A recycling plant uses an electric motor to drive a conveyor belt that separates magnetic materials from non-magnetic waste. Explain how the electric motor produces rotation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An electric motor contains a coil of wire placed between two permanent magnets. When a direct current is passed through the coil, the motor begins to rotate, driving the conveyor belt.
Model answer (5 marks)
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 field are acted on by forces in opposite directions, creating a torque. This torque causes the coil to rotate, turning the conveyor belt. Fleming’s left‑hand rule is used to determine the direction of the force on each side of the coil. The magnitude of the force – and therefore the speed of rotation – depends on the magnetic flux density of the field and the size of the current.
Examiner tips
- Use the word ‘force’ and ‘torque’ to show understanding of the motor effect.
- Mention Fleming’s left‑hand rule explicitly.
- Show that the force is proportional to I and B to justify the speed dependence.
- Keep the answer concise – 5 marks only.
Common mistakes
- Confusing the motor effect with the generator effect.
- Failing to mention that the forces are opposite on the two sides of the coil.
- Using the wrong hand rule (right‑hand rule) for a motor.
Mark scheme (5 marks)
- A current-carrying wire in a magnetic field experiences a force (motor effect / force on a wire)
- The two sides of the coil that are perpendicular to the magnetic field experience forces in opposite directions
- These opposing forces cause 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
- The magnitude of the force (and therefore the speed of rotation) depends on the magnetic flux density of the field and/or the size of the current
Key terms in this question
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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