦ/Δt, with ΔΦ measured in webers and Δt in seconds.AQA
Rate of change of momentum: The resultant force equals this quantity, which for constant mass reduces to F = ma.OCR
Rate of emission: Above the threshold frequency, the number of photoelectrons released per second, directly proportional to the intensity of the incident radiation.OCR
Rate of energy transfer: Power is the rate of doing work or transferring energy, measured in watts (W), where 1 W = 1 J s⁻¹.OCR
Ratio method: Forming a ratio such as P₁/P₂ = (r₁²T₁⁴)/(r₂²T₂⁴) so that σ and 4π cancel, then solving for the unknown quantity.AQA
Ray box: Produces a narrow beam of light to investigate refraction and total internal reflection.OCR
Rayleigh criterion: Gives the minimum angular resolution θ of a telescope as θ ≈ λ ÷ D, where λ is the wavelength and D is the diameter of the objective aperture.AQA
RC time constant: Of the junction capacitance and load resistance determines response time, so a smaller load resistor gives a faster but smaller signal.AQA
RC timing network: An external resistor–capacitor network that sets the running frequency of an astable rather than the supply voltage.AQA
Reaction time: The time before the brakes are applied, during which the thinking distance is travelled; thinking distance divided by speed gives it.OCR
Real forces providing circular motion: The force is provided by a real interaction such as tension, friction or gravity, not by a new outward force.OCR
Real image: An inverted, diminished image formed on the retina, which acts as the fixed image plane.AQA
Real meters: Have small but non-zero ammeter resistance and large but finite voltmeter resistance, which cause small measurement errors.AQA
Real-time communication system: Transmits information with negligible delay, so the receiver reproduces the signal almost instantaneously.AQA
real-time viewing: Fluoroscopic image intensification produces a bright output image that a camera or screen can display, allowing real-time viewing in fluoroscopy.AQA
Rearrangement of ρ = m/V: The equation can be rearranged to m = ρV and V = m/ρ to find mass or volume.OCR
Rearranging C = Q/V: The defining equation can be rearranged to Q = CV and V = Q/C to find charge or potential difference.OCR
Reasonableness checking: Comparing the value against everyday experience or a known reference before committing to the answer.OCR
Receiving aerial: Must be aligned parallel to the electric field oscillations to detect the signal strongly; rotating it towards perpendicular reduces reception.AQA
Reception: Incoming ultrasound waves deform the crystal, generating a small alternating potential difference that can be detected and processed.OCR
Reciprocal relationship: Frequency and period are reciprocals of each other, so f = 1/T and T = 1/f.OCR
Reciprocal relationship between period and frequency: Frequency is the reciprocal of period, f = 1/T, with the period measured as the horizontal distance for one complete cycle.AQA
Reciprocal relationship for capacitance: The reciprocal of the total capacitance equals the sum of the reciprocals of the individual capacitances: 1/C = 1/C₁ + 1/C₂ + ...OCR
Reciprocals: Period and frequency are reciprocals: T = 1/f and f = 1/T, so a larger period corresponds to a smaller frequency, and vice versa.OCR
Recombination: After roughly 380 000 years the universe cooled enough for electrons and nuclei to combine into neutral atoms, so radiation could travel freely.OCR
Red giant: A star whose outer layers expand and cool after core hydrogen is exhausted, giving a larger radius and lower surface temperature.OCR
Red giant branch: The region reached when the core contracts while outer layers expand and cool, giving higher luminosity and lower temperature.AQA
Red giants: Cool but luminous stars that sit above the main sequence on the right.OCR
red shift: Calculated from observed and rest wavelengths using z = (λ_observed − λ_rest) / λ_rest, and for cosmological red shifts it indicates recession and distance.AQA, OCR
Red-shift and blue-shift: For a receding source the observed wavelength increases and frequency decreases, while for an approaching source the wavelength decreases and frequency…OCR
Redshift: A shift of spectral lines that gives the recession speed, often using Δλ/λ₀ = v/c for speeds much less than c.AQA
Reference tone: A tone at 1 kHz set to a chosen sound intensity level, such as 40 dB, against which a test tone is judged.AQA
Reflection and transmission of ultrasound at boundaries: When ultrasound meets a boundary between two media, part of the wave is reflected and part is transmitted, with the reflected fraction depending on the…OCR
Reflection at boundaries: The change of direction at a boundary, where the angle of incidence equals the angle of reflection, both measured from the normal.OCR
Reflection at curved and plane surfaces: Incoming parallel rays reflect off the concave primary mirror and are redirected by a secondary mirror, convex for Cassegrain or plane for Newtonian.AQA
Refraction: The gradual bending of sky waves as electron density changes through the ionosphere, which can return them to Earth.AQA, OCR
Refraction and wave speed: A change in wave speed upon entering a new medium, altering wavelength and direction while frequency remains constant.OCR
Refraction in rectangular blocks: Tracing the incident, internal and emergent rays and measuring angles of incidence and refraction from the normal to verify Snell's law.OCR
Refractive index: Defined as n = c/v, the ratio of the speed of light in a vacuum to its speed in the medium; it is dimensionless and always ≥ 1.OCR
Regular solid: A solid that has symmetry, so its centre of mass is at its geometric centre.AQA
Regulation: What happens once breakdown begins, because small changes in current produce almost no change in voltage.AQA
Rejection of absolute space and time: Together the postulates force us to abandon absolute time.AQA
Relationship between current and drift velocity: The current is I = nAvq, where n is the number density of charge carriers, A is the cross-sectional area, v is the mean drift velocity and q is the charge on…OCR
Relationship between half-life and decay constant: Half-life is related to the decay constant by T½ = ln2/λ.OCR
Relationship between linear and angular quantities: The linear speed v is related to the angular velocity ω and the radius r by v = ωr, where v is in m s⁻¹, ω in rad s⁻¹ and r in m.OCR
Relationship between linear and angular speed: Since v = ωr, substituting gives a = (ωr)²/r = ω²r, so a = ω²r.OCR
Relationship between nuclear mass and energy transfer: Energy transfer is most efficient when the moderator nucleus mass is comparable to the neutron mass, so light nuclei such as hydrogen are effective.AQA
Relationship between potential difference and separation: The magnitude of the field strength is given by E = V/d, where d is the plate separation.OCR
Relationship between radius, volume and density: Because volume depends on the cube of the radius, substituting R gives V = (4/3)πr₀³A, and since nuclear mass is also approximately proportional to A, the…AQA
Relationship between timing quantities, speed and radius: Period and frequency are related to speed v and radius r by the equation v = 2πr/T, so knowing only the period or frequency does not uniquely determine the…OCR
Relationship between work done and potential energy change: The area under the graph between two positions equals the change in electric potential energy between those positions.OCR
Relationship E_p = Vq: Using E_p = Vq and substituting V = Q/(4πε₀r) gives E_p = Qq/(4πε₀r).OCR
Relative atomic mass: The abundance-weighted mean of the isotope masses, calculated as the sum of (isotope abundance × isotope mass) divided by the total abundance.AQA
Relative hazard: Alpha is dangerous if inhaled or ingested because it ionises densely over a short range, beta harms skin and eyes, and gamma is highly penetrating and needs…AQA
Relative intensity: The comparison of intensity levels in which equal ratios of intensity produce equal differences in decibels, reflecting relative rather than absolute changes.AQA
Relative motion: Movement of a source of electromagnetic radiation relative to an observer, with the relative speed taken along the line of sight.OCR
Relative permittivity: The dielectric constant of the material, denoted εr, which is dimensionless.OCR
Relativistic jets: Jets along the rotation axis into which material from the surrounding accretion disc can be funnelled, emitting intense gamma rays.AQA
Relativistic jets and magnetic fields: Magnetic fields produce collimated relativistic jets that emit radio waves via synchrotron radiation.AQA
Relativistic kinetic energy and the Lorentz factor: The relativistic expression Eₖ = (γ − 1)mc² fits the experimental data perfectly, while energy increased steeply as v approached c.AQA
Remote handling: Needed because spent or irradiated fuel emits intense beta and gamma radiation, so workers use manipulators, lead or concrete flasks and remote cameras.AQA
Repeatability: The closeness of repeated readings taken by the same person, using the same method and equipment, over a short time.AQA
Repeated elution and regrowth: Because the parent half-life is much longer than the 6-hour daughter half-life, the generator can be milked repeatedly as technetium-99m regrows.AQA
Replication and reliability: Independent groups repeat the investigation, which strengthens confidence in a claim.AQA
Reproducibility: The closeness of readings when the measurement is repeated by a different person, or with different equipment or conditions.AQA
Reset input: Forces all outputs to zero, clearing the count to 0000 regardless of the current state, either asynchronously or synchronously.AQA
Reset pin: The input that clears the counter to zero when the decoded signal reaches it.AQA
Resistance: Measures how difficult it is for current to flow through a component, defined by R = V/I, where V is the potential difference across the component and I is…OCR
