A student connects a battery to a resistor and measures the potential difference across the resistor and the current through it. State what is meant by 'potential difference' and explain why the voltmeter must be connected in parallel with the resistor rather than in series.

Cambridge International IGCSE Physics (0625) — 4.3 Electric circuits · State and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Potential difference is the work done per unit charge passing through the resistor.
It is measured in volts (V).
A voltmeter in series would change the current in the circuit because it adds resistance.
A voltmeter in parallel allows it to measure the p.d. across the resistor without significantly affecting the current.

Examiner tips

  • Use the exact definition of potential difference. Mention volts as the unit. Explain that series adds resistance and reduces current. Show that parallel measures the same p.d. without altering current.

Common mistakes

  • Saying the voltmeter must be in series. Forgetting to mention the unit volts. Confusing voltage with current or resistance.

Mark scheme (4 marks)

  1. Potential difference is the work done per unit charge passing through the resistor / component
  2. Measured in volts (V)
  3. A voltmeter connected in series would change / reduce the current in the circuit because it would add resistance to the circuit
  4. A voltmeter in parallel allows it to measure the p.d. across the resistor without significantly affecting the current through the resistor / circuit

Key terms in this question

potential difference · voltmeter · parallel

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