A student connects a battery to a resistor in a circuit. Explain the difference between the electromotive force (e.m.f.) of the battery and the potential difference across the resistor.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The e.m.f. of the battery is the electrical work done by the source in moving a unit charge around the complete circuit – it is the energy supplied to the charges. The potential difference across the resistor is the work done by a unit charge as it passes through that component – it is the energy transferred from the charges to the resistor. Thus e.m.f. represents the input of energy, while the potential difference represents the energy released by the component. Both are measured in volts (V).
Examiner tips
- Use the terms ‘e.m.f.’ and ‘potential difference’ explicitly; explain the direction of energy flow (input vs. output).
- Show the relationship between work, charge and voltage (W=QV).
- Mention that both are measured in volts.
Common mistakes
- Confusing the e.m.f. with the voltage drop across the resistor; treating them as the same quantity.
- Forgetting to state that e.m.f. is the energy supplied by the source, whereas the potential difference is the energy released by the component.
Mark scheme (4 marks)
- E.m.f. is the electrical work done by the battery / power source in moving a unit charge around the complete circuit
- Potential difference is the work done by a unit charge passing through a component / the energy transferred from electrical energy by unit charge in the resistor
- E.m.f. refers to energy given to charges by the source / input of energy, whereas potential difference refers to energy transferred away by / released in a component
- Both e.m.f. and potential difference are measured in volts (V)
Key terms in this question
electromotive force · potential difference
Related
- All Cambridge International IGCSE Physics (0625) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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