A DC electric motor contains a rectangular coil of wire suspended between the poles of two permanent magnets. Describe how the motor produces a continuous rotation, including the role of the split ring commutator.

Pearson Edexcel International GCSE Physics (4PH1) — 6.2 Electromagnetism · Describe · 4 marks · View as Markdown

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

A student builds a simple DC electric motor using a rectangular coil with 50 turns wound on a former. The coil is connected to a battery via a split ring commutator and carbon brushes placed inside a uniform magnetic field.

Model answer (4 marks)

A DC motor has a rectangular coil carrying a direct current. The two sides of the coil that are perpendicular to the magnetic field experience magnetic forces that are equal in magnitude but opposite in direction, producing a torque that turns the coil.

When the coil is at a vertical position the forces on the two sides are opposite, so the coil would stop and then reverse if the current direction stayed the same.

A split‑ring commutator connected to the coil reverses the current direction in the coil every half turn. This keeps the forces on each side of the coil in the same direction relative to the magnetic field, so the torque continues to act in the same sense and the coil rotates continuously.

Thus the commutator prevents the coil from stopping or reversing and allows continuous rotation.

Examiner tips

  • Use the word "torque" or "turning effect" to show understanding of forces. Show the sequence: coil → forces → torque → commutator → reversal → continuous rotation. Mention that the commutator changes current direction every 180°.
  • common_mistakes
  • :
  • Forgetting to explain why the commutator is needed (i.e., that without it the coil would stop at the vertical position). Using vague terms like "the coil keeps turning" without linking to forces and current reversal. Mixing up the direction of forces or not specifying that the two sides are perpendicular to the field.

Mark scheme (4 marks)

  1. Direct current in the coil means the two sides perpendicular to the magnetic field experience forces in opposite directions
  2. These opposite forces produce a turning effect / cause the coil to rotate
  3. Without the commutator the coil would stop / reverse at the vertical position (every half turn the current direction in the coil would cause the forces to oppose rotation)
  4. The split ring commutator reverses the direction of current in the coil every half turn, so the forces on each side remain in the same direction and rotation continues

Key terms in this question

split ring commutator

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