Resistance as the ratio V/I: The constant ratio of potential difference to current, giving R = V/I.AQA
Resistance calculation from coordinates: Reading the coordinates directly from the graph and using R = V/I, whether the line is curved or straight.AQA
Resistance ratio and potential difference: The supply p.d.AQA
Resistivity: Defined by ρ = RA/L, where R is resistance, A is cross-sectional area and L is length, with unit the ohm metre (Ω m).AQA, OCR
Resolution: Replaces one vector by two perpendicular vectors whose combined effect is identical, not extra forces but a mathematical replacement.AQA
Resolution and number of bits: A group of bits can represent a finite set of values, so more bits increase the number of possible levels or values.AQA
Resolution and precision: Identifying the smallest scale division of an instrument and recording the value with a sensible number of significant figures, estimating between divisions…OCR
Resolution and wavelength: Resolution is limited by wavelength; a higher frequency shortens wavelength and improves resolution but increases attenuation in tissue, reducing penetration…AQA
Resolution limits of electron microscopes: Shorter de Broglie wavelengths allow finer detail to be resolved, though practical electron microscopes use much higher voltages to resolve finer details.AQA
Resolution limits of microscopes: Because electrons have a much shorter de Broglie wavelength than visible light, the resolution is far better than an optical microscope.AQA
Resolution of forces: The cos θ factor selects the component of force along the displacement, so the force is resolved along the displacement before multiplying.AQA, OCR
Resolution of vectors: The reverse process of resolving a single vector into two perpendicular components using sine and cosine.AQA
Resolution of vectors into perpendicular components: The initial velocity is resolved into horizontal and vertical components, each direction treated separately.AQA
Resolved forces: Splitting every force into perpendicular components, usually horizontal and vertical, then applying ΣFₓ = 0 and ΣF_y = 0.AQA
Resolving components: Resolve each vector into perpendicular components, add components, then recombine to find the resultant.AQA
Resolving power: Depends on aperture diameter and wavelength, so a radio dish needs a much larger diameter than an optical mirror for the same resolution.AQA
Resonance: The condition where the driving frequency equals a natural frequency of the system, giving maximum energy transfer per cycle and a maximum steady-state…AQA, OCR
Resonance and maximum amplitude: Occurs when a periodic driving force is applied at a frequency equal to the natural frequency, producing a maximum amplitude of the steady-state oscillation.OCR
Resonance and natural frequency: Both systems oscillate at a natural frequency, with the LC circuit's resonant angular frequency ω = 1/√(LC) compared with ω = √(k/m).AQA
Resonance in LC circuits: A larger capacitance gives slower oscillations for a given inductance, as a softer spring gives a longer period, with angular frequency ω = 1/√(LC).AQA
Resonance in mechanical systems: A mechanical example must identify the driver, the oscillating system, and the natural frequency that is matched or approached.AQA
Resonance in receiver circuits: Detection relied on tuning the receiver to the transmitter frequency so that resonance produced detectable sparks.AQA
Resonant frequency: The driving frequency that matches a harmonic of the string, identified by observing the largest-amplitude stationary wave with clear nodes and antinodes.AQA
Resonant frequency f₀: The frequency at which the response is maximum or minimum, depending on circuit type.AQA
Resonant peak: The peak of the curve, occurring at the driving frequency closest to the natural frequency of the oscillator, where the resonant amplitude is large at low…OCR
Responsible laboratory practice: Following local rules, such as warning signs and restricted access, and never looking directly into the beam or its reflection.AQA
Responsivity against wavelength: The spectral response curve plots responsivity, or photocurrent at fixed irradiance, against wavelength at fixed reverse bias.AQA
Rest energy: The energy equivalent of rest mass, calculated using E = mc², and in particle physics it is usually quoted in megaelectronvolts, MeV.AQA
Rest energy and energetically allowed decays: The proton has the lowest rest energy of all baryons, so no further baryon decay is energetically possible.AQA
Rest masses of atomic constituents: Protons, neutrons and electrons have specific rest masses, handled exclusively in kilograms using the values provided on the data sheet.AQA
Restoring acceleration: Acceleration that always acts towards the equilibrium position, shown by the minus sign in the defining equation.OCR
Restoring force and acceleration towards equilibrium: Acceleration is opposite in sign to displacement and always points towards the equilibrium position, opposing velocity only when moving away from equilibrium.AQA
Restoring force and simple harmonic motion: The capacitor provides the restoring effect that drives oscillations, just as a spring provides the restoring force.AQA
Resultant force: The vector sum of all forces acting on an object; if it is non-zero, the object accelerates in its direction.AQA, OCR
Resultant force and free-body diagrams: The force in the equation is the resultant of all forces acting on the object, not a single applied force, and a free-body diagram is drawn and forces…OCR
Resultant force and motion: A body remains at rest or moves with constant velocity unless acted on by a resultant force, so zero net force means zero acceleration.OCR
Resultant magnitude and direction: A single vector with the same effect as the original vectors, stated as a magnitude with unit and a direction relative to a stated reference.AQA
Resultant torque: The sum of the moments about any point, which must be zero for equilibrium, with clockwise moments equalling anticlockwise moments.OCR
Resultant vectors: The single vector found by combining two or more vectors, taking direction into account.OCR
Reverse bias: The condition where the cathode is positive relative to the anode, and the diode blocks until the reverse voltage reaches the Zener voltage.AQA
Reverse breakdown: The state in which a Zener diode is normally operated, where the voltage across it stays almost constant even when the current through it changes.AQA
Reverse breakdown operation: The Zener is connected in reverse bias, with its cathode to the positive line and anode to the negative line, so it operates in breakdown.AQA
Reverse-biased p-n junction operation: A photodiode is operated in reverse bias, so the p-n junction has a wide depletion region and a strong internal field.AQA
Reversed heat engines: A heat pump or refrigerator, in which work input moves thermal energy from a cold region to a hot region.AQA
Reversed temperature axis: A deliberate convention that places hot blue stars on the left and cool red stars on the right, with temperature decreasing to the right.AQA
Rhodopsin: A pigment contained in rods which bleaches in bright light, so rod vision dominates in dim conditions.AQA
Right-hand grip rule: The rule giving the field direction: thumb along conventional current, fingers curl in the field direction.OCR
Rigid rotating body: A body rotating about a fixed axis for which angular velocity is the same for all points, even though points further from the axis move faster in a…OCR
Rigid-body rotation: Because ω is the same for all points on a rigid body, v is proportional to r, so points further from the axis move faster in a straight-line sense.OCR
Ring of detectors and transmitted intensity: A ring of detectors surrounds the patient and measures the transmitted X-ray intensity at many angles.OCR
Risk reduction in practical work: Never directing a laser beam towards anyone's face, keeping the beam below eye level, removing reflective objects from the beam path and using the lowest…AQA
Risk–benefit of ionising radiation: The technique exposes the patient to ionising radiation, so benefit must be weighed against risk.OCR
Risk–benefit reasoning in applied physics: Assessment compares risk and benefit for a named use rather than declaring radiation simply good or bad.AQA
Rod cells: Highly sensitive in dim light but colour-blind, dominating at low intensity and peaking near 500 nm.AQA
Role of experimental evidence: Rutherford's alpha-scattering results showed that positive charge and most mass are concentrated in a tiny nucleus, so the plum-pudding model was replaced.AQA
Root mean square speed: The square root of the mean square speed, a representative speed for the particles that is not the same as the average speed.OCR
Root mean square value: The steady direct value that would dissipate the same average power in a given resistor.AQA
Rotating X-tube and fan-shaped beam: The X-tube rotates around the patient and produces a thin fan-shaped X-ray beam that passes through a narrow slice of the body.OCR
Rotational equilibrium: A state in which an object has zero angular acceleration, meaning it is either not turning or rotating at a constant angular velocity.AQA
Rotational form of Newton's second law: The resultant torque produces angular acceleration according to T = Iα, the rotational analogue of F = ma.AQA
Rotational kinetic energy: Energy stored by a rotating mass, given by Eₖ = ½Iω², so storage increases with moment of inertia and with the square of angular speed.AQA
Rules for drawing field diagrams: To map a field, draw lines with arrows showing direction and vary their spacing to show how strength changes with position.OCR
Rutherford scattering: Alpha particles are directed at a very thin gold foil inside an evacuated chamber, and a detector records where particles arrive.OCR

