A student is investigating how the resistance of a wire changes with its length. She plans to measure the current through the wire and the voltage across it for five different lengths. Describe how the student should carry out this investigation to obtain valid and reliable results, and explain how she should use her measurements to find the resistance for each length.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The student has access to a power supply, ammeter, voltmeter, connecting leads, a metre ruler, and nichrome wire of uniform cross-sectional area.
Model answer (5 marks)
1. Measure the length of the wire with the metre ruler and mark the five lengths.
2. For each length, connect the wire between the power supply terminals using the connecting leads, and place the ammeter in series and the voltmeter across the wire.
3. Keep the material, cross‑sectional area and temperature of the wire constant throughout the experiment.
4. Record the voltage (V) and current (I) for each length, taking at least two readings per length and calculate the mean to improve reliability.
5. Calculate the resistance for each length by dividing the mean voltage by the mean current: R = V ÷ I.
2. For each length, connect the wire between the power supply terminals using the connecting leads, and place the ammeter in series and the voltmeter across the wire.
3. Keep the material, cross‑sectional area and temperature of the wire constant throughout the experiment.
4. Record the voltage (V) and current (I) for each length, taking at least two readings per length and calculate the mean to improve reliability.
5. Calculate the resistance for each length by dividing the mean voltage by the mean current: R = V ÷ I.
Examiner tips
- Use the correct order of instruments (ammeter in series, voltmeter in parallel).
- State that the wire’s temperature should be monitored or kept constant.
- Show the mean calculation and the formula R = V/I.
Common mistakes
- Failing to keep temperature constant, leading to varying resistance.
- Using the voltmeter in series or the ammeter in parallel.
Mark scheme (5 marks)
- Measure and record the length of wire using the metre ruler / set a specific length of wire between the two connection points
- Keep variables that could affect resistance constant, such as the material / cross-sectional area / temperature of the wire
- Record the voltmeter reading (voltage) and ammeter reading (current) for each length
- Repeat readings for each length and calculate a mean / take repeat measurements to improve reliability
- Divide the voltage by the current (V ÷ I) to calculate the resistance for each length
Key terms in this question
resistance · valid · reliable · voltage · current
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
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- Decode the mark scheme abbreviations →
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