A student is investigating how the length of a wire affects its resistance. Describe how the student should make their investigation a fair test and explain why repeating measurements and calculating a mean improves the reliability of their results.

OCR GCSE Physics A: Gateway Science (J249) — P9 Practical skills · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A student sets up a circuit to measure the resistance of different lengths of resistance wire. They plan to change the length of the wire and measure the resistance each time.

Model answer (4 marks)

1. Keep all other variables constant – use the same type of wire, same diameter and same temperature for every trial.
2. Only change the length of the wire (independent variable) and record the resistance (dependent variable).
3. Repeat each measurement several times to minimise random errors.
4. Calculate the mean of the repeated measurements to obtain a more reliable value for the resistance.

Examiner tips

  • Use numbered points to show each mark point clearly.
  • Mention the independent and dependent variables explicitly.
  • Explain that the mean reduces the influence of outliers.

Common mistakes

  • Failing to state that only the length is varied.
  • Not recognising the need to keep temperature constant.
  • Calculating a mean but not explaining why it improves reliability.

Mark scheme (4 marks)

  1. Keep all other variables constant / the same (e.g. same type of wire, same thickness/diameter of wire, same temperature)
  2. Only change the independent variable (length of wire) and measure the dependent variable (resistance)
  3. Repeating measurements reduces the effect of random errors / anomalous results
  4. Calculating a mean gives a more reliable / accurate value for the resistance

Key terms in this question

fair test · mean · reliability

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