Fundamental particles — OCR A-Level Physics
Test yourself on Fundamental particles with OCR A-Level practice questions.
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Fundamental particles explained
Every particle has a corresponding antiparticle.
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The electron's antiparticle is the positron; the proton's is the antiproton; the neutron's is the antineutron; and each neutrino has a corresponding antineutrino. In OCR A Level Physics A, you must recognise these pairs and their symbols, for example e⁻ and e⁺, p and p̄, n and n̄, ν and ν̄. When a particle meets its antiparticle they can annihilate, producing photons. In pair production, a photon can create a particle–antiparticle pair. For the MCQ, identify the correct antiparticle partner or the particle–antiparticle pair from the options, checking charge and baryon number conservation where relevant.
(b) particle and its corresponding antiparticle have same mass; electron and positron have opposite charge; proton and antiproton have opposite charge
A particle and its antiparticle have exactly the same mass but opposite values of charge and other quantum numbers such as baryon number and lepton number. For example, the electron and positron both have mass 9.11 × 10⁻³¹ kg, but the electron has charge −1.60 × 10⁻¹⁹ C while the positron has charge +1.60 × 10⁻¹⁹ C. Similarly, the proton and antiproton have the same mass (1.67 × 10⁻²⁷ kg) but opposite charge: proton +1.60 × 10⁻¹⁹ C, antiproton −1.60 × 10⁻¹⁹ C. In MCQs, you may be asked to compare masses or charges, or to identify which property is opposite and which is the same.
(c) classification of hadrons; proton and neutron as examples of hadrons; all hadrons are subject to both the strong nuclear force and the weak nuclear force
Hadrons are particles that experience the strong nuclear force. They are not fundamental; they are made of quarks. The proton and neutron are the most common examples of hadrons, specifically baryons (made of three quarks). All hadrons are subject to both the strong nuclear force and the weak nuclear force. In MCQs, you may be asked to classify a particle as a hadron or lepton, or to identify which forces act on hadrons. Remember that the strong force acts between quarks and between hadrons, while the weak force is responsible for beta decay and acts on quarks and leptons.
(d) classification of leptons; electron and neutrino as examples of leptons; all leptons are subject to the weak nuclear force but not the strong nuclear force
Leptons are fundamental particles that do not experience the strong nuclear force. The electron and the neutrino are examples of leptons. All leptons are subject to the weak nuclear force, which is responsible for processes such as beta decay. Leptons are not made of quarks. In MCQs, you may be asked to identify a lepton from a list, or to state which forces act on leptons. Remember that there are three flavours of charged lepton (electron, muon, tau) and corresponding neutrinos, but the specification only requires electron and neutrino as examples.
(e) simple quark model of hadrons in terms of up (u), down (d) and strange (s) quarks and their respective anti-quarks
Hadrons are particles that feel the strong interaction, and the simple quark model explains them as composites of quarks. Baryons such as the proton and neutron contain three quarks (qqq), while mesons such as the pion contain a quark and an anti-quark (qq̄). The model uses up (u), down (d) and strange (s) quarks, each with a corresponding anti-quark: anti-up (ū), anti-down (d̄) and anti-strange (s̄). Anti-quarks carry opposite charge, baryon number and strangeness to their quarks. For example, a proton is uud and a neutron is udd; a π⁺ meson is ud̄. Quark composition determines the hadron's charge, baryon number and strangeness, so you can deduce these properties from the quark content alone.
(f) quark model of the proton (uud) and the neutron (udd)
The proton and neutron are baryons, each built from three quarks in the simple quark model. The proton has composition uud: two up quarks and one down quark. The neutron has composition udd: one up quark and two down quarks. Using quark charges of u = +2/3 e and d = −1/3 e, the proton charge is (+2/3) + (+2/3) + (−1/3) = +1 e, and the neutron charge is (+2/3) + (−1/3) + (−1/3) = 0. Both have baryon number +1 because each quark has baryon number +1/3. Neither contains a strange quark, so both have strangeness 0. These compositions explain why the proton is charged and the neutron is neutral despite similar masses.
