A hospital uses a large electric motor to drive a ventilation fan. The motor contains a coil of wire that rotates between two permanent magnets. Explain how the electric motor produces rotation.

OCR A-Level Physics A (H556) — 4.5 Quantum physics · Explain · 5 marks · View as Markdown

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

Electric motors are used in many devices in hospitals, from ventilation fans to pumps. Inside each motor, a coil of current-carrying wire is placed between the poles of permanent magnets.

Model answer (5 marks)

Current flows through the coil, creating a magnetic field around it.
The coil’s field interacts with the field of the permanent magnets.
A force is exerted on each side of the coil (motor effect).
The forces on opposite sides act in opposite directions.
These opposing forces cause the coil to rotate.

Examiner tips

  • Use the exact terms: ‘current flows’, ‘magnetic field’, ‘motor effect’, ‘opposite forces’, ‘rotate’.
  • Show the sequence of cause–effect to cover all 5 points.
  • Keep each point brief but complete – one sentence per point is enough.

Common mistakes

  • Confusing the direction of the magnetic field with the direction of the force.
  • Omitting the motor effect or the interaction between the coil and the magnets.

Mark scheme (5 marks)

  1. Current flows through the coil of wire, producing a magnetic field around it
  2. The magnetic field of the coil interacts with the magnetic field of the permanent magnets
  3. A force is exerted on each side of the coil (motor effect / force on a current-carrying wire in a magnetic field)
  4. The forces on opposite sides of the coil act in opposite directions
  5. These opposing forces cause the coil to rotate

Key terms in this question

electric motor · permanent magnet

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