A student places a current-carrying wire between the poles of a permanent magnet. The wire experiences a force. Explain how the student could increase the size of this force without changing the magnetic field strength, and state the effect of reversing the direction of the current on the direction of the force.

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)

Increase the current through the wire – the force is proportional to the current, so a larger current gives a larger force.
Increase the length of wire that lies in the magnetic field – the force is proportional to the length of wire in the field, so a longer segment gives a larger force.
Reversing the direction of the current reverses the direction of the force (the force changes to the opposite direction).

Examiner tips

  • Mention the proportionality to current and length explicitly. Use the formula F = I L B sinθ to justify. State that reversing I changes the sign of F, i.e. the direction.
  • common_mistakes
  • :
  • Saying only the magnetic field can be changed. Forgetting to mention that the length of wire in the field matters. Claiming the force changes magnitude when current is reversed, instead of direction.

Mark scheme (4 marks)

  1. Increase the size of the current (through the wire)
  2. Increasing the current increases the force because force depends on (is proportional to) the current
  3. Increase the length of wire (in the magnetic field)
  4. Reversing the current reverses / changes the direction of the force (to the opposite direction)

Key terms in this question

current · magnetic field · force

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