A bicycle dynamo contains a permanent magnet that spins near a coil of wire. As the magnet spins faster, a larger voltage is produced across the coil. Explain how the dynamo produces a voltage and why spinning the magnet faster increases the voltage produced.

Cambridge International IGCSE Physics (0625) — 4.5 Electromagnetic effects · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

A spinning magnet creates a changing magnetic field through the coil.
The changing field induces an electromotive force (e.m.f.) in the coil.
If the magnet spins faster the magnetic field changes more rapidly.
A greater rate of change of magnetic field produces a larger induced voltage.

Examiner tips

  • Use the term ‘changing magnetic field’ and ‘induced e.m.f.’
  • Explain that speed increases the rate of change of field
  • Show the causal link: faster spin → greater dB/dt → higher voltage

Common mistakes

  • Confusing the magnet’s motion with the coil’s motion
  • Using ‘current’ instead of ‘voltage’ or e.m.f.

Mark scheme (4 marks)

  1. The spinning magnet produces a changing magnetic field through the coil
  2. The changing magnetic field induces a voltage (e.m.f.) in the coil
  3. Spinning faster means the magnetic field changes more rapidly / the rate of change of magnetic field is greater
  4. A greater rate of change of magnetic field induces a larger voltage

Key terms in this question

permanent magnet

Related

More Electromagnetic effects questions

▶ Try answering this question with AI marking (free) →