A student uses a cell to drive a current through a resistor. Describe the difference between electromotive force (e.m.f.) and potential difference (p.d.), and state the unit in which both are measured.

Cambridge International IGCSE Physics (0625) — 4.2 Electrical quantities · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

Electromotive force (e.m.f.) is the electrical work done by the cell per unit charge as the charge completes the whole circuit. Potential difference (p.d.) is the work done per unit charge as it passes through a particular component, such as a resistor. Thus e.m.f. represents the energy supplied by the source, while p.d. represents the energy transferred by a component; e.m.f. drives the circuit, p.d. is the energy output at a component.
Both are measured in volts (V).

Examiner tips

  • Use the terms e.m.f. and p.d. exactly; explain e.m.f. as source work and p.d. as component work.
  • Show the relationship: e.m.f. drives the circuit, p.d. is the energy transferred.
  • Mention the unit volts for both.
  • Keep the answer concise to fit 4 marks.

Common mistakes

  • Confusing e.m.f. with p.d. or vice versa; mixing up source vs component.
  • Omitting the unit volts for one or both quantities.
  • Using vague language like ‘energy’ without specifying work per unit charge.

Mark scheme (4 marks)

  1. E.m.f. is the electrical work done by the power source (cell) per unit charge moved around the complete circuit.
  2. P.d. is the work done per unit charge passing through a component (e.g. the resistor).
  3. E.m.f. relates to energy input from the source / p.d. relates to energy output at a component, so e.m.f. drives the circuit whereas p.d. is the energy transferred by a component.
  4. Both e.m.f. and p.d. are measured in volts (V).

Key terms in this question

electromotive force · potential difference

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