S

S.I. base quantities: Mass, length, time, amount of substance, thermodynamic temperature and electric current, each with its associated SI unit.AQA, OCR
S.I. base unit: The metre (symbol m) is the S.I.OCR
S.I. base units: The seven units including the metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol) and candela (cd).OCR
Safety: Considered for both patient and operator, including dose, exposure time, need for contrast agents and any non-ionising alternatives.AQA
Safety applications of impulse: Increasing the contact time for a given momentum change reduces the force, which is why crumple zones and airbags reduce injury.OCR
Safety procedures: Precautions including minimising exposure time, maximising distance and using shielding.OCR
Sagittarius A*: The supermassive black hole at the centre of the Milky Way, with a mass of about 4 million solar masses.AQA
Sampling: Measures the analogue voltage at regular time intervals, with the sampling rate being the number of samples taken per second, measured in hertz.AQA
Sampling rate: The number of samples taken per second, measured in hertz; increasing it reduces the time between samples and allows faster variations to be represented.AQA
Saturation: The output voltage is limited by the positive and negative supply rails, so the output clips rather than following the input further.AQA
Scalar: Electric potential is a scalar that is positive near a positive charge and negative near a negative charge.AQA
Scalar addition of potential energy contributions: Energy is a scalar, so contributions from several charges add algebraically.OCR
Scalar nature of potential: Gravitational potential is a scalar quantity, measured in joules per kilogram (J kg⁻¹).AQA, OCR
Scalar nature of potential and algebraic addition: Potential is a scalar, so potentials from several charges add algebraically, keeping signs.OCR
Scalar nature of work: Work is a scalar quantity measured in joules (J), where 1 J = 1 N m.OCR
Scalar quantity: A quantity that has magnitude only, such as mass, time, temperature, speed, distance, energy and power.OCR
Scale and order of magnitude: Nuclear radii are of the order of 10⁻¹⁵ m to 10⁻¹⁴ m, commonly quoted in femtometres where 1 fm = 10⁻¹⁵ m.AQA
Scale and orders of magnitude: A typical atomic diameter is of the order of 10⁻¹⁰ m, while a typical nuclear diameter is of the order of 10⁻¹⁵ m to 10⁻¹⁴ m.OCR
Scale and structure of the nucleus: The constant-density conclusion shows that nuclear material has a constant density, independent of the size of the nucleus.AQA
Scale drawing: A method for non-perpendicular vectors: choose a scale, draw each vector tip-to-tail at the correct angle, then measure the resultant length and direction.AQA, OCR
Scale of the atom: Atomic radius is typically about 10⁻¹⁰ m, while nuclear radius is about 10⁻¹⁵ m to 10⁻¹⁴ m, so nuclear volume is smaller by a factor of about 10¹⁵.OCR
Scale selection: The scale should be chosen so that the data points occupy at least half of the axis length, making the graph easy to read and interpret.OCR
Schwarzschild radius: The radius r_s = 2GM/c² obtained by setting the escape velocity greater than c, also called the event horizon.AQA
Scientific authority: Newton’s authority and his particle or corpuscular theory dominated for over a century, which delayed acceptance of the wave theory.AQA
Scientific community: The collective body that debates and validates a change, so acceptance is social as well as empirical.AQA
Scientific method and evidence: Theories are accepted or rejected on the basis of available evidence as well as the standing of their proponents.AQA
Scientific models: The Rutherford model and the plum-pudding model, compared using the scattering evidence.AQA
Scintillation crystal: A large thallium-doped sodium iodide crystal in which an absorbed gamma photon produces a burst of visible photons proportional to the energy deposited.AQA
Scintillator: A layer that absorbs incident X-rays and re-emits visible light photons, acting as a converter between X-ray and optical energy.AQA, OCR
Scope of the specification: Questions on internal gate structure are outside the scope of this specification.AQA
Sealed source: Radioactive material that is contained so that it does not enter the patient's general circulation.AQA
Second law of thermodynamics: It forbids a device that converts a single thermal reservoir entirely into work with no sink, so efficiency is always below 1.AQA
Secondary maxima: Features on each side of the central maximum that are narrower and fainter, with intensity decreasing as the order increases so the pattern fades away from…AQA
Security and interception: The risk of a medium being tapped or intercepted, a required consideration when comparing media.AQA
Selection of appropriate equation: Each form is useful depending on which quantities are known, and choosing the right form avoids unnecessary intermediate calculations.OCR
Selection of the appropriate equation: Choose the equation that matches the data and the unknown, since each equation omits one quantity.OCR
Semiconductor charge-carrier model: The explanation that heating releases many more charge carriers, so the number density n increases and resistance falls.OCR
Semiconductor diode: Conducts easily in forward bias above about 0.6 V, but has very high resistance in reverse bias, giving an asymmetric curve.AQA
Sensing and switching applications: Circuits in which a potential divider with a variable component is used, for example a light- or temperature-sensitive output p.d.OCR
Sensor circuits: A thermistor acts as a temperature sensor in a potential divider.AQA
Sensor resistance and physical conditions: Warm or cool the thermistor, or change the light intensity on the LDR, and record how the sensor resistance and the output voltage change.OCR
Sensor transduction: A sensor converts a non-electrical physical quantity, such as temperature, light intensity, strain or pressure, into an electrical signal.AQA
Sensors in potential divider circuits: A thermistor or light-dependent resistor can form part of the circuit so that the output p.d.AQA
Sequential circuits: Systems such as latches and flip-flops that are analysed by treating each gate as a block with defined behaviour.AQA
Sequential logic: Circuit advanced by the clock pulse that the astable supplies.AQA
Sequential logic circuits: Circuits that produce outputs depending on present inputs and stored past states, so reading outputs means tracking how each stage changes over time.AQA
Series and parallel capacitor rules: The series rule 1/C_total = 1/C₁ + 1/C₂ gives a smaller total capacitance, while the parallel rule C_total = C₁ + C₂ gives a larger total.OCR
Series and parallel combinations: For a circuit with a series part and a parallel part, first simplify the parallel section to a single equivalent resistance, then add series resistances.OCR
Series and parallel connections: The ways components are joined in a circuit, which you must identify when interpreting a diagram or choosing the diagram that correctly represents a described…OCR
Series and parallel meter connection: An ammeter is connected in series, while a voltmeter is connected in parallel.OCR
Series and parallel p.d.: In a series circuit the p.d.OCR
Series and parallel topology: Reproducing the series and parallel connections shown in the diagram rather than an approximate layout, connecting series paths first and then adding parallel…OCR
Series circuit potential differences: In a series circuit the potential differences across the components sum to the supply electromotive force.OCR
Series circuits and p.d. sharing: A potential divider consists of two or more components in series across a supply, so the supply p.d.OCR
Series combination of capacitors: A connection in which the same charge Q is stored on each capacitor and the potential differences add to the supply p.d.OCR
Series resistor design: A series resistor is connected between the input supply and the Zener to limit current and drop the excess voltage.AQA
Set square: A tool that helps align a ruler vertically or check right angles, reducing parallax and alignment errors.OCR
Shared wave behaviours and differing photon energies: All regions can be reflected, refracted, diffracted, interfere and be polarised, while regions differ in photon energy E = hf and in how they interact with…OCR
Sharpness of resonance: A measure of how narrow the peak is, often compared using the width of the curve at a chosen fraction of the peak amplitude.AQA
Sharpness of resonance in practical contexts: Damping broadens each resonance, so the stationary-wave response is less sharply tuned to a single driving frequency.AQA
Shielding: Thick concrete, water or lead that absorbs gamma radiation and neutrons, reducing dose to workers and the public.AQA
Short GRBs: Bursts associated with merging compact objects rather than supergiant collapse.AQA
Short-range interactions: The force is short-range, becoming negligible beyond about 3 fm, so only neighbouring nucleons interact strongly.OCR
SI prefixes: Scale a unit by powers of ten, and their values must be known and used fluently.AQA
SI unit of charge: The coulomb, symbol C, is the SI unit of electric charge.OCR
SI units: Resistance in ohms and capacitance in farads are used so that τ is in seconds.AQA
SI units and conversions: All quantities should be converted to SI units first, for example converting a velocity given in km h⁻¹ to m s⁻¹ by dividing by 3.6 before using P = Fv.OCR
SI units and the newton: All quantities are checked to be in SI units, converting grams to kilograms and so on, and the newton is defined so that 1 N = 1 kg m s⁻².OCR
SI units and unit consistency: Speed v is in metres per second (m s⁻¹), frequency f in hertz (Hz), and wavelength λ in metres (m).OCR
SI units and unit conversion for volume: Density is in kg m⁻³, mass in kg and volume in m³, converting cm³ by multiplying by 10⁻⁶.OCR
Sign and direction: Area below the time axis counts as negative displacement for velocity–time graphs, and speed–time graphs give distance because direction is lost.OCR
Sign and direction in energy calculations: If θ = 180°, cos 180° = −1, so W = −Fx, meaning energy is transferred away from the object.OCR
Sign and direction in motion graphs: The sign of the gradient of a displacement–time graph is included in the velocity, and a velocity–time graph that crosses the time axis shows a change of…AQA
Sign and direction of potential change: The sign of the potential difference indicates the direction of potential change between the two points.AQA
Sign convention for like and unlike charges: Like charges give positive potential energy that decreases as r increases, while unlike charges give negative potential energy that becomes more negative as r…OCR
Sign convention for positive and negative charges: A positive Q gives positive V; a negative Q gives negative V.OCR
Sign conventions and the zero of potential at infinity: Gravitational potential is negative because the zero is defined at infinity, and V is always negative and rises towards zero as 1/r.AQA
Sign conventions and units: Define a positive direction consistently, assign the sign of g according to the chosen positive direction, and always check units and the sign of each vector.OCR
Sign conventions for expansion and compression: For expansion the gas does work on the surroundings and the area is positive; for compression work is done on the gas and the area is negative relative to…AQA
Sign conventions in loop analysis: A loop direction is chosen and electromotive forces and potential differences are treated consistently, so their algebraic sum around the loop is zero.OCR
Sign conventions in one dimension: Area below the time axis represents negative impulse, reducing momentum in the chosen positive direction, so those areas are subtracted.OCR
Sign conventions in projectile calculations: Consistent signs are used for upward and downward quantities, taking care with the sign of g.AQA
Signal detection and computer image reconstruction: Excited protons emit RF signals as they de-excite, these signals are detected by a receiver coil, and a computer processes them to reconstruct a visual image.AQA
Signal processing and noise reduction: The machine must reject common-mode noise and have a high input impedance, with a time base and display producing a voltage–time trace.AQA
Simple (coherent) scatter: A photon is redirected with negligible energy change, contributing little to attenuation.OCR
Simple harmonic motion: Oscillatory motion where the acceleration is directly proportional to displacement from equilibrium and always directed towards that equilibrium point,…AQA, OCR
Simple harmonic motion analogy: The inductor behaves like the inertial mass and the capacitor like the spring, with current the analogue of velocity and charge the analogue of displacement.AQA
Simultaneous equations in circuit analysis: Assign a current direction to each branch, write loop equations using the e.m.f.s and the p.d.s across resistors, and solve the simultaneous equations.OCR
sin C = 1/n: At the critical angle, n sin C = 1 × sin 90° = 1, giving sin C = 1/n where n is the refractive index of the denser medium relative to the less dense medium.OCR
Sine form: The solution x = A sin(ωt), which starts at equilibrium moving in the positive direction.OCR
Single-slit diffraction: Light spreads after passing through one narrow slit, forming a central maximum with weaker side maxima.AQA
Sink: The cold reservoir that receives rejected energy Q_C at lower temperature T_C.AQA
Sinusoidal description of oscillations: A description of an oscillation such as x = A cos(ωt) or x = A sin(ωt), to which angular frequency links the time description.OCR
Sinusoidal waveforms: The relationships V_rms = V₀/√2 and I_rms = I₀/√2 apply only to these; other waveforms need their own rms calculation.AQA