(g) charges of the up (u), down (d), strange (s), anti-up (ū), anti-down (d̄) and the anti-strange (s̄) quarks as fractions of the elementary charge e
Quark charges are given as fractions of the elementary charge e. The up quark (u) has charge +2/3 e; the down quark (d) has charge −1/3 e; the strange quark (s) has charge −1/3 e. Anti-quarks carry opposite charge: anti-up (ū) has charge −2/3 e; anti-down (d̄) has charge +1/3 e; anti-strange (s̄) has charge +1/3 e. These fractional charges combine to give integer charges for hadrons. For example, a proton uud has charge (+2/3) + (+2/3) + (−1/3) = +1 e, and a π⁻ meson dū has charge (−1/3) + (−2/3) = −1 e. Knowing these values lets you deduce the charge of any hadron from its quark composition.
(h) beta-minus (β⁻) decay; beta-plus (β⁺) decay
Beta decay is a weak interaction process in which a neutron or proton changes quark flavour. In beta-minus (β⁻) decay, a neutron converts to a proton: one down quark changes to an up quark, emitting an electron and an anti-neutrino. The quark change is d → u, so the nucleon changes from udd to uud. In beta-plus (β⁺) decay, a proton converts to a neutron: one up quark changes to a down quark, emitting a positron and a neutrino. The quark change is u → d, so the nucleon changes from uud to udd. In both cases charge, baryon number and lepton number are conserved. Beta decay explains how unstable nuclei adjust their neutron-to-proton ratio.
(i) β⁻ decay in terms of a quark model; d → u + ⁰₋₁e + ν̄
Beta-minus decay happens when a neutron-rich nucleus converts a neutron into a proton. In the quark model the neutron is udd and the proton is uud, so one down quark must change into an up quark. The transformation is d → u + ⁰₋₁e + ν̄, where ⁰₋₁e is the beta-minus particle (an electron) and ν̄ is the electron antineutrino. Charge must balance: the down quark has charge −1⁄3, while the up quark (+2⁄3), the electron (−1) and the antineutrino (0) sum to −1⁄3. Lepton number also balances because the electron has lepton number +1 and the antineutrino has −1. The emitted electron and antineutrino are not quarks; they are leptons produced by the weak interaction. A useful check is to write the quark content of the parent and daughter nucleon and confirm that only one quark flavour changes.
(j) β⁺ decay in terms of a quark model; u → d + ⁰₊₁e + ν
Beta-plus decay occurs in proton-rich nuclei when a proton converts into a neutron. In quark terms the proton is uud and the neutron is udd, so an up quark changes into a down quark. The transformation is u → d + ⁰₊₁e + ν, where ⁰₊₁e is the positron (the antiparticle of the electron) and ν is the electron neutrino. Charge balance is essential: the up quark has charge +2⁄3, while the down quark (−1⁄3), the positron (+1) and the neutrino (0) sum to +2⁄3. Lepton number balances because the positron has lepton number −1 and the neutrino has +1. The positron and neutrino are leptons emitted by the weak interaction, not quarks. A reliable method is to write the parent and daughter quark contents and confirm that exactly one quark flavour changes.
(k) balancing of quark transformation equations in terms of charge
Quark transformation equations must balance in charge. Each quark has a fractional charge: up-type quarks have charge +2⁄3 and down-type quarks have charge −1⁄3, while antiquarks have the opposite sign. Leptons such as the electron have charge −1, the positron +1, and neutrinos zero. To balance an equation, add the charges on the left and set them equal to the sum on the right. For example, in d → u + ⁰₋₁e + ν̄ the left side is −1⁄3 and the right side is +2⁄3 + (−1) + 0 = −1⁄3. A systematic method is to write the charge above each symbol, sum each side, and adjust the lepton charges until they match. This check also helps identify whether a proposed transformation is physically possible.
