A bicycle dynamo contains a coil of wire that rotates inside a magnetic field. As the cyclist pedals faster, the dynamo produces a larger voltage. Explain why a voltage is produced in the coil and why pedalling faster increases the size of this voltage.

WJEC A-Level Physics (Wales) — 4.5 Electromagnetic induction · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

A rotating coil cuts through magnetic field lines, changing the magnetic flux through the coil. This change in flux induces an electromotive force (voltage) in the coil.

When the cyclist pedals faster the coil rotates more quickly. The faster rotation means the coil cuts the field lines more often, so the rate of change of magnetic flux is greater. According to Faraday’s law, a larger rate of change of flux produces a larger induced voltage.

Examiner tips

  • Use the phrase ‘changing magnetic flux’ to show understanding of Faraday’s law
  • Explain that faster pedalling → higher angular speed → higher flux‑change rate
  • Link the rate of change directly to the induced voltage

Common mistakes

  • Saying the voltage is produced by a ‘moving magnet’ instead of a rotating coil
  • Confusing the direction of induced voltage with its magnitude
  • Omitting the link between pedalling speed and the rate of change of flux

Mark scheme (5 marks)

  1. The rotating coil cuts through magnetic field lines (or the magnetic field through the coil changes)
  2. This induces a voltage (e.m.f.) in the coil
  3. Pedalling faster increases the speed of rotation of the coil
  4. The coil cuts field lines more frequently / the rate of change of the magnetic field through the coil increases
  5. A greater rate of change of the magnetic field induces a larger voltage

Key terms in this question

magnetic field · coil

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