Slew rate: A limit on how quickly the output voltage can change, causing distortion of fast or large signals.AQA
Slit separation: The centre-to-centre distance s between the two slits, used in w = λD / s and not to be confused with the width of one slit.AQA
Slit spacing: The spacing d between adjacent slits, which with the diffraction angle θ and wavelength λ gives the grating equation.AQA
Slit width and diffraction spreading: For a given wavelength, increasing the slit width makes the central maximum narrower, so a wider slit gives a tighter pattern.AQA
Small-angle approximation: The condition that is part of the model, because at large amplitudes a real pendulum departs from simple harmonic motion and its period begins to increase.AQA, OCR
Smoke particles as visible evidence for molecular motion: The smoke particles are much larger than the air molecules, so their motion provides evidence for the random motion of the invisible molecules rather than…OCR
Smoothing: Absorbing energy when the driving torque exceeds the load and releasing it when the load exceeds the drive, so output speed varies less.AQA
Smoothing of motion: A flywheel reduces speed fluctuations during each cycle, keeping the crankshaft turning between power strokes in a vehicle.AQA
Software to process data: Software such as a spreadsheet used to process and analyse data, including calculating derived quantities, plotting graphs and identifying trends.OCR
Solar system: A star together with the planets, satellites, comets and other bodies gravitationally bound to it.OCR
Solid: Particles closely spaced and arranged in a regular, ordered lattice, vibrating about fixed mean positions rather than moving from place to place.OCR
Sound intensity level: A quantity in dB given by 10 log₁₀(I / I₀), where I₀ is 1 × 10⁻¹² W m⁻².AQA
Sound stationary waves: A loudspeaker facing a hard surface or a resonance tube creates them, and nodes and antinodes in air can be located using a microphone or by resonance.AQA
Source: The terminal from which charge carriers enter the channel; in an n-channel device electrons flow from source to drain.AQA
Spacing: How far apart the atoms or molecules are, which is close together in solids and liquids but far apart in gases.OCR
Spatial encoding using gradient fields: Gradient field coils superimpose small position-dependent fields so that the precession frequency differs between small regions of the patient.AQA
Spatial resolution: The smallest separation between two points that the eye can distinguish as separate.AQA
Specific charge: Charge divided by mass, with units C kg⁻¹.AQA
Specific charge calculations: Calculations of the specific charge of ions and nuclei use mass values strictly in kilograms, avoiding the atomic mass unit until Year 2.AQA
Specific heat capacity: The energy needed per unit mass per unit temperature change, linking energy transfer to temperature change for a fixed mass with no phase change.AQA, OCR
Specific latent heat: Used in ΔE = mL when a substance changes state at constant temperature, where L is specific latent heat.AQA
Specific latent heat of fusion: The energy needed to change unit mass of a substance from solid to liquid at its melting point without a change in temperature.OCR
Specific latent heat of vaporisation: The energy needed to change unit mass from liquid to vapour at its boiling point without a change in temperature.OCR
Specimen preparation and vacuum requirements: The specimen must be extremely thin so electrons can transmit through it, and the whole column is evacuated so electrons are not scattered by air.AQA
spectral class: A labelling of the horizontal axis as OBAFGKM, where O is hottest on the left and M is coolest on the right, matching the temperature axis.AQA
Spectral fingerprint: The characteristic pattern of spectral lines produced by the unique set of photon energies an element can emit or absorb.OCR
Spectral response: Chosen so it matches the wavelength of the optical source, for example a silicon photodiode for near-infrared or visible light.AQA
Spectroscopic evidence: The pattern of lines, such as the Balmer series in hydrogen, can be used to deduce the energy level spacings.AQA
Spectroscopy and spectral analysis: Using a grating to separate light into a spectrum so that its component wavelengths can be analysed.AQA
Spectroscopy as a remote sensing tool: Recording the spectrum of a star and matching the wavelengths of the lines present against laboratory spectra of known elements, revealing composition without…OCR
Spectrum: The component wavelengths of light separated by a grating, which can then be analysed.AQA
Spectrum ordering by wavelength and frequency: The spectrum order from longest to shortest wavelength is radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.OCR
Spherical aberration: Arises when a spherical mirror brings marginal rays to a different focus from paraxial rays.AQA
Spherical component: The power that corrects the refractive error in one principal meridian, given in DS in standard notation.AQA
Spherical symmetry and equivalent point charge: For a uniformly charged sphere, charge may be considered to be at the centre, so it can be treated as a point charge for external points.AQA
Spontaneity of radioactive decay: The decay is not triggered by external factors such as temperature, pressure or chemical state, and cannot be predicted or controlled.OCR
Spread: The scatter of the repeats, used to judge whether a value is genuinely anomalous.OCR
Spreadsheet simulations: Applying the iterative calculation row by row for successive time intervals to generate a decay curve.OCR
Spring constant: The constant of proportionality k, defined by F = k∆L, where ∆L is the extension; a stiffer spring has a larger k.AQA, OCR
standard candles: Type 1a supernovae have a standardised peak luminosity, so comparing observed peak brightness with known absolute magnitude allows distance estimation for…AQA
Standard circuit symbols: The standardised shorthand used to draw electrical circuits, which you must recognise and reproduce for named components.OCR
Standard form: Written as a × 10ⁿ, where 1 ≤ a < 10, keeping very large and very small values readable and reducing place-value errors.AQA
Standard SI units in atomic physics: Calculations involving specific charge or basic mass comparisons at this stage should rely on standard SI units, with masses handled in kilograms.AQA
Standard truth tables of logic gates: The standard truth table of each gate is used as its definition when analysing or building circuits.AQA
Stationary wave: A pattern formed when two waves of the same frequency travelling in opposite directions superpose, with nodes and antinodes in fixed positions.AQA
Stationary wave formation: A stationary wave forms when two progressive waves of the same frequency and similar amplitude travel in opposite directions through the same medium.OCR
Stationary wave on a string: Forms when two identical progressive waves travelling in opposite directions superpose, with each end of a string fixed at both ends being a node.AQA
Stationary waves: Formed by the superposition of two progressive waves of the same frequency travelling in opposite directions.OCR
Stationary waves and boundary conditions: In tubes, the resonant frequencies depend on whether the ends are open or closed, because the boundary conditions determine the allowed stationary-wave…AQA
Stationary waves on strings: A wave reflects at a fixed end and interferes with the incident wave, with nodes at fixed ends and antinodes at free positions, and the string can vibrate in…AQA
Statistical behaviour of large samples: The simulation shows decay is random for individual nuclei but predictable in the statistical behaviour of a large sample.OCR
Steady-state conditions: Conditions where a temperature difference is maintained by a continuous energy input, so a continuous net energy flow occurs.OCR
Stefan constant: The constant σ in Stefan's law, which cancels along with 4π when a ratio is formed for two stars.AQA, OCR
Stefan's law: A black body radiates power P = σAT⁴, where σ is the Stefan constant, A is surface area and T is absolute temperature in kelvin.AQA, OCR
Stellar composition and absorption spectra: Absorption lines form when cooler gas in the star's atmosphere absorbs specific wavelengths from the continuous spectrum beneath, so comparing line patterns…OCR
Stellar evolution: The path of a star similar to our Sun on the HR diagram from formation to white dwarf.AQA
Stellar parallax: The apparent shift in a nearby star's position against more distant background stars as the Earth moves from one side of its orbit to the other.OCR
Step-index optical fibre: A core of higher refractive index surrounded by cladding of lower refractive index, with an abrupt change of index at the core–cladding boundary.AQA
Step-up transformer: A transformer with more secondary turns, which raises voltage while lowering current.OCR
Stokes’ Law: The viscous force on a small sphere is F = 6πηrv, where η is the fluid viscosity, r is the sphere radius and v is the speed.AQA
Stopping distance: The total distance a vehicle travels from the moment the driver needs to stop until it is at rest, being the sum of thinking distance and braking distance.OCR
Stopping potential: The reverse potential difference that just prevents the fastest emitted electrons from reaching the collector, making the photocurrent zero.AQA
Straight conductor in a magnetic field: The BLv result follows because the conductor sweeps area at rate Lv, so the rate of change of flux linkage is B × Lv for a single conductor.AQA
Strain: The fractional change in length, ε = x/L, where x is the extension and L the original length; it has no unit.OCR
Strange particles: Particles that are produced through the strong interaction and decay through the weak interaction, for example kaons.AQA
Strange quark: Carries s = −1, while a strange antiquark carries s = +1.AQA
Strangeness: A quantum number, symbol s, invented to explain why certain particles such as kaons and hyperons appear only in pairs when produced.AQA
Strangeness and conservation laws: Strangeness is conserved in strong interactions, but in the weak interaction it is not conserved.AQA
Strangeness and the weak interaction: Strangeness is conserved in strong and electromagnetic interactions but may change in weak interactions.AQA
Strangeness of nucleons: Neither the proton nor the neutron contains a strange quark, so both have strangeness 0.OCR
Streamlining and shape: A streamlined shape reduces drag compared with a blunt shape because it affects how smoothly air flows around the object.OCR
Stress: The force acting per unit cross-sectional area, σ = F/A, measured in pascals (Pa), where 1 Pa = 1 N m⁻².OCR
Stress, strain and the Young modulus: The Young modulus is found from E = kL/A using the gradient, or from E = FL/(AΔL) from a single consistent set of readings.OCR
Strong interaction: The interaction through which strange particles are produced, where strangeness is conserved.AQA
Strong interactions: These act on quarks and cannot change a strange quark into a non-strange quark or create a strange quark without its antiquark.AQA
Strong nuclear force: Acts between nucleons and holds the nucleus together against electrostatic repulsion between protons, being much stronger than the electrostatic force at…AQA, OCR
Structure of the atom: A small, dense, positively charged nucleus containing protons and neutrons, surrounded by electrons, with nearly all the mass in the nucleus but nearly all…AQA
Sum-of-products: Each row giving output 1 contributes a product term, and the expression is the OR of those terms.AQA
Superconductivity: A property of certain materials which have zero resistivity at and below a critical temperature which depends on the material.AQA
Supermassive black holes: Objects with masses of millions to billions of solar masses that reside at the centres of most large galaxies, including the Milky Way.AQA
Supernova: An event showing a rapid increase in absolute magnitude, meaning the absolute magnitude becomes more negative over a short time.AQA, OCR
Superposition: The overlapping of waves from the two slits at the screen, which the fringe pattern is evidence for.AQA
Superposition of fields: Between two masses the pattern shows attraction, and the resultant field direction at a point can be interpreted.AQA
Superposition of fields from multiple charges: The resultant field at a point is the vector sum of the fields due to each source charge.OCR
Superposition of oppositely travelling waves: A stationary wave forms when two progressive waves of the same frequency and similar amplitude travel in opposite directions through the same medium and…OCR
Superposition of waves: Displacements of two waves add at every point.AQA
Surface potential barrier: Barrier by which electrons that are free from individual atoms remain bound within the metal.AQA
Surface temperature axis: The horizontal axis spanning roughly 50 000 K on the left down to about 2 500 K on the right, so the scale is reversed compared with a normal graph.AQA
Synchronisation: Transmitter and receiver must stay synchronised so that samples are not sent to the wrong destination.AQA
Synchronous orbit: An orbit whose orbital period equals the rotation period of the body it orbits, so the satellite returns to the same position in the sky after each rotation.AQA
Synchrotron and thermal radiation: The hot accretion disc radiates thermal emission across visible, UV and X-ray bands, while magnetic fields produce relativistic jets that emit radio waves via…AQA
Systematic checking before powering a circuit: Checking that the circuit is complete and correctly connected, and confirming the supply voltage suits the components, before switching on the supply.OCR
Systematic errors: Consistent shifts of all readings from the true value, caused by apparatus faults such as a zero error, miscalibration or a flawed technique like parallax.AQA, OCR

