A technician is testing a bicycle dynamo. When the cyclist pedals faster, the dynamo produces a larger potential difference. Explain why increasing the speed of the rotating magnet inside the dynamo causes a larger potential difference to be induced in the coil.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A bicycle dynamo contains a magnet that spins inside a coil of wire. The spinning magnet induces a potential difference in the coil, which lights the bicycle's lamp.
Model answer (4 marks)
A changing magnetic field induces a potential difference in the coil.
Increasing the speed of the magnet makes the magnetic field through the coil change more rapidly.
The rate of change of the magnetic field is therefore greater.
A greater rate of change of the magnetic field induces a larger potential difference in the coil.
Increasing the speed of the magnet makes the magnetic field through the coil change more rapidly.
The rate of change of the magnetic field is therefore greater.
A greater rate of change of the magnetic field induces a larger potential difference in the coil.
Examiner tips
- Use the word ‘induce’ and ‘rate of change’ to show understanding of Faraday’s law.
- Show the logical sequence: speed → rate of change → potential difference.
- Keep each point short and directly linked to the marks scheme.
- Use UK spelling (induce, potential).
Common mistakes
- Confusing the magnetic field itself with the induced emf; the field changes, not the field strength alone.
- Using vague terms like ‘more electricity’ instead of ‘larger potential difference’.
- Omitting the link that the emf is proportional to the rate of change of the magnetic field.
Mark scheme (4 marks)
- A changing/moving magnetic field induces a potential difference in the coil
- Increasing speed means the magnetic field through the coil changes more rapidly / the rate of change of the magnetic field increases
- A greater rate of change of the magnetic field induces a greater / larger potential difference
- This is because the induced potential difference depends on / is proportional to the rate of change of the magnetic field (or rate at which field lines are cut)
Key terms in this question
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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