A current-carrying wire is placed between the poles of a magnet. Describe how the direction of the force on the wire changes when both the direction of the current and the direction of the magnetic field are reversed at the same time.

Eduqas GCSE Physics — 8.2 Magnetic effects of currents and the motor effect · Describe · 4 marks · View as Markdown

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

A horizontal wire carries a current from left to right. It is placed in a magnetic field directed from north to south. The wire experiences an upward force.

Model answer (4 marks)

Reversing the current alone would reverse the force.
Reversing the magnetic field alone would also reverse the force.
When both the current and the field are reversed, the two reversals cancel, so the force remains in the original upward direction.
The magnitude of the force is unchanged.

Examiner tips

  • Use the right-hand rule to justify the direction changes.
  • Show the two separate reversals before combining them.
  • Mention that the force magnitude depends on I, B and length, not on direction.

Common mistakes

  • Confusing the direction of the magnetic field with the pole names.
  • Saying the force reverses when both are reversed, instead of staying the same.
  • Ignoring that the magnitude stays the same.

Mark scheme (4 marks)

  1. Reversing the current alone would reverse the direction of the force
  2. Reversing the magnetic field alone would also reverse the direction of the force
  3. When both are reversed, the two reversals cancel out / the force is in the same direction as before
  4. The size / magnitude of the force is unchanged

Key terms in this question

force · current · magnetic field · direction

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