T

Tachometer: Measures the rotational speed of a shaft or wheel, typically in revolutions per minute (rpm).AQA
Tangent: Drawn at the required point, with two well-separated points read on it to calculate the gradient rather than using two data points on a curve.AQA, OCR
Tangential direction of velocity: The direction of v is always tangent to the circle.OCR
Tangential motion when force is removed: If the centripetal force is removed, the object moves in a straight line tangent to the circle.AQA
Technetium-99m: A metastable excited state of technetium-99 that decays by gamma emission with a half-life of about six hours and is used as a medical tracer.AQA
Technetium–99m: A tracer that emits gamma radiation, has a half-life of about 6 hours, and can be attached to many pharmaceuticals that target specific organs.OCR
Temperature: A measure of mean random kinetic energy, so a constant temperature confirms kinetic energy is unchanged.AQA
Temperature and average kinetic energy: The model links temperature to the average kinetic energy of the particles.OCR
Temperature and resistance: In a metal conductor, heating increases lattice ion vibration, so electrons collide more often and resistance rises; in a negative temperature coefficient…AQA
Temperature as a measure of internal energy: Internal energy depends only on the temperature and on the number of molecules, not on the type of gas, so doubling the thermodynamic temperature of a fixed…OCR
Temperature change: Denoted by ∆θ, it is the difference between final and initial temperature, in kelvin or degrees Celsius, not a single reading.AQA
Temperature dependence of conductivity: In semiconductors n rises steeply with temperature as more electrons are freed, so resistance falls, while in metals n is approximately constant with…OCR
Temperature dependence of resistance: A filament lamp's resistance rises as it heats with increasing current, whereas a thermistor's resistance falls as temperature rises.AQA, OCR
Temperature dependence of resistivity: In a metal, increasing temperature makes the lattice ions vibrate more, so conduction electrons collide more often and resistivity increases; in a…OCR
Temperature rise and energy transfer: A rise in temperature increases the average random kinetic energy of the particles, so the internal energy of the body increases.OCR
Temperature sensing: Using a thermistor whose resistance changes predictably with temperature so that it can act as a sensor in a circuit.AQA
Temporary versus continuous energy supply: A capacitor is a temporary store, not a source of continuous power, because it discharges as charge is removed, so its voltage falls.OCR
Tensile deformation: Occurs when forces stretch the material, producing an extension, which is an increase in length.OCR
Tensile strain: The fractional extension ε = ΔL/L, where ΔL is the extension and L the original length; it is dimensionless, though it may be quoted as a percentage.AQA, OCR
Tensile stress: The force F applied perpendicular to a specimen's cross-sectional area A, so σ = F/A, measured in pascals (Pa), where 1 Pa = 1 N m⁻².AQA, OCR
Tension: A pulling force transmitted through a string, rope or cable when it is stretched.OCR
Terminal potential difference: The energy transferred per unit charge by the source to the external circuit, which is less than the e.m.f.AQA, OCR
Terminal potential difference and lost volts: The terminal potential difference V is the p.d.OCR
Terminal speed: Reached when a falling object's drag force grows until it balances the object's weight, so the resultant force and the acceleration are zero.AQA
Terminal velocity: A constant maximum speed reached when drag equals weight and the resultant force is zero.OCR
Terminal velocity and force balance: Without the electric field, the droplet falls at terminal speed when the resultant force is zero, with weight balanced by viscous drag and upthrust ignored.AQA
Test bodies and field direction: The direction of the vector represents the direction of the force on a suitable test body, such as a positive test charge or a compass needle.AQA
The aether and the medium for light: The wave theory appeared to require a medium, the aether, whose properties were uncertain.AQA
The collaborative nature of modern science: Particle physics relies on large teams of scientists and engineers working together over long periods, with shared expertise, funding and international…AQA
The effect of damping on the resonant peak: Lighter damping gives a taller, narrower peak, while heavier damping gives a lower, broader one.OCR
The electronvolt as an energy unit: An energy unit where 1 eV = 1.60 × 10⁻¹⁹ J, and energy in eV must be converted to joules before calculating.OCR
The equation R = ρL/A: Relates resistance in ohms to length in metres and cross-sectional area in square metres, and rearranges to ρ = RA/L.OCR
The joule: One joule is the work done when a force of one newton moves an object one metre in the direction of the force, so 1 J = 1 N m.OCR
The minus sign in the induction equation: In E = −N ΔΦ/Δt it expresses Lenz's law and indicates the opposing direction of the induced e.m.f.OCR
The role of engineering in experimental physics: Engineers design and maintain accelerators, detectors, magnets, vacuum systems and data-handling equipment, contributing precision components and computing…AQA
The volt: The unit in which e.m.f.OCR
The volt as joules per coulomb: Because 1 V = 1 J C⁻¹, the product of volts and coulombs is joules.OCR
The wave equation linking speed, frequency and wavelength: Measured wave quantities are related using the wave equation, with frequency in hertz and wavelength in metres.OCR
Theory change in physics: The particle model was not displaced until decisive evidence for interference and diffraction was performed and understood.AQA
Thermal energy transferred: Denoted by Q and measured in joules, it is found from Q = mc∆θ when a substance changes temperature without changing state.AQA
Thermal equilibrium: The state reached when two or more objects in thermal contact have no net transfer of thermal energy between them, their temperatures being equal.OCR
Thermal management in X-ray tubes: The anode rotates so the electron beam strikes a moving area, preventing local overheating and allowing a higher tube current and continuous operation.AQA
Thermal neutrons and moderation: Fast neutrons must be slowed by a moderator, for example graphite or water, to become thermal neutrons with energies comparable to the thermal motion of…AQA
Thermal store: The store to which friction and air resistance transfer some energy, so real speeds and heights are lower than ideal calculations predict.AQA
Thermionic emission: The release of electrons from the surface of a filament when it is heated by a low-voltage supply.AQA, OCR
Thermionic emission and electron acceleration: Electrons are emitted from a heated cathode by thermionic emission and accelerated by a high potential difference towards the anode.OCR
thermistor: A component whose resistance falls as temperature rises, so in a divider its output p.d.AQA, OCR
Thermistors in sensing circuits: Because the change is large and reproducible, NTC thermistors are used as temperature sensors and in potential-divider circuits.OCR
Thermodynamic temperature: The temperature used in the gas laws, measured in kelvin, where T/K = θ/°C + 273.AQA
Thin lens equation: 1/f = 1/u + 1/v, used with v fixed near 2.5 cm to show how f must change as u changes.AQA
Thinking distance: The distance travelled during the driver's reaction time, before the brakes are applied; it equals speed multiplied by reaction time.OCR
Three-dimensional image reconstruction: The computer collects many coincidence events and reconstructs a three-dimensional image of tracer concentration.OCR
Threshold frequency: The minimum frequency of the radiation, depending on the metal, at or above which emission occurs; below it no emission occurs however intense the radiation is.AQA, OCR
Threshold of hearing: The lowest equal loudness curve, most sensitive around 2–5 kHz and rising sharply at low and high frequencies.AQA
Threshold voltage: The gate-source voltage above which electrons are attracted to the surface, forming an induced n-channel that connects source and drain.AQA, OCR
Threshold voltage of an LED: The voltage at which the LED just begins to emit light or conduct appreciably, so that the minimum energy supplied per electron is eV.OCR
Threshold wavelength: The corresponding wavelength, given by λ₀ = c/f₀ = hc/φ.OCR
Time and the second: Time is an S.I.OCR
Time constant: The product of resistance and capacitance, τ = RC, equal to the time for charge, potential difference or current to fall to 1/e of its initial value during…AQA, OCR
Time constant and component choice: A resistor and capacitor are chosen so that τ = RC is long enough to observe.OCR
Time constant τ = RC: The time for charge or current to fall to 1/e of its starting value, about 37%, found by multiplying resistance in ohms by capacitance in farads.OCR
Time dilation: A consequence of special relativity in which a moving clock runs slow relative to the observer, becoming significant only when v is an appreciable fraction of…AQA
Time slots: Short, repeating slices of time in which each signal is allocated the whole channel.AQA
Time-base: The control that sets the horizontal scale, so the horizontal width of one cycle in divisions multiplied by the time-base gives the period.OCR
Time, distance and shielding: Principles used for staff and patient safety, plus monitoring and controlled handling of the source.AQA
Timebase control: Sets the horizontal sweep speed in time per division, so it controls the time scale of the display.AQA
Timing circuits: The resistor and capacitor in the timing network set the frequency, so changing them changes the clock rate.AQA
Timing diagram: A drawing in which each clock pulse is labelled, the state of every flip-flop after each active edge is marked, and combinational gates are then evaluated.AQA
Timing over multiple oscillations: Measuring the total time for multiple oscillations and dividing by the number of oscillations to reduce the effect of reaction time.OCR
Tip-to-tail addition: A method for non-perpendicular vectors in which the vectors are drawn tip-to-tail and the cosine rule or a scale drawing is used.OCR
Tip-to-tail method: Placing the second vector starting from the tip of the first, preserving each vector's direction.OCR
Torque: Measures the turning effect of a force about an axis, given by T = Fr, where r is the perpendicular distance from the axis to the line of action.AQA
Torque and its unit: A pure turning effect whose unit is the newton metre (N m), the same unit as moment.OCR
Torque of a couple: The product of one of the forces and the perpendicular distance between the lines of action of the two forces, given by torque = Fd.OCR
Total energy output: Found using E = P t, with power in watts and time converted to seconds, giving roughly 1.2 × 10⁴⁴ J for the Sun over its lifetime.AQA
Total input energy: The total energy supplied to the system, including energy eventually dissipated as heat or sound.OCR
Total internal energy: For N atoms, U = (3/2)NkT, which also equals (3/2)nRT for n moles.AQA
Total internal reflection: Occurs when light travels towards a less optically dense medium and the angle of incidence exceeds C, so no light refracts out and all the light is reflected…AQA, OCR
Total internal reflection in semi-circular blocks: Directing the ray into the curved face along a radius so it hits the exact centre of the flat face, then increasing the angle of incidence beyond the critical…OCR
Total mass: Doubling the mass while keeping the distribution the same doubles the moment of inertia.AQA
Tracer: A substance taken up by an organ whose emitted gamma photons are detected outside the body, and which must be non-toxic and have a known biological pathway.AQA
Tracking mount: Used by both telescopes to follow a source across the sky.AQA
Transducer damping and coupling: A backing material damps vibrations so the pulse is short, and gel couples the transducer to skin to reduce reflection at the air boundary.AQA
Transducer stages: Convert between electrical signals and radiated waves or sound, as at the input and output of the system.AQA
Transformer action and turns ratio: Alternating current in the primary coil produces a changing magnetic flux in the core, which links the secondary coil and induces an alternating e.m.f.OCR
Transformer action in the national grid: Transformers step up voltage for transmission and step down voltage for safe domestic use.AQA
Transit depth: A quantity that relates approximately to the square of the ratio of planet radius to star radius, so a deeper transit indicates a larger planet relative to…AQA
Transit method: A detection technique in which a small, regular decrease in the star's apparent brightness is detected when the planet passes across the stellar disc.AQA
Transmission diffraction grating: Consists of many equally spaced parallel slits, through which monochromatic light passes to produce sharp maxima.OCR
Transmitting aerial: Emits an electromagnetic wave whose electric field oscillates along the aerial's direction.AQA
Transparent medium: The intervening space is empty, with no absorption or scattering, so that all emitted radiation reaches the observer.AQA
Transverse: Describes a wave in which the oscillations are perpendicular to the direction of energy transfer, as with electromagnetic waves and waves on a string.AQA
Transverse wave: A wave in which the oscillations are perpendicular to the direction of energy travel, for example a wave on a string or an electromagnetic wave.AQA, OCR
Transverse wave nature: Electromagnetic waves are transverse oscillations of electric and magnetic fields and do not require a medium.OCR
Triangle of forces: The representation of three forces in magnitude and direction by the sides of a closed triangle, taken in order.OCR
Trigonometry: Used to project a vector onto each axis, giving F cos θ along the axis and F sin θ perpendicular to it.AQA
Trigonometry in physical problems: Using sine rule, cosine rule or right-angled trigonometry on the closed triangle to determine an unknown force or angle.AQA
Triple point of water: A fixed point on the scale, defined as 273.16 K.OCR
Truth table: Lists every input combination and the resulting output, so it defines a circuit's behaviour completely.AQA
Truth tables: Tables that list the output of each basic gate for all input combinations, and can be constructed and completed for circuits containing several gates.AQA
Turning effect of forces: The moment of a force about a point is the turning effect of that force, calculated as the product of the force and the perpendicular distance from the point…AQA
Turning effect without translation: A couple tends to rotate a body without translating it because the vector sum of the equal and opposite forces is zero.AQA
Two-source interference: Coherent light passes through two narrow slits separated by a small distance a and falls on a screen a distance D away, producing an interference pattern.OCR
Types of field: Electric fields act on charge, magnetic fields on moving charges or magnetic poles, and nuclear fields on nucleons.OCR

