Explain how a microphone converts sound into a varying electrical current.

WJEC GCSE Physics (Wales) — 1.9 Electromagnetism · Explain · 4 marks · View as Markdown

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

A microphone is used to pick up a singer's voice during a live performance. The microphone is connected to a recording device.

Model answer (4 marks)

Sound waves produce pressure variations that make the microphone diaphragm vibrate.
The diaphragm is attached to a coil of wire that moves with it inside a permanent magnetic field.
As the coil moves through the field the generator effect induces a potential difference across the coil.
The induced current therefore varies in the same way as the pressure variations, giving a current that mirrors the sound wave.

Examiner tips

Use the word ‘induced’ to show understanding of the generator effect.
Show the sequence: pressure → diaphragm → coil movement → magnetic field → induced EMF → varying current.
Mention that the coil is in a magnetic field produced by a permanent magnet.
Use the phrase ‘same way as the pressure variations’ to link sound to current variation.

Common mistakes

Forgetting to mention the magnetic field or the coil’s movement.
Describing the microphone as a simple resistor rather than an electromagnetic transducer.
Using the word ‘generated’ instead of ‘induced’ for the EMF.

Mark scheme (4 marks)

Sound waves cause pressure variations that make the diaphragm vibrate
A coil of wire is attached to the diaphragm and moves with it inside a magnetic field
The generator effect causes a potential difference to be induced across the coil
The induced current varies in the same way as the pressure variations in the sound, producing a corresponding current variation

Key terms in this question

microphone