A bicycle dynamo generates electricity to power a front lamp. The dynamo works by rotating a permanent magnet inside a fixed coil of wire. A cyclist increases their speed, causing the magnet to rotate more quickly. Explain how the dynamo produces a voltage and why the voltage increases when the magnet rotates more quickly.

Pearson Edexcel International GCSE Physics (4PH1) — 6.3 Electromagnetic induction · Explain · 4 marks · View as Markdown

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

A bicycle dynamo consists of a small permanent magnet that spins inside a fixed coil of wire when the wheel turns. When the dynamo is connected to the bicycle lamp, the lamp glows. As the cyclist pedals faster, the lamp becomes noticeably brighter.

Model answer (4 marks)

A relative motion between the rotating permanent magnet and the fixed coil changes the magnetic flux through the coil. According to Faraday’s law, this changing flux induces an electromotive force (voltage) in the coil – the generator effect. When the cyclist pedals faster, the magnet turns more rapidly, so the magnetic flux through the coil changes at a higher rate. The induced voltage is proportional to the rate of change of flux, so a faster rotation produces a larger voltage, making the lamp brighter.

Examiner tips

  • Show the link between relative motion and changing flux, then Faraday’s law, then the speed–rate relationship.
  • Use the exact terms: magnetic flux, Faraday’s law, induced voltage, generator effect.
  • Mention the lamp brightness as a consequence of higher voltage.

Common mistakes

  • Confusing the magnet’s rotation with the coil’s rotation – the coil is fixed.
  • Failing to state that the voltage is proportional to the rate of change of flux.
  • Using vague phrases like ‘more electricity’ instead of ‘greater induced voltage’.

Mark scheme (4 marks)

  1. There is relative motion / movement between the (rotating) magnet and the coil, so the magnetic field through the coil changes.
  2. This changing magnetic field induces a voltage in the coil (generator effect).
  3. When the magnet rotates more quickly, the magnetic field through the coil changes more quickly / at a faster rate.
  4. The greater the rate of change of the magnetic field, the greater the induced voltage, so the voltage (and brightness of the lamp) increases.

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