U

Ultimate tensile strength: The maximum stress a material can withstand before it breaks when being stretched.OCR
Ultimate tensile stress: The maximum stress the material can withstand before necking, distinct from the stress at which it fractures.AQA
Ultrasound: A longitudinal wave with a frequency greater than 20 kHz, the upper limit of normal human hearing.OCR
Ultrasound transducer: A device that emits and receives ultrasound, containing a piezoelectric crystal such as quartz or lead zirconate titanate.OCR
Ultraviolet catastrophe: The classical prediction that intensity rises without limit as wavelength decreases, implying infinite energy emission in the ultraviolet region.AQA
Uncertainties and improvements: Repeat readings, use a wide range of values, allow thermal equilibrium, avoid parallax when reading volume, and consider the dead volume in tubing or syringe.AQA
Uncertainty reduction and repeat readings: The mean diameter from readings at several places and orientations gives the cross-sectional area, and the wire is tapped to remove kinks.OCR
Uneven collisions and resultant random forces: Air molecules move rapidly and randomly and collide unevenly with the much larger smoke particles, so the resultant force on a smoke particle changes randomly…OCR
Uniform acceleration: Shown by a straight line on a velocity–time graph, and by a parabolic curve on a displacement–time graph.AQA
Uniform acceleration in a gravitational field: The only force after launch is weight, producing a vertical acceleration of constant magnitude g, while the horizontal acceleration is zero.AQA
Uniform and non-uniform acceleration: A straight sloping velocity–time line means uniform acceleration, a horizontal line means zero acceleration, and a curve means non-uniform acceleration.AQA
Uniform and radial fields: Between oppositely charged parallel plates the lines are straight, parallel and equally spaced, representing a uniform field, while near an isolated point…AQA
Uniform electric field: A field in which the electric force on a charged particle is constant, so the acceleration is constant.OCR
Uniform electric fields: The field between parallel plates, related to the potential difference V and plate separation d by E = V/d.AQA
Uniform field: Between parallel plates, given by E = V/d, where V is the potential difference across the plates and d is their separation.AQA
Uniform field inside a solenoid: A long solenoid produces a strong, nearly uniform field inside along its axis.OCR
Uniform gravitational field: A field in which g is constant over the height moved, such as near the Earth's surface.OCR
Uniform gravitational field near the Earth's surface: Close to the Earth's surface, over distances small compared with the Earth's radius, the gravitational field strength is effectively uniform: its magnitude…OCR
Uniform solid: A solid that has the same density throughout its volume.AQA
Uniform sphere interior fields and potentials: Inside a uniform sphere the magnitude of g is proportional to r and reaches zero at the centre, while V is finite and becomes more negative towards the centre.AQA
Unit analysis: A check on the meaning of a gradient or area, since the gradient's unit is the vertical unit divided by the horizontal unit and the area's unit is the product…AQA
Unit consistency in the Wien constant: The constant 2.9 × 10⁻³ has units of metre kelvin, so λmax must be in metres and T in kelvin.OCR
Unit consistency: Wb s⁻¹ equals V: The unit of ΔΦ/Δt is Wb s⁻¹, which is equivalent to the volt.OCR
Unit conversion: To convert a length from kilometres to metres, multiply by 1000, or equivalently multiply by 10³; to convert metres to kilometres, divide by 1000.AQA, OCR
Unit conversion and prefixes: Prefixed values are converted to base units before substituting, such as 25 cm to 0.25 m or 2.5 kJ to 2500 J.AQA
Unit conversion between astronomical and SI units: Because 1 Mpc = 3.09 × 10¹⁹ km, dividing km s⁻¹ Mpc⁻¹ by 3.09 × 10¹⁹ km cancels the km and gives s⁻¹.OCR
Unit conversion between atomic-scale and SI units: Multiplying by 1.60 × 10⁻¹⁹ changes eV to J and dividing changes J to eV, so the result can be expressed in eV or J as required.AQA
Unit conversion between parsecs, light-years and metres: Multiply parsecs by 3.26 to get light-years, or multiply light-years by 0.307 to get parsecs; one parsec equals about 3.09 × 10¹⁶ m.OCR
Unit conversion in calculations: Convert all quantities to base units before substituting, then present the answer with a sensible prefix.AQA
Unit conversion in cosmology: Multiplying the number of AU or ly by the appropriate metre value to convert the distance to metres.OCR
Unit conversion in spectroscopy calculations: Convert nanometres to metres before substituting, multiplying by 10⁻⁹.OCR
Unit conversion of time: Convert minutes to seconds by multiplying by 60 before substituting, since seconds give joules while hours give watt-hours.OCR
Unit conversions: Charge is often expressed in coulombs, and units such as millicoulombs (1 mC = 1 × 10⁻³ C) or microcoulombs (1 μC = 1 × 10⁻⁶ C) must be converted when…OCR
Unit of flux linkage: The weber-turn (Wb-turn), though often quoted in webers (Wb) because N is dimensionless.AQA
Unit of resistivity: The ohm metre (Ω m), obtained from Ω × m² / m.OCR
Unit prefixes and conversions: Capacitance values may be given in microfarads (1 μF = 1 × 10⁻⁶ F), nanofarads (1 nF = 1 × 10⁻⁹ F) or picofarads (1 pF = 1 × 10⁻¹² F).OCR
Units and conversions: Mass must be in kilograms and Δθ is the temperature change in kelvin or degrees Celsius, where a temperature difference in kelvin equals the same difference…OCR
Units and dimensions: The SI unit is N kg⁻¹, equivalent to m s⁻².OCR
Units and unit conversion in cosmology: H₀ must be converted from km s⁻¹ Mpc⁻¹ into SI of s⁻¹ using 1 Mpc = 3.09 × 10²² m before inverting.OCR
Units of field strength: The unit is newtons per coulomb, N C⁻¹, which is equivalent to volts per metre, V m⁻¹.OCR
Units of impulse: The SI unit of impulse is N s, equivalent to kg m s⁻¹.OCR
Units of the time constant: Because 1 F × 1 Ω = 1 s, the product CR has units of seconds.OCR
Universal attraction between all matter: Gravity is an attractive force that acts between all matter, anywhere in the universe, from atoms to galaxies.AQA
Universal laws: The same physical principles apply throughout space and time, allowing astronomers to interpret distant spectra and light curves using laboratory physics.OCR
Unpolarised transverse wave: A wave whose oscillation direction is randomly distributed around the line of travel, so every perpendicular plane contains oscillations.AQA
Unstable nucleus: A nucleus with an imbalance of protons and neutrons, or too much energy, that decays randomly to become more stable.AQA
Up-link: The link from Earth to satellite, which uses a different carrier frequency from the down-link.AQA
Up/down input: Selects the direction of counting: at one logic level the counter increments, and at the other level it decrements.AQA
Uplink and downlink: A signal is sent from an Earth station to a satellite, amplified and retransmitted on a different frequency to avoid interference.AQA
upper estimate: A value that overestimates the radius because the point-charge and strong-force approximations are ignored and the alpha never touches the nuclear surface.AQA
Upper limit for the nuclear radius: The calculated distance of closest approach, which is an upper limit because the alpha particle does not penetrate the nucleus.AQA
Upthrust: A force acting upwards through the fluid, which together with drag balances weight at terminal velocity.AQA, OCR
Use of a metal grille with microwaves: A metal grille placed between transmitter and receiver transmits or blocks the wave depending on whether its bars are parallel or perpendicular to the…OCR
Use of dividers to obtain smaller p.d.s: Potential dividers are used to obtain a smaller p.d.OCR
Use of familiar physical benchmarks: Recalling or constructing a familiar reference value, such as adult mass about 70 kg or air density about 1 kg m⁻³, and scaling it to the quantity being…AQA
Use of polarising filters with light: Unpolarised light is passed through a polarising filter, then through a second filter, and rotating the second filter changes the transmitted intensity.OCR
Use of Q = CV to transform expressions: Substituting Q = CV into W = ½QV gives W = ½CV², and substituting V = Q/C gives W = ½Q²/C.OCR
Use of standard results: Standard expressions are not derived; where a question needs one, it will be supplied, and the correct expression must be selected.AQA
Use of the elementary charge: Treating 1.60 × 10⁻¹⁹ as the conversion factor between the two energy units rather than as a power of ten adjustment.AQA
Useful and wasted transfers: Useful energy moves a vehicle, while unwanted transfers include heating due to friction, air resistance and braking.AQA
Useful output energy: The energy transferred to the intended form, such as kinetic energy or gravitational potential energy.OCR
Uses of geostationary satellites: Telecommunications, television broadcasting and weather monitoring, because their fixed position allows a ground antenna to point permanently at the satellite.OCR
Using and rearranging divider equations: Applying Vout = [R₂/(R₁ + R₂)]Vin to find the output voltage across the lower resistor R₂, and V₁/V₂ = R₁/R₂ to compare the potential differences across two…OCR
Using ratios to compare wave intensities: The ratio of two intensities equals the square of the ratio of their amplitudes, I₁/I₂ = (A₁/A₂)².OCR

