The potential difference across a length of wire is 3.0 V and the current through it is 0.25 A. Calculate its resistance.
[2 marks]
Total for this question: 2
Physics required practicals
Resistance of a wire
AQA 4.2 · RP3
GCSE Physics (8463) · Required practical 3 — method, variables, the marks examiners report students losing.
Board and spec code confirmed against AQA 8463 · registry checked 2026-07-17How this checking works
Investigate the factors affecting resistance: how the resistance of a wire depends on its length at constant temperature, and how resistors combine in series and parallel.
Apparatus
- Length of resistance wire (e.g. constantan) taped to a metre ruler
- Two crocodile-clip leads to tap off different lengths
- Ammeter (in series) and voltmeter (in parallel with the wire)
- Cell or power supply, switch and connecting leads
- For part (b): fixed resistors to connect in series and parallel
Method
- 1Set up a circuit with the wire, ammeter, switch and power supply; connect the voltmeter across the length of wire being tested.
- 2Clip the crocodile clips 0.10 m apart; close the switch briefly and record the potential difference V and current I.
- 3Resistance R = V / I for that length.
- 4Open the switch, move a clip to a new length (e.g. 0.20 m, 0.30 m ... 1.00 m) and repeat, switching off between readings so the wire does not heat up.
- 5Plot resistance against length; for part (b), measure the total resistance of resistors connected in series, then in parallel.
Variables
Independent
Length of the wire
Dependent
Resistance of the wire (from R = V / I)
Control
- The wire itself (material and thickness / cross-sectional area)
- Temperature of the wire
- The power supply / potential difference
Results & processing
- Calculate R = V / I in ohms for each length; a graph of resistance against length is a straight line through the origin, so resistance is proportional to length.
- In series the total resistance adds up (R = R1 + R2); in parallel the total resistance is less than the smallest single resistor.
Where students lose marks
Leaving the current flowing between readings.
Fix: The wire heats up, which raises its resistance; take readings quickly and switch off in between so temperature stays constant.
Putting the voltmeter in series or the ammeter in parallel.
Fix: Ammeter in series (measures the current through the wire); voltmeter in parallel across the wire (measures the p.d. across it).
Measuring the length carelessly.
Fix: Read the length against the ruler at the point the clips touch, and use a wide range of lengths for a clear line.
Improve the method
- Keep the current low and take readings quickly (or use a switch) so the wire's temperature stays constant.
- Repeat each length and take a mean resistance.
- Use a wide range of lengths to draw a reliable line.
Try it — exam-style
Easy
ORIGINAL
Medium
ORIGINAL
Two 4 Ω resistors are connected in parallel. Calculate the total resistance.
[2 marks]
Total for this question: 2
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
Stuck on resistance of a wire?
Circuit practicals reward getting the meters in the right place and controlling temperature — I drill the setup and the maths. Free intro call, then a free first lesson.