(l) decay of particles in terms of the quark model.
Particle decay can be described by showing how quark flavours change. In β⁻ decay a down quark becomes an up quark, so a neutron becomes a proton and an electron plus an electron antineutrino are emitted. In β⁺ decay an up quark becomes a down quark, so a proton becomes a neutron and a positron plus an electron neutrino are emitted. In each case the weak interaction changes quark flavour, and the emitted leptons conserve charge and lepton number. To describe a decay, write the quark content of the parent particle, show the flavour change, and list the products. For example, the neutron udd becomes the proton uud with the transformation d → u + ⁰₋₁e + ν̄. This approach explains why the nucleon's identity changes and why leptons appear even though they are not constituents of the nucleon.
Your focus
- Name the antiparticle for a given particle from the required list.
- Write or recognise the symbols for particles and antiparticles.
- Apply conservation of charge and baryon number to simple particle–antiparticle processes.
Show all 36 objectives
- State that particle and antiparticle have the same mass.
- State that electron and positron have opposite charge.
- State that proton and antiproton have opposite charge.
- Define a hadron as a particle subject to the strong nuclear force.
- Give proton and neutron as examples of hadrons.
- State that all hadrons are subject to both the strong and weak nuclear forces.
- Define a lepton as a fundamental particle not subject to the strong nuclear force.
- Give electron and neutrino as examples of leptons.
- State that all leptons are subject to the weak nuclear force but not the strong nuclear force.
- Describe hadrons as baryons or mesons in terms of quarks and anti-quarks.
- Identify the quark content of a given hadron using u, d, s and their anti-quarks.
- Deduce charge, baryon number and strangeness from quark composition.
- Recall the quark composition of the proton and neutron.
- Calculate the charge of a nucleon from its quark content.
- Explain why the neutron is neutral despite containing charged quarks.
- Recall the charges of u, d, s, ū, d̄ and s̄ as fractions of e.
- Calculate the total charge of a hadron from its quark composition.
- Apply the rule that anti-quarks have opposite charge to their quarks.
- Describe beta-minus and beta-plus decay in terms of quark changes.
- Identify the emitted particles in each type of beta decay.
- Apply conservation of charge, baryon number and lepton number to beta decay.
- Write the quark-level equation for β⁻ decay.
- Verify charge and lepton number conservation in a β⁻ decay equation.
- Explain how a neutron changes into a proton in terms of quark flavour change.
- Write the quark-level equation for β⁺ decay.
- Verify charge and lepton number conservation in a β⁺ decay equation.
- Explain how a proton changes into a neutron in terms of quark flavour change.
- Assign correct charges to quarks, antiquarks and leptons.
- Balance a quark transformation equation in terms of charge.
- Use charge conservation to identify an invalid transformation.
- Describe β⁻ and β⁺ decay using quark transformations.
- Write balanced quark-level decay equations.
- Explain how conservation laws constrain the products of particle decay.
Fundamental particles exam tips
Marking Points
- Electron and positron are a particle–antiparticle pair.
- Proton and antiproton are a particle–antiparticle pair.
- Neutron and antineutron are a particle–antiparticle pair.
- Neutrino and antineutrino are a particle–antiparticle pair.
- Annihilation occurs when a particle meets its antiparticle, often producing photons.
- Pair production is the creation of a particle–antiparticle pair from a photon.
- Particle and antiparticle have identical mass.
- Electron and positron have opposite charge.
- Proton and antiproton have opposite charge.
- Other quantum numbers (e.g. baryon number, lepton number) are also opposite.
- Charge is conserved in particle–antiparticle creation and annihilation.
- Hadrons are particles that interact via the strong nuclear force.
- Proton and neutron are examples of hadrons.
- All hadrons are subject to the strong nuclear force.
- All hadrons are subject to the weak nuclear force.
