Explain how an electric motor produces rotation.

WJEC GCSE Physics (Wales) — 1.9 Electromagnetism · Explain · 4 marks · View as Markdown

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

An electric motor consists of a coil of wire placed inside a magnetic field. When a current flows through the coil, the motor begins to rotate.

Model answer (4 marks)

An electric motor works because a current‑carrying coil in a magnetic field experiences a force – the motor effect. The two sides of the coil that are perpendicular to the magnetic field are pushed in opposite directions, so the coil is subjected to a torque. This torque causes the coil to rotate. Fleming’s Left‑Hand Rule can be used to predict the direction of the force on each side of the coil.

Examiner tips

Use the term ‘motor effect’ and ‘torque’ to show understanding of the force on the coil.
Show that the forces on opposite sides are opposite in direction and that this produces rotation.
Mention Fleming’s Left‑Hand Rule to demonstrate how the direction of the force is determined.
Keep the answer concise – 4 marks can be earned with 3–4 short sentences.

Common mistakes

Confusing the magnetic field direction with the force direction; students often state the force is along the field rather than perpendicular.
Failing to explain that the forces on opposite sides are opposite, so a torque is produced.
Using vague language such as ‘the coil moves’ instead of ‘the coil rotates’ or ‘torque is produced’.

Mark scheme (4 marks)

The current-carrying coil experiences a force / the motor effect causes a force on the coil
The two sides of the coil that are perpendicular to the magnetic field experience forces in opposite directions
Because the forces act in opposite directions, this causes the coil to rotate
Fleming's Left-Hand Rule can be used to determine the direction of the force on each side of the coil

Key terms in this question

electric motor