Explain how the generator effect is used in an alternator to produce an alternating 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).

Model answer (4 marks)

A coil of wire is rotated inside a magnetic field (or a magnetic field is rotated around a stationary coil). The relative motion between the conductor and the field induces a potential difference across the coil ends. As the coil turns, the induced voltage changes sign, so the direction of the induced current reverses periodically. This continual reversal at a fixed frequency produces an alternating current.

Examiner tips

State the relative motion between conductor and magnetic field first. Explain that the induced EMF changes sign as the coil rotates. Mention the fixed frequency of reversal. Use the term "alternating current" explicitly.

Common mistakes

Failing to mention the coil’s rotation or relative motion. Describing the induced EMF but not its sign change. Using “direct current” instead of “alternating current”.

Mark scheme (4 marks)

A conductor/coil rotates within a magnetic field (or there is relative motion between a conductor and a magnetic field)
This relative motion induces a potential difference across the ends of the conductor
As the coil rotates, the direction of the induced current reverses / changes direction periodically
This continual reversal of current direction at a set frequency produces alternating current

Key terms in this question

generator effect · alternating current · alternator