Current, resistance and potential difference — AQA GCSE Combined Science

Potential difference is the energy transferred per unit charge between two points in a circuit, measured in volts (V). One volt equals one joule per coulomb, so a 6 V battery transfers 6 J of energy to each coulomb of charge passing through it. In exam questions, the term used will be potential difference, but answers that correctly use voltage are equally acceptable. When solving problems, identify the two points across which the energy transfer occurs, such as across a resistor or a lamp, then apply V = IR or energy transferred = charge × potential difference. Always give the unit V and show substitution clearly.

The relationship between current, potential difference and resistance is summarised by V = IR, where V is potential difference in volts (V), I is current in amperes (A) and R is resistance in ohms (Ω). Rearranging gives I = V ÷ R and R = V ÷ I. To use the equation, identify the two known quantities, substitute values with units, then rearrange and calculate. For example, a 12 V supply driving 3 A through a resistor gives 12 = 3 × R, so R = 12 ÷ 3 = 4 Ω. This equation can be used to calculate the resistance of any component at a given potential difference and current, including non-ohmic components like filament lamps whose resistance changes.

This relationship links the potential difference across a component, the current through it and its resistance. Potential difference (V) is the energy transferred per unit charge, measured in volts (V). Current (I) is the rate of flow of charge, measured in amperes (A). Resistance (R) is the opposition to current, measured in ohms (Ω). For a fixed resistance, increasing the potential difference increases the current in direct proportion. For example, a resistor of 20 Ω with a current of 0.30 A has V = 0.30 × 20 = 6.0 V. Rearranged, I = V ÷ R and R = V ÷ I. This applies to ohmic conductors at constant temperature; filament lamps and diodes behave non-linearly, so the equation describes the value at a particular point rather than a constant ratio.

This is the symbolic form of the relationship between potential difference (V), current (I) and resistance (R). V is measured in volts, I in amperes and R in ohms. The equation allows any one quantity to be found when the other two are known. For example, a 12 V supply driving 0.50 A through a resistor gives R = V ÷ I = 12 ÷ 0.50 = 24 Ω. Rearranged forms are I = V ÷ R and R = V ÷ I. The equation describes ohmic behaviour at constant temperature; for a filament lamp the resistance rises as the current increases, so V and I are not directly proportional across the whole range. When using the equation, convert mA to A and kΩ to Ω first, then substitute and give the answer with the correct unit.

Potential difference (p.d.) is the energy transferred per unit charge passing between two points in a circuit, measured in volts (V). One volt equals one joule per coulomb, so a 6 V battery transfers 6 J of energy to each coulomb of charge that passes through it. In circuit diagrams, p.d. is measured across a component using a voltmeter connected in parallel. For a fixed resistor, p.d. and current are directly proportional at constant temperature, giving a straight line through the origin on a V–I graph. The symbol V stands for both the quantity and its unit, so write V = 4.5 V. When two components are in series, the p.d.s across them add to the supply p.d.; in parallel, each branch has the same p.d.

Electric current is the rate of flow of electrical charge, measured in amperes (A), where one ampere equals one coulomb per second (1 A = 1 C/s). The symbol I represents current, so write I = 0.50 A. In a series circuit the current is the same at every point because charge is conserved and there is only one path. In parallel branches, the current splits so that the total current from the supply equals the sum of the branch currents. Current is measured with an ammeter connected in series with the component. For a fixed resistor at constant temperature, current is directly proportional to potential difference, so the current–potential difference graph is a straight line through the origin.

Resistance measures how strongly a component opposes the flow of electric charge. Resistance is defined by R = V ÷ I, where V is potential difference in volts and I is current in amperes. The unit of resistance is the ohm, symbol Ω; one ohm allows one ampere when one volt is applied. In a series circuit, adding resistors increases total resistance, so current falls for the same supply potential difference. In parallel, adding branches decreases total resistance because there are more paths. Resistance changes with temperature in filament lamps and with light intensity in LDRs, so R = V ÷ I gives the resistance at that particular operating point. When calculating, substitute values carefully. For example, a 12 V supply driving 3 A gives R = 12 ÷ 3 = 4 Ω.

In this required practical you build circuits from circuit diagrams and test how resistance depends on factors such as the length of a wire, the number of resistors in series or parallel, and the type of component. A typical method uses a battery or power supply, an ammeter in series, a voltmeter in parallel with the test component, and a variable resistor to change current. Record potential difference and current, then calculate resistance using R = V ÷ I. For a wire, measure resistance at several lengths and plot a graph; resistance should increase with length. For series and parallel combinations, compare measured total resistance with predictions. Control variables such as temperature and wire material. Assess the circuit before closing the switch, check meter ranges and connections, and repeat readings to improve reliability.

For a metallic wire kept at constant temperature, resistance is directly proportional to length. Doubling the length doubles the resistance because the charge carriers must travel through twice as many lattice ions, so they make more collisions that oppose their flow. This relationship is described by R ∝ L, and it holds only while temperature and the material and thickness of the wire stay the same. A useful method is to connect a wire in series with an ammeter and a cell, place a voltmeter in parallel across a measured length, and calculate R = V ÷ I for several lengths. Plotting resistance against length should give a straight line through the origin.

Resistors can be connected in series or in parallel, and the total resistance depends on the arrangement. In series, the same current flows through each resistor, the potential differences add to give the supply potential difference, and the total resistance is the sum of the individual resistances: R_total = R₁ + R₂. In parallel, the potential difference across each branch is the same, and the currents in the branches add to give the total current. Adding a parallel branch gives more paths for charge, so total resistance falls. The total resistance of two resistors in parallel is less than the resistance of the smallest individual resistor. Students are not required to calculate the exact total resistance of parallel circuits using the reciprocal formula.