- Hadrons are composite particles made of quarks.
- Baryons (e.g. proton, neutron) are a sub-class of hadrons.
- Leptons are fundamental particles.
- Electron and neutrino are examples of leptons.
- All leptons are subject to the weak nuclear force.
- Leptons are not subject to the strong nuclear force.
- Leptons are not made of quarks.
- Lepton number is conserved in interactions.
- Hadrons are particles that interact via the strong force and are not fundamental.
- Baryons are made of three quarks (qqq); mesons are made of a quark and an anti-quark (qq̄).
- The simple model uses up (u), down (d) and strange (s) quarks.
- Each quark has a corresponding anti-quark: ū, d̄ and s̄.
- Anti-quarks have opposite charge, baryon number and strangeness to their quarks.
- Quark composition determines a hadron's charge, baryon number and strangeness.
- Proton quark composition is uud.
- Neutron quark composition is udd.
- Proton charge is +1 e from (+2/3) + (+2/3) + (−1/3).
- Neutron charge is 0 from (+2/3) + (−1/3) + (−1/3).
- Both proton and neutron have baryon number +1.
- Neither proton nor neutron contains a strange quark, so strangeness is 0.
- Up quark charge is +2/3 e.
- Down quark charge is −1/3 e.
- Strange quark charge is −1/3 e.
- Anti-up quark charge is −2/3 e.
- Anti-down quark charge is +1/3 e.
- Anti-strange quark charge is +1/3 e.
- Hadron charge is the sum of its quark charges.
- Beta-minus decay: a neutron changes into a proton.
- In β⁻ decay, a down quark changes to an up quark (d → u).
- β⁻ decay emits an electron and an anti-neutrino.
- Beta-plus decay: a proton changes into a neutron.
- In β⁺ decay, an up quark changes to a down quark (u → d).
- β⁺ decay emits a positron and a neutrino.
- Charge, baryon number and lepton number are conserved in both decays.
- Identifies that in β⁻ decay a down quark transforms into an up quark, changing a neutron into a proton.
- States the transformation d → u + ⁰₋₁e + ν̄ with the electron and electron antineutrino as products.
- Confirms charge conservation: −1⁄3 = +2⁄3 + (−1) + 0.
- Recognises that the emitted electron and antineutrino are leptons, not quarks, and that the weak interaction mediates the change.
- Distinguishes β⁻ decay from β⁺ decay by the direction of quark flavour change and the particle emitted.
- Identifies that in β⁺ decay an up quark transforms into a down quark, changing a proton into a neutron.
- States the transformation u → d + ⁰₊₁e + ν with the positron and electron neutrino as products.
- Confirms charge conservation: +2⁄3 = −1⁄3 + (+1) + 0.
- Recognises that the positron and neutrino are leptons, not quarks, and that the weak interaction mediates the change.
- Distinguishes β⁺ decay from β⁻ decay by the direction of quark flavour change and the emitted particles.
- States the fractional charges of up-type (+2⁄3) and down-type (−1⁄3) quarks and the opposite charges of their antiquarks.
- States the charges of the electron (−1), positron (+1) and neutrinos (0).
- Adds charges on each side of a quark transformation equation and shows that the totals are equal.
- Uses charge balance to test whether a proposed quark transformation is valid.
- Applies the method to both β⁻ and β⁺ transformations without confusing the sign of the emitted lepton.
- Describes a neutron as udd and a proton as uud, and identifies the quark that changes in β⁻ decay.
- Describes a proton as uud and a neutron as udd, and identifies the quark that changes in β⁺ decay.
- Writes the quark-level transformation for β⁻ decay as d → u + ⁰₋₁e + ν̄.
- Writes the quark-level transformation for β⁺ decay as u → d + ⁰₊₁e + ν.
- Explains that the weak interaction changes quark flavour and that the emitted leptons conserve charge and lepton number.
