Explain why a current-carrying conductor experiences a force when placed in a magnetic field, and describe how the direction of this force can be reversed.

OCR GCSE Physics A: Gateway Science (J249) — P3.3 Magnetism and the motor effect · Explain · 4 marks · View as Markdown

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

An engineer is testing a simple electric motor. She notices that when current flows through a wire inside a magnetic field, the wire moves. She wants to understand why this happens and how to control the direction of movement.

Model answer (4 marks)

A current‑carrying conductor produces its own magnetic field.

This magnetic field interacts with the external magnetic field.

The interaction of the two fields produces a force on the conductor – the motor effect.

The direction of the force can be reversed by reversing either the current direction or the magnetic field direction (but not both).

Examiner tips

Use the phrase ‘motor effect’ to show understanding of the force on a conductor in a magnetic field.
Show the two ways to reverse the force – change current or field, not both.
Keep the answer concise – one sentence per point for 4 marks.

Common mistakes

Confusing the direction of the magnetic field with the direction of the force.
Saying the force can be reversed by changing both current and field, which cancels the effect.
Using vague terms like ‘magnetic influence’ instead of ‘interaction of magnetic fields’.

Mark scheme (4 marks)

The current-carrying conductor produces its own magnetic field
This magnetic field interacts with the external magnetic field
The interaction of the two fields produces a force on the conductor (the motor effect)
The direction of the force can be reversed by reversing either the current direction or the direction of the magnetic field (not both)

Key terms in this question

magnetic field · current-carrying conductor · force