Explain why the current through a resistor connected in a circuit increases when the potential difference across it is increased, using a model of charge carriers.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A simple circuit consists of a battery connected to a single resistor. The battery is replaced by one of greater electromotive force (emf), causing the current in the circuit to increase.
Model answer (4 marks)
The battery supplies an electric field that does work on the charge carriers, giving them energy. A larger emf creates a larger potential difference across the resistor, so each electron gains more energy per unit charge. This stronger field accelerates the electrons more, increasing their drift velocity. A higher drift velocity means a larger amount of charge passes any cross‑section of the resistor per unit time, so the current, defined as the rate of flow of charge, increases.
Examiner tips
- Use the word ‘drift velocity’ to link field strength with carrier speed.
- Show the chain: emf → electric field → energy per charge → drift velocity → current.
- Mention that current is the rate of charge flow, not the charge itself.
- Avoid vague phrases like ‘more current’ – explain the mechanism.
Mark scheme (4 marks)
- The battery (source of emf) acts as an energy source that does work on charge carriers, giving them energy / creating an electric field that drives charge carriers through the resistor.
- A greater potential difference means more energy is transferred per unit charge / a stronger electric field acts on the charge carriers.
- The charge carriers (electrons) therefore move with greater drift velocity / are accelerated more strongly through the resistor.
- A greater drift velocity means more charge passes a given cross-section per unit time, so the current (defined as rate of flow of charge) is greater.
Key terms in this question
potential difference · current · charge carrier
Related
- All IB DP Physics Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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