A student places a straight wire horizontally between the poles of a permanent magnet so that the wire is at right angles to the magnetic field. When a current is switched on, the wire moves. When the student reverses the direction of the current, the wire moves in the opposite direction. Describe what determines the direction of the force on the wire, and explain what would happen to the force if the current in the wire were switched off.

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

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

In an investigation into the motor effect, a straight wire is connected to a power supply and placed between the poles of a permanent magnet so that the wire is perpendicular to the magnetic field lines.

Model answer (4 marks)

The force on the wire is given by the motor effect: it depends on the direction of the electric current in the wire and on the direction of the magnetic field produced by the magnet. When the current is switched on, the wire experiences a force perpendicular to both the current and the field (right‑hand rule). If the current is reversed, the force reverses direction. If the current is switched off, there is no force on the wire because a current‑carrying conductor in a magnetic field is required for the motor effect.

Examiner tips

  • Use the right‑hand rule to link current and field to force direction
  • Mention that force is perpendicular to both current and field
  • State that removing the current removes the force
  • Keep answer concise and to the point

Common mistakes

  • Confusing the direction of the magnetic field with the direction of the force
  • Forgetting to mention that both current and field are required
  • Saying the force disappears without explaining why

Mark scheme (4 marks)

  1. The direction of the force depends on the direction of the current (in the wire)
  2. The direction of the force also depends on the direction of the magnetic field
  3. If the current is switched off there is no force on the wire
  4. A force on a current-carrying conductor in a magnetic field (motor effect) requires both a current AND a magnetic field to be present

Key terms in this question

magnetic field · force

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