A bicycle dynamo produces electricity to power the bike's lights. Inside the dynamo, a small magnet rotates near a coil of wire. Explain why an alternating current (a.c.) is produced in the coil, and describe two ways the cyclist could increase the size of the induced voltage.

Eduqas A-Level Physics — 4.5 Electromagnetic induction · Explain · 5 marks · View as Markdown

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

A bicycle dynamo contains a permanent magnet that spins inside a coil of wire when the wheel turns. The faster the wheel turns, the faster the magnet rotates.

Model answer (5 marks)

As the magnet spins, the magnetic field through the coil changes direction continuously – the field lines cut the coil in opposite directions on each half‑turn. This changing magnetic flux induces a voltage (electromagnetic induction). Because the field direction reverses each half‑turn, the induced voltage reverses direction each half‑turn, producing alternating current.

To increase the induced voltage the cyclist could:
1. Spin the wheel faster, increasing the speed of rotation of the magnet.
2. Use a stronger permanent magnet or increase the number of turns in the coil.

Examiner tips

  • Use the phrase ‘changing magnetic flux’ to show understanding of induction.
  • Explain that the reversal of field direction gives an a.c. output.
  • Mention both speed and coil/magnet strength as ways to raise voltage.

Common mistakes

  • Saying the voltage is constant or that it is direct current.
  • Forgetting to explain why the voltage reverses each half‑turn.
  • Using vague terms like ‘more electricity’ instead of ‘higher induced voltage’.

Mark scheme (5 marks)

  1. As the magnet rotates, the magnetic field through the coil continuously changes direction / the field lines cut the coil in opposite directions on each half-turn
  2. This changing magnetic field induces a voltage / current in the coil (electromagnetic induction)
  3. Because the field direction reverses each half-turn, the induced voltage / current reverses direction each half-turn, producing alternating current
  4. First method to increase induced voltage: increase the speed of rotation of the magnet (cycle faster / increase wheel speed)
  5. Second method to increase induced voltage: use a stronger magnet OR increase the number of turns on the coil

Key terms in this question

alternating current · induced voltage

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