An electric motor is used to drive a conveyor belt in a food packaging factory. Explain how the electric motor produces continuous rotation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An electric motor contains a coil of wire that is free to rotate between two permanent magnets. When a direct current is passed through the coil, the motor begins to spin and keeps spinning as long as the current flows.
Model answer (5 marks)
A current‑carrying coil in a magnetic field experiences a force (Lorentz force). The two sides of the coil that are perpendicular to the field feel forces in opposite directions, creating a torque that rotates the coil. Fleming’s left‑hand rule is used to predict the direction of each force. The coil keeps turning because a split‑ring commutator reverses the current every half turn, so the torque always acts in the same rotational sense.
Examiner tips
- Use the term ‘Lorentz force’ and ‘torque’.
- Show the opposite forces on the two sides of the coil.
- Mention Fleming’s left‑hand rule.
- Explain the role of the commutator.
Common mistakes
- Confusing the magnetic field direction with the force direction.
- Forgetting to mention the commutator.
Mark scheme (5 marks)
- A current-carrying coil of wire in a magnetic field experiences a force
- 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 motor continues to rotate because a split-ring commutator reverses the current direction every half turn, maintaining the direction of rotation
Key terms in this question
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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