V

Vacuum speed of electromagnetic waves: All electromagnetic waves travel at the same speed in a vacuum, c = 3.00 × 10⁸ m s⁻¹, regardless of frequency or wavelength.AQA
Validation of new knowledge: A collective process in which evidence must be shared, tested and judged, not the decision of one scientist.AQA
Variable control of p.d.: Use a potential divider or variable resistor to control the p.d.OCR
variable resistor: A component that changes its resistance by hand, so the output p.d.AQA
Variable resistors and potentiometers: Replacing a fixed resistor with one of these allows the output p.d.AQA
Variables and control: In calculations you may be told which factors are held constant so that you can isolate the effect of one variable.OCR
Vector: A physical quantity that has both magnitude and direction, such as displacement, velocity, acceleration, force, momentum or electric field strength.AQA
Vector addition: Done by calculation or scale drawing, accounting for direction; for perpendicular vectors, resolve into components and use Pythagoras.AQA
Vector addition and resolution: Used to test equilibrium by resolving forces in two perpendicular directions and checking each component sum is zero, or by drawing a closed vector triangle.AQA
Vector addition at any angle: Drawing each vector tip-to-tail at its correct angle using a protractor, with arrows showing direction, when vectors act at angles other than 90°.AQA
Vector addition of displacements: Displacement is a vector, so the direction of each wave's displacement must be taken into account when combining them.OCR
Vector addition of perpendicular components: The resultant velocity at any instant is the vector sum of the horizontal and vertical components, with direction found from the tangent of the angle between…OCR
Vector direction: Acceleration is in the same direction as the resultant force, and a negative acceleration is deceleration or acceleration opposite to the chosen positive…AQA
Vector directions and the left-hand rule: The force is always perpendicular to both the velocity and the field, and the direction follows Fleming's left-hand rule for positive charges; for negative…AQA
Vector field direction: E is a vector directed radially away from positive charge and towards negative charge.OCR
Vector fields: Field lines never cross, because the field has a single direction at each point, and their density is proportional to field strength.AQA
Vector nature of electrostatic force: Coulomb’s law gives the magnitude, and the direction must be determined from the signs of the charges, the force acting along the line joining them.OCR
Vector nature of field strength: Field strength is a vector, so direction matters: the force on a positive charge is in the direction of E, while the force on a negative charge is opposite to…OCR
Vector nature of fields: Fields are vector quantities and obey superposition.OCR
vector nature of force: Force is a vector quantity, so its direction matters and must be given as well as its magnitude.OCR
Vector nature of force, velocity and field: The magnitude is given by F = Bqv sin θ, where θ is the angle between velocity and field, and the force is zero when the particle moves parallel to the field.AQA
Vector nature of g: It is a vector quantity, directed towards the centre of the mass creating the field.OCR
Vector nature of gravitational fields: The field is directed towards the mass, so the vector form carries a minus sign when r is measured outwards from the mass.OCR
Vector nature of momentum: Momentum is a vector, so a sign convention is used consistently and oppositely directed velocities have opposite signs.AQA
Vector nature of velocity and acceleration: Because velocity is a vector, its direction changes continuously, so the body accelerates even though its speed is constant.AQA, OCR
Vector quantity: A quantity that has both magnitude and direction, such as displacement, velocity, acceleration, force, momentum and weight.OCR
Vector representation of fields: A force field can be represented as a vector whose magnitude gives the strength of the field at that point and whose direction gives the direction of the…AQA
Vector resolution: Splitting one vector into two perpendicular components that together have the same effect, replacing the original vector rather than adding to it.OCR
Vector resolution and closed triangles: For three coplanar forces, resolve in two perpendicular directions and set the sum of components in each direction to zero, or draw the forces tip-to-tail to…AQA
Vector resolution of velocity: Splitting an initial velocity at an angle into perpendicular components, such as v cos θ horizontally and v sin θ vertically.OCR
Vector resolution of velocity and force: A method of drawing the velocity vector, then drawing drag antiparallel to it and lift at 90° to it, before resolving.AQA
Vector resolution with trigonometry: Resolving a vector into perpendicular components, so a force F at angle θ has components F cos θ and F sin θ.OCR
Vector subtraction as reversed addition: Adding the negative of the vector, a − b = a + (−b), where −b has the same magnitude as b but the opposite direction.OCR
Vector triangle construction: A diagram in which the first vector is drawn to scale in its direction and the second is drawn starting from the tip of the first, keeping its own direction.OCR
Vehicle dynamics: Factors affecting maximum speed include engine power, air resistance, frontal area, shape, rolling resistance and gradient.AQA
Vehicle safety engineering: Crumple zones, seat belts and airbags increase the time taken for the vehicle and occupants to change momentum, reducing the average force for the same…AQA
Velocity: The vector rate of change of displacement, so a car on a roundabout can have constant speed but changing velocity.AQA, OCR
Velocity selection: Only particles with speed v = E/B travel undeflected, while slower or faster particles are deflected one way or the other.OCR
Velocity-displacement relationship in simple harmonic motion: The speed at displacement x is given by v = ±ω√(A² - x²), where A is the amplitude and ω is the angular frequency.OCR
Verification: Checking that the finished circuit reproduces every row of the table and correcting any row that does not match.AQA
Verification of predicted total capacitance: Measured total capacitance is compared with the value predicted by the series or parallel rule, and readings verify the combination rules.OCR
Vernier calipers: An instrument used to measure internal, external and depth dimensions, typically to 0.1 mm or 0.05 mm.OCR
Vertical gain control: Sets the vertical scale in volts per division, so it controls the voltage scale of the display.AQA
Vertical position control: Shifts the trace up or down to set a convenient zero line, without changing the measured potential difference.AQA
Vibrating length: The length L between the fixed end and the vibration generator, which actually contains the stationary wave.AQA
Virtual earth: The condition of the inverting input when the non-inverting input is held at zero volts, so the inverting input sits at zero volts.AQA
Virtual exchange particles: Particles that exist only during the interaction and are not permanent constituents of the nucleus.AQA
Virtual particles: A particle emits one, which is absorbed by another, transferring momentum; such particles exist only during the interaction.AQA
Virtual photons: Not directly detectable and exist only for the brief time of the interaction, transmitting the electromagnetic force between charged particles.AQA
Viscosity: The quantity η in Stokes’ Law, whose accurate value is needed because the radius depends on it.AQA
Visual representation of fields: Electric field lines are a visual way to represent an electric field.OCR
Vitrification: A storage method in which high-level waste is turned into glass blocks, cooled in water ponds for decades to dissipate heat, and destined for deep geological…AQA
Voltage division proportional to resistance: In a series divider the current is the same through each resistor, so the potential difference across each resistor is proportional to its resistance.OCR
Voltage reference: The steady voltage across a Zener diode in reverse breakdown, against which other parts of a circuit compare their signals or use as a fixed level.AQA
Voltage regulation: The circuit acts as a constant voltage source because the Zener voltage changes only slightly as current through it varies.AQA
Voltage-controlled device: A device in which the gate voltage, not a gate current, controls the drain current, because the oxide insulates the gate.AQA
Voltmeter: Connected in parallel across the component being measured, because it must compare the energy at two points.OCR
Volts/division: The control that sets the vertical scale, so the peak-to-peak height in divisions multiplied by the volts/division gives the peak-to-peak voltage.OCR
Volume ratios: Because volume depends on the cube of the radius, the atom's volume is about 10¹⁵ times the nuclear volume.OCR

