A battery is connected to two resistors in series. State what is meant by 'electromotive force' (e.m.f.) of the battery and explain how the potential difference across each resistor relates to the e.m.f. of the battery.

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)

Electromotive force is the electrical work done by the battery in moving a unit charge around the complete circuit.
The potential difference across each resistor is the work done by a unit charge passing through that resistor.
The potential differences across the two resistors add up to equal the e.m.f. of the battery.
The p.d. across each resistor is less than the e.m.f. (because the e.m.f. is divided between the two resistors).

Examiner tips

  • Use the exact definition of e.m.f. as work per unit charge. Show the series relationship: V_total = V1 + V2. Explain that each V is smaller than the total e.m.f. because the battery’s energy is shared.
  • common_mistakes
  • :
  • Confusing e.m.f. with the battery’s terminal voltage. Saying the potential differences are equal to the e.m.f. instead of adding up to it. Using non‑UK spelling such as ‘potential difference’ as ‘voltage’ only.

Mark scheme (4 marks)

  1. Electromotive force is the electrical work done by the battery in moving a unit charge around the complete circuit
  2. The potential difference across each resistor is the work done by a unit charge passing through that resistor
  3. The potential differences across the two resistors add up to equal the e.m.f. of the battery
  4. The p.d. across each resistor is less than the e.m.f. (because the e.m.f. is divided between the two resistors)

Key terms in this question

electromotive force · potential difference · series

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