Explain why a current-carrying wire placed in a magnetic field experiences a force, and state two ways in which the size of this force can be increased.

AQA GCSE Physics (8463) — 4.7.2 The motor effect · 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 current‑carrying wire has its own magnetic field, which interacts with the magnetic field of the permanent magnet, producing a force on the wire.

The force can be increased by:
1. Increasing the current through the wire.
2. Increasing the magnetic flux density (strength of the magnetic field).

Examiner tips

Use the phrase ‘produces its own magnetic field’ and ‘interacts with the magnetic field of the permanent magnet’ to show understanding of the motor effect.
Show the two methods of increasing the force explicitly – current and magnetic flux density – as the mark scheme expects.
Keep the answer concise and use UK spelling (e.g., ‘magnetic field’).

Common mistakes

Confusing the magnetic field of the wire with the external field; students sometimes say the wire’s field alone causes the force.
Failing to mention both ways to increase the force, or only mentioning one method such as increasing current.

Mark scheme (4 marks)

The current in the wire produces its own magnetic field (around the wire)
The magnetic field of the wire interacts with the magnetic field of the permanent magnet
Increasing the current increases the force
Increasing the magnetic flux density (strength of the magnetic field) increases the force

Key terms in this question

magnetic field · current