Examiner Tips
- 💡Learn the symbols and charges for each pair, including the bar over antiparticles.
- 💡In MCQs, check that charge and baryon number are conserved in any reaction or decay.
- 💡Remember that neutrino–antineutrino pairs are distinct even though both have zero charge.
- 💡Compare masses first: they are equal for particle and antiparticle.
- 💡Check the sign of charge: it is opposite for particle and antiparticle.
- 💡Use conservation of charge to check if a proposed reaction is possible.
- 💡Learn the definition: hadrons are particles that feel the strong nuclear force.
- 💡Remember proton and neutron as the key examples of hadrons.
- 💡In MCQs, if a particle is made of quarks, it is a hadron.
- 💡Learn the definition: leptons are fundamental particles that do not feel the strong force.
- 💡Remember electron and neutrino as the required examples.
- 💡In MCQs, if a particle is not made of quarks and does not feel the strong force, it is a lepton.
- 💡Learn the quark content of the proton (uud) and neutron (udd) as anchors for baryon questions.
- 💡When given a hadron's quark composition, work out charge, baryon number and strangeness systematically.
- 💡Check whether a particle is a baryon or meson before applying conservation rules.
- 💡Practise writing anti-quark symbols clearly to avoid losing marks.
- 💡Memorise uud for proton and udd for neutron as a pair.
- 💡Show the charge sum explicitly when asked to verify proton or neutron charge.
- 💡Remember baryon number is +1 for both nucleons.
- 💡Check for strange quarks before stating strangeness.
- 💡Learn the six charges as a table: u +2/3, d −1/3, s −1/3, ū −2/3, d̄ +1/3, s̄ +1/3.
- 💡When summing charges, convert to a common denominator to avoid arithmetic slips.
- 💡Check that the total charge of a hadron is an integer multiple of e.
- 💡Use the anti-quark rule: opposite sign to the corresponding quark.
- 💡Write the quark-level change first, then the emitted particles.
- 💡Check conservation of charge, baryon number and lepton number in each decay.
- 💡Remember β⁻ increases proton number by 1 and β⁺ decreases it by 1.
- 💡Use the mnemonic: beta-minus makes a proton, beta-plus makes a neutron.
- 💡Write the quark-level equation and then check charge on both sides before selecting an answer.
- 💡Check lepton number as well as charge; a correct-looking charge balance can still have the wrong neutrino type.
- 💡Remember that the nucleon changes because its quark content changes, not because a proton is added from outside.
- 💡Write the quark-level equation and check charge on both sides before choosing an option.
- 💡Check lepton number as well as charge; the neutrino type must match the emitted lepton.
- 💡Remember that β⁺ decay reduces the proton number by one and increases the neutron number by one.
- 💡Write the charge above every symbol before adding, so no term is missed.
- 💡Check both charge and lepton number; charge balance alone does not guarantee a valid equation.
- 💡If the totals do not match, look first at the sign of the emitted lepton and the quark flavour charge.
- 💡Write the quark content of the parent and daughter particles before writing the transformation.
- 💡Check charge and lepton number on both sides of the equation.
- 💡Use the direction of quark flavour change to decide whether the decay is β⁻ or β⁺.
Common Mistakes
- Thinking the positron is a proton: the positron has the same mass as an electron but positive charge, while the proton is much more massive.
- Believing the antineutron has negative charge: the antineutron is neutral, like the neutron, but has opposite baryon number.
- Assuming a neutrino and antineutrino are identical: they are distinct antiparticles with opposite lepton number.
- Confusing annihilation with pair production: annihilation converts mass into energy (photons), while pair production converts a photon into a particle–antiparticle pair.
- Thinking antiparticles have negative mass: mass is always positive and identical for particle and antiparticle.
- Assuming all antiparticles are negatively charged: the antiproton is negative, but the antineutron is neutral and the positron is positive.