W

Wave equation: c = fλ relates speed, frequency and wavelength for electromagnetic waves in a vacuum.AQA
Wave equation linking c, f and λ: Because c = fλ, substituting f = c/λ gives E = hc/λ, useful when wavelength is quoted.OCR
Wave speed on a stretched string: Given by v = √(T/μ), where T is the tension and μ is the mass per unit length.AQA
Wave speed on a string: Given by v = √(T/μ), where T is the tension and μ is the mass per unit length.AQA
Wave speed, frequency and wavelength relationship: The wave equation links the speed of a wave to its frequency and wavelength: v = fλ.OCR
Wave-particle duality: Einstein's photons explained the photoelectric effect, which wave theory could not, and de Broglie then proposed matter waves, confirmed by electron…AQA
Wave-particle duality and the de Broglie wavelength: Accelerated electrons have a much shorter de Broglie wavelength than visible light, which gives a far better resolution than an optical microscope.AQA
Wave-particle relationship for electromagnetic radiation: Because for light in a vacuum c = fλ, substituting f = c/λ gives ΔE = hc/λ, which is useful when a wavelength is known.OCR
Wave–particle duality: Current physics accepts both wave and photon behaviour depending on the experiment, with interference and diffraction showing wave behaviour and emission and…AQA, OCR
Wave–particle duality of electromagnetic radiation: Electromagnetic radiation has both wave and particle properties; the photon model is needed for emission and absorption, while wave behaviour explains…AQA
Waveform interpretation and clinical relevance: The P wave is atrial depolarisation, the QRS complex is ventricular depolarisation and the T wave is ventricular repolarisation; comparing a patient's trace…AQA
Waveform interpretation and graphical representation: On an oscilloscope or waveform diagram, AM is recognised by a changing envelope with evenly spaced carrier cycles; FM by constant peak height with varying…AQA
Wavelength: For a transverse wave, the distance between successive crests; for a longitudinal wave, the distance between successive compressions.AQA, OCR
Wavelength dependence of diffraction: For a given slit width, increasing the wavelength makes the central maximum wider, so red light produces a broader central maximum than blue light.AQA
Wavelength determination: The wavelength is found using the relationship λ = 2 × (node separation), measuring across several node spacings and dividing to reduce uncertainty.OCR
Wavelength measurement: Measurements of node or antinode positions can be used to determine wavelength, with a node spacing of half a wavelength.AQA
Wavelength separation by a grating: Higher orders spread out more, so the grating separates wavelengths, which is why it is used in spectrometry.OCR
Weak and electromagnetic interactions: The correct exchange particle is chosen for each: a virtual photon for electromagnetic and W⁺ or W⁻ for weak charged-current interactions.AQA
Weak interaction: The interaction responsible for muon decay, in which a negative muon transforms into an electron plus a muon neutrino and an electron antineutrino.AQA
Weak interaction and beta decay: A free neutron is unstable and undergoes beta-minus decay, in which a down quark changes into an up quark and a W⁻ boson is emitted.AQA
Weak interaction and particle decay: The emitted electron and antineutrino are not quarks; they are leptons produced by the weak interaction.OCR
Weak interaction decays: Kaons decay by the weak interaction, in which strangeness is not conserved.AQA
Weak interactions: These do not conserve strangeness, which is why strange particles can decay singly.AQA
Weak nuclear force: A force to which all leptons are subject, and which is responsible for processes such as beta decay.OCR
Weber: The unit of flux and of flux linkage; flux linkage may also be quoted in weber-turns.OCR
Weber (Wb): The SI unit of flux, where 1 Wb = 1 T m².AQA
Weight resolves on an inclined plane: For a plane at angle θ to the horizontal, the weight W resolves into W sin θ down the slope and W cos θ perpendicular into the slope.AQA
Weighted addition: Changing an input resistor changes the weighting of that input, allowing weighted addition of the input voltages.AQA
White dwarf: The remaining core after the outer layers are ejected, supported by electron degeneracy pressure rather than fusion, which gradually cools and fades over…AQA, OCR
White dwarfs: Stars in the lower left region with hot surfaces but very low luminosity, which requires very small radii.AQA, OCR
White light: Light whose wavelengths each produce their own pattern, so the central maxima overlap to give a white centre while the secondary maxima spread into coloured…AQA
Whole-life environmental impact and recyclability: A designer compares options using data, considering whole-life energy use, recyclability and the effect on different groups of people.AQA
Wien's displacement law: λmaxT = 2.9 × 10⁻³ m K, where λmax is the wavelength of peak emission, linking a star’s peak-emission wavelength to its surface temperature.AQA, OCR
Wire parallel to the field: Experiences no force, and for other angles the component of length perpendicular to the field is used.AQA
Work as a transfer of energy: Work done on an object transfers energy to it, and work done by it transfers energy away.OCR
Work as energy transfer: The energy transferred when a force moves its point of application.OCR
Work as the area under a p–V graph: For a reversible process, the work done by a gas equals the area under the p–V path, because work is the integral of pressure with respect to volume.AQA
Work done: The energy transferred when a force moves its point of application through a displacement in the direction of the force.AQA, OCR
Work done and energy transfer: The work done in stretching or compressing the material equals the area under the graph, because work is the product of force and the displacement it produces.OCR
Work done as area under a force–distance graph: Work done equals the area under the force–distance graph between the two separations, found by integration or square-counting because the graph is curved.OCR
Work done by a constant torque: The product of a constant torque T and the angular displacement θ, given by W = Tθ.AQA
Work done by a force: Work is done when a force moves its point of application through a displacement.OCR
Work done by a force at an angle to the displacement: If the force acts at an angle θ to the displacement, only the component along the displacement does work, W = Fs cos θ.OCR
Work done by a gas: At constant pressure, work done by the gas is W = pΔV.AQA
Work done by a magnetic force: The force is always perpendicular to velocity, so it does no work and changes direction but not speed.AQA
Work done in electric fields: Given by W = QΔV, so it is zero when ΔV is zero, regardless of the magnitude of Q.AQA
Work done on charged particles: Given by W = QV, using the charge in coulombs and the potential difference in volts, and expressed in eV or J as required.AQA
Work done per unit mass: The energy transferred per unit mass in moving a small test mass between two points, given by the magnitude of the potential difference.AQA
Work function: The minimum energy required to remove an electron from the surface of a particular metal; it is a property of the metal and is often quoted in electronvolts.AQA, OCR

X

X-ray opaque contrast medium: A material used to enhance contrast in a chosen organ, as illustrated by the barium meal technique.AQA
X-ray production: Occurs when accelerated electrons are stopped at a metal target, the anode metal where X-rays are produced.OCR
X-ray production at the target: The target metal is set into the anode and is where the accelerated electrons are stopped, producing X-rays.OCR

Y

Young modulus: E = σ/ε, equivalently FL/(AΔL), in Pa; it applies only within the elastic limit, where stress is proportional to strain.AQA, OCR

Z

Zener breakdown voltage: The voltage, labelled V_Z, at the sharp knee on the reverse characteristic where reverse breakdown begins.AQA
Zener diode: A diode designed to operate safely in reverse breakdown, where the voltage across it stays nearly constant over a range of reverse currents.AQA
Zero errors: A systematic error where an instrument does not read zero when it should, such as a top-pan balance reading 0.02 g with nothing on it.OCR
Zero of electric potential: Defined at infinity, where the electric field is zero.AQA
Zero potential at infinity: At infinity, the potential is defined as zero.OCR
Zero resultant force: The condition meaning zero acceleration, not zero velocity, so the object moves at constant velocity.OCR
Zero total energy: At escape the total energy is zero, so ½mv² = GMm/r, the boundary between bound and unbound motion.AQA
Zero value at infinity: The zero value is chosen at infinity, where the field strength is zero.AQA
Zero-order maximum: The maximum at θ = 0°, where all wavelengths overlap.AQA
Zinc plate and ultraviolet radiation: When ultraviolet radiation is shone on the zinc plate, electrons are emitted from the surface, the negative charge is reduced and the leaf falls.OCR

Other terms

½mc² = 3/2 kT: Gives the mean translational kinetic energy of a molecule of mass m and mean-square speed c².OCR
λ t₁/₂ = ln 2: The equation relating the decay constant to half-life, where ln 2 is the natural logarithm of 2, approximately 0.693.OCR
Φ = BA cos θ: The expression used when B is at an angle θ to the normal, taking the component of B perpendicular to the area as B cos θ.AQA

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