A rectangular coil of wire is connected to a sensitive galvanometer and is rotated steadily inside a uniform magnetic field produced by two permanent magnets. After several complete rotations, the coil is brought to rest with its plane parallel to the magnetic field. Describe how the induced voltage changes as the coil is rotated, and explain three separate ways in which the maximum induced voltage could be increased.

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

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

A rectangular coil of wire is connected to a sensitive galvanometer and rotated steadily inside a uniform magnetic field. The coil completes several full rotations before being brought to rest.

Model answer (4 marks)

As the coil turns, the magnetic flux through it changes continuously, so the induced emf oscillates – the galvanometer needle deflects back and forth. The emf is greatest when the coil’s plane is parallel to the field (maximum flux) and zero when the plane is perpendicular (no flux). To increase the maximum emf you can: 1. Use a stronger magnet or increase the magnetic field strength; 2. Rotate the coil faster so the flux changes more quickly; 3. Increase the length of wire in the field – e.g. add more turns, enlarge the coil area or use a longer wire.

Examiner tips

  • Use the exact phrase ‘magnetic flux’ and ‘emf’ – examiners look for correct terminology; show the needle deflects back and forth; list each method clearly; keep answer within 4 marks.
  • Mention the coil’s plane relative to the field (parallel = max, perpendicular = zero) – this is a key point.
  • Use concise, bullet‑style points for the three methods to hit each mark.

Mark scheme (4 marks)

  1. As the coil rotates, the induced voltage continuously changes / alternates (accept: the galvanometer needle deflects back and forth); the voltage is greatest when the plane of the coil is parallel to the magnetic field and zero when the plane is perpendicular to the field.
  2. Use a stronger magnet / increase the strength of the magnetic field.
  3. Increase the speed of rotation of the coil (so the magnetic field through the coil changes more quickly).
  4. Increase the length of wire inside the magnetic field — for example by increasing the number of turns on the coil, using a larger coil area, or using a longer wire.

Key terms in this question

induced voltage · magnetic field

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