A rescue team uses an electric motor to winch injured climbers up a cliff face. The motor contains a coil of wire that rotates between two permanent magnets. Explain how the motor produces a continuous rotation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An electric motor converts electrical energy into kinetic energy by making use of the force experienced by a current-carrying wire placed in a magnetic field.
Model answer (5 marks)
A current is supplied to the coil, so a current‑carrying wire lies in the magnetic field of the two permanent magnets.
1. The two sides of the coil that are perpendicular to the magnetic field each experience a magnetic force (F = Iℓ×B).
2. The forces on opposite sides of the coil act in opposite directions, producing a torque.
3. This torque causes the coil to rotate.
4. As the coil turns, the contacts in the commutator reverse the current direction in the appropriate coil segment.
5. Fleming’s Left‑Hand Rule is used to predict the direction of the force and therefore the direction of rotation.
1. The two sides of the coil that are perpendicular to the magnetic field each experience a magnetic force (F = Iℓ×B).
2. The forces on opposite sides of the coil act in opposite directions, producing a torque.
3. This torque causes the coil to rotate.
4. As the coil turns, the contacts in the commutator reverse the current direction in the appropriate coil segment.
5. Fleming’s Left‑Hand Rule is used to predict the direction of the force and therefore the direction of rotation.
Examiner tips
- Use the exact phrase "current‑carrying wire" and "magnetic field". Include the commutator step to show continuous rotation. Mention Fleming’s Left‑Hand Rule to demonstrate understanding of force direction.
Common mistakes
- Forgetting to mention the commutator and current reversal. Using the wrong hand rule (Right‑Hand Rule) or not specifying which rule. Omitting the fact that the forces are on opposite sides of the coil.
Mark scheme (5 marks)
- A current flows through the coil of wire (in the magnetic field)
- The two sides of the coil that are perpendicular to the magnetic field each experience a force
- The forces on the two opposite sides of the coil act 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 / rotation
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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