Current, resistance and potential difference — AQA GCSE Physics
Test yourself on Current, resistance and potential difference with AQA GCSE practice questions.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Current, resistance and potential difference explained
Two quantities decide the current in a component: the potential difference across it and its resistance.
Read the full explanation
Potential difference is the push, so raising it drives a bigger current; resistance opposes the flow, so raising it cuts the current down. Both act together through V = I R, which rearranges to I = V ÷ R. That is why a circuit can be controlled in two separate ways. You can change the push, by adding or removing cells or by turning up a variable power supply, or you can change the opposition, by adjusting a variable resistor or adding further resistors. Both appear in practical questions asking how the potential difference across a component could be varied, and both are credited.
The greater the resistance of the component the smaller the current for a given potential difference (pd) across the component.
For a fixed potential difference, current and resistance work against each other: double the resistance and the current halves, because I = V ÷ R. That single relationship explains most circuit questions. Close a switch that brings a second heating element into parallel and the total resistance of the circuit falls, so the current from the battery rises, and since the potential difference across the battery is unchanged the power output rises too, because P = I V. Add resistors in series instead and the total resistance increases, so the current falls. The same reasoning explains why a long thin wire passes a smaller current than a short thick one of the same material at the same potential difference.
Questions will be set using the term potential difference. Students will gain credit for the correct use of either potential difference or voltage.
The board sets its questions using the term potential difference, but you may answer with either potential difference or voltage and gain full credit. What is not interchangeable is the preposition. Potential difference is always across a component, because it compares two points, while current is always through a component, because it passes along the path. An answer that says the voltage through the resistor or the current across the lamp risks the mark even when the physics behind it is right. The same care pays off in calculations: values carried forward from an earlier part are credited under the errors carried forward rule, so a wrong number early on does not automatically cost every mark that follows.
Current, potential difference or resistance can be calculated using the equation: potential di f f erence = current × resistance V = I R potential difference, V, in volts, V current, I, in amperes, A (amp is acceptable for ampere) resistance, R, in ohms, Ω Required practical activity 3: Use circuit diagrams to set up and check appropriate circuits to investigate the factors affecting the resistance of electrical circuits. This should include: the length of a wire at constant temperature combinations of resistors in series and parallel.
The defining equation of this topic is potential difference = current × resistance, V = I R, with V in volts, I in amperes and R in ohms, Ω; it rearranges to R = V ÷ I and I = V ÷ R. The required practical puts it to work. Build a circuit with the test wire, a battery and an ammeter in series and a voltmeter across the wire, include a variable resistor to change the readings, and calculate R = V ÷ I for each length of wire. Resistance is proportional to length, provided the wire does not warm up, so keep the current low and switch off between readings. Then repeat for resistors in series, where resistance adds, and in parallel, where the total is less than the smallest.
Your focus
- Deduce the current through a 12 Ω component connected across a 6 V supply, then state the effect of doubling its resistance.
- Give two practical ways of varying the potential difference across a component in a test circuit.
- Calculate the current through a component from its resistance and the pd across it.
Show all 16 objectives
- Predict the effect on current of changing the supply or the resistance in a named circuit.
- Explain why a larger resistance gives a smaller current when the potential difference is fixed.
- Predict how closing a switch that adds a parallel branch changes the total resistance and the current.
- Link a change in current at constant potential difference to a change in power output.
- Compare the currents in two wires of different resistance connected to the same supply.
- Use potential difference and voltage interchangeably while keeping across and through correct.
- Rewrite a loosely worded circuit statement so that each quantity is attached to the right component.
- Apply an earlier calculated value correctly in a later part of a structured question.
- Identify the ambiguity in an answer that relies on the word it and correct it.
- Find the resistance of a component that passes 0.25 A when 6.0 V is across it, and give the unit.
- Describe a method for finding how the resistance of a wire depends on its length.
- Explain why the wire must be kept at a constant temperature during the investigation.
- Predict the total resistance of two resistors connected first in series and then in parallel.
Current, resistance and potential difference exam tips
Marking Points
- one mark for identifying potential difference and resistance as the two quantities that set the current
- one mark for a method of varying the potential difference, such as adjusting a variable resistor
- one mark for an alternative method, such as changing the number of cells or the supply setting
- one mark for using I = V ÷ R correctly to support the reasoning
- one mark for stating that a greater resistance gives a smaller current at the same potential difference
- one mark for identifying whether the total resistance of the circuit rises or falls in the situation described
- one mark for deducing the resulting change in current
- one mark for linking that change to the power output using P = I V with the pd unchanged
- one mark for using potential difference or voltage consistently for the quantity across a component
- one mark for using through for current and across for potential difference
- one mark for a clear statement of which component the quantity refers to
- one mark for carrying an earlier value forward correctly into a later calculation
- one mark for the battery in series with the component and the ammeter
- one mark for the voltmeter connected in parallel with the component
- one mark for including a variable resistor, variable power supply or potentiometer
- one mark for calculating resistance as potential difference divided by current with the unit ohms
- one mark for keeping the temperature of the wire constant, for example by switching off between readings
Examiner Tips
- 💡Give one way of varying the pd precisely; adding another cell and adjusting a variable resistor are both accepted.
- 💡Keep the direction of each effect straight: more pd gives more current, more resistance gives less.
- 💡Quote I = V ÷ R when explaining a change so the examiner sees the reasoning, not just the conclusion.
- 💡Work through the chain in order: total resistance, then current, then power, each with its reason.
- 💡The second mark usually depends on the first, so an error about the resistance costs the rest.
- 💡State explicitly that the supply potential difference stays the same, because it is often the missing step.
- 💡Name the component every time: the potential difference across the lamp, the current through the lamp.
- 💡Answers using it are credited only when it clearly refers to the right subject, so repeat the noun.
- 💡If an early answer feels wrong, still use it in the next part; the errors carried forward rule protects the later marks.
- 💡In a six-mark method, state the circuit, the range of lengths, the readings taken and the equation used.
- 💡Say that repeat readings are taken at each length and a mean resistance calculated.
- 💡Check any parallel answer against the rule that the total must be smaller than the smallest single resistance.
Common Mistakes
- naming only the battery as the thing that controls the current, ignoring resistance altogether
- suggesting you change the component being investigated in order to vary the potential difference across it
- writing use the variable resistor without saying it changes the resistance and so the pd
- assuming current stays fixed when extra components are added to the circuit
- saying that adding a parallel branch increases the total resistance
- assuming the potential difference across the battery changes when a switch is closed
- reversing the relationship, so claiming more resistance means more current
- quoting P = I² R without any quantitative comparison, which the mark scheme ignores
- writing voltage through a component or current across a component
- using the word it without making clear which quantity or component is meant
- switching between voltage and potential difference mid-answer so the meaning becomes unclear
- abandoning later parts of a question after an early numerical slip, when errors carried forward would still earn marks
- leaving the circuit switched on so the wire heats up, which raises the resistance as the length increases
- measuring the length of wire from the end rather than between the crocodile clips
- adding resistances in parallel as though they combine like series resistors
- recording the ammeter reading in milliamperes but using it as though it were amperes