A student investigates how the resistance of a wire changes with the length of the wire. She plans to measure the current through the wire and the voltage across it for six different lengths. Explain how the student should carry out a valid and reliable investigation, including how she would use her results to find the resistance at each length.

Edexcel A-Level Physics (9PH0) — 1.1 Practical and investigative skills · Explain · 5 marks · View as Markdown

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

A student investigates how the resistance of a wire changes with the length of the wire. She uses a circuit with a power supply, an ammeter, a voltmeter, and a length of resistance wire.

Model answer (5 marks)

1. Keep all other variables constant – use the same wire material, same cross‑sectional area and the same temperature (allow the wire to cool between readings). 2. Repeat each measurement at least twice or three times and calculate a mean value for the current or voltage. 3. Connect the voltmeter in parallel across the wire and the ammeter in series with the wire to measure voltage and current accurately. 4. Use the equation V = IR, rearranged as R = V/I, to calculate the resistance for each length. 5. Identify and discard any anomalous results before calculating the mean, or check for anomalies in the data set.

Examiner tips

  • Use the exact wording ‘keep all other variables constant’ to show control of variables.
  • Show the mean calculation and the use of R = V/I to earn the calculation marks.
  • Mention the parallel/series connection to demonstrate correct circuit set‑up.

Common mistakes

  • Failing to keep temperature constant, leading to variable resistance.
  • Not repeating measurements or not calculating a mean, reducing reliability.
  • Using the voltmeter in series or the ammeter in parallel, giving incorrect readings.

Mark scheme (5 marks)

  1. Keep all other variables constant / control variables (e.g. same wire material, same cross-sectional area, same temperature / allow wire to cool between readings)
  2. Repeat each measurement (at least twice / three times) and calculate a mean value for current or voltage
  3. Use the voltmeter connected in parallel across the wire and the ammeter connected in series to measure voltage and current
  4. Use the equation V = IR (rearranged as R = V/I) to calculate resistance at each length
  5. Identify and discard anomalous results before calculating the mean / check for anomalies in the data

Key terms in this question

resistance · valid · reliable

Related

More Practical and investigative skills questions

▶ Try answering this question with AI marking (free) →