Explain how a moving-coil microphone converts sound waves into an electrical signal.

Eduqas GCSE Physics — 8.4 Microphones, speakers, oscillating currents in detection · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Sound waves make the diaphragm vibrate.
The diaphragm is attached to a coil that moves back and forth inside a permanent magnet.
As the coil moves through the magnetic field it cuts magnetic field lines, inducing an electromotive force in the coil.
The induced voltage (and current) is alternating, with the same frequency as the sound wave.

Examiner tips

  • Use the exact sequence: diaphragm → coil movement → magnetic field → induced voltage. Show that the voltage is alternating and matches the sound frequency. Keep each point brief but complete.

Common mistakes

  • Confusing the coil with the diaphragm; they are separate. Forgetting to mention that the induced voltage is alternating. Using vague terms like "creates a signal" without explaining the induction process.

Mark scheme (4 marks)

  1. Sound waves cause the diaphragm to vibrate
  2. The vibrating diaphragm causes the attached coil to move (back and forth) inside a magnetic field
  3. The moving coil cuts through magnetic field lines, inducing a voltage (e.m.f.) in the coil
  4. The induced voltage (and current) is alternating / oscillating, matching the frequency of the original sound wave

Key terms in this question

coil

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