- Believing the positron has the same charge as a proton: the positron has charge +1.60 × 10⁻¹⁹ C, same magnitude as the electron but opposite sign, while the proton also has +1.60 × 10⁻¹⁹ C but a much larger mass.
- Forgetting that lepton number and baryon number are opposite for antiparticles, not just charge.
- Thinking leptons are hadrons: leptons do not feel the strong force and are fundamental.
- Believing hadrons only feel the strong force: they also feel the weak force and, if charged, the electromagnetic force.
- Assuming all hadrons are baryons: mesons are also hadrons but consist of a quark–antiquark pair.
- Confusing the range of the strong force: it acts over about 10⁻¹⁵ m, within the nucleus.
- Thinking leptons feel the strong force: they do not, because they are not made of quarks.
- Believing neutrinos are hadrons: neutrinos are leptons and are fundamental.
- Assuming all leptons are charged: neutrinos are neutral leptons.
- Confusing lepton number with baryon number: they are separate conservation laws.
- Thinking mesons contain two quarks rather than a quark and an anti-quark; correct this by stating mesons are qq̄.
- Believing anti-quarks have the same charge as their quarks; correct this by noting anti-quarks have opposite charge.
- Assuming all hadrons are baryons; correct this by recalling mesons are also hadrons.
- Forgetting that strangeness is carried by the strange quark and its anti-quark; correct this by tracking s and s̄ in compositions.
- Swapping the compositions, giving the proton udd and neutron uud; correct this by remembering proton is uud.
- Adding quark charges incorrectly and getting a fractional charge for the proton; correct this by summing +2/3, +2/3 and −1/3 to get +1.
- Thinking the neutron has no quarks because it is neutral; correct this by noting it contains udd.
- Assigning strangeness to the proton or neutron; correct this by noting neither contains s or s̄.
- Giving the strange quark charge as +2/3 e; correct this by noting s has charge −1/3 e.
- Forgetting to reverse the sign for anti-quarks; correct this by taking the opposite of the quark charge.
- Treating quark charges as whole numbers; correct this by using fractions of e.
- Mixing up anti-down and anti-strange charges; correct this by noting both are +1/3 e.
- Saying beta-minus decay emits a positron; correct this by noting β⁻ emits an electron and an anti-neutrino.
- Confusing the quark change direction; correct this by remembering β⁻ is d → u and β⁺ is u → d.
- Forgetting the neutrino or anti-neutrino; correct this by including it to conserve lepton number.
- Thinking beta decay involves the strong interaction; correct this by identifying it as a weak interaction.
- Writing the antineutrino as a neutrino: the β⁻ product is the electron antineutrino ν̄, which balances lepton number against the electron.
- Treating the electron as a quark or as part of the nucleon: it is a lepton emitted from the nucleus, not a constituent of the neutron.
- Balancing charge incorrectly by ignoring the down quark's negative charge: the down quark charge is −1⁄3, not +1⁄3 or zero.
- Writing the neutrino as an antineutrino: the β⁺ product is the electron neutrino ν, which balances lepton number against the positron.
- Using an electron instead of a positron: β⁺ decay emits a positron, the antiparticle of the electron.
- Balancing charge incorrectly by treating the up quark as negative: the up quark charge is +2⁄3, not −2⁄3.
- Forgetting that quark charges are fractional: treating an up quark as +1 and a down quark as −1 gives incorrect totals.
- Ignoring the charge of the emitted lepton: the electron or positron contributes ±1 and must be included in the sum.
- Reversing the sign of the antineutrino or neutrino: neutrinos have zero charge, so their sign does not affect charge balance, but their particle type still matters for lepton number.
- Thinking that the whole neutron turns into a proton without any quark change: the change is a single quark flavour transformation.
- Believing that electrons or positrons are quarks: they are leptons produced in the decay and are not part of the nucleon.
- Mixing up the neutrino types: β⁻ emits an electron antineutrino and β⁺ emits an electron neutrino.