An electric motor uses a current-carrying coil of wire placed between the poles of two permanent magnets. Explain how the motor produces continuous rotation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Electric motors are found in many everyday devices, from electric toothbrushes to electric vehicles. They rely on the interaction between a magnetic field and a current-carrying conductor.
Model answer (5 marks)
The current‑carrying coil is placed in the magnetic field between the two permanent magnets. According to the motor effect, a force
F = Iℓ×B acts on each side of the coil that is perpendicular to the field. The two sides experience forces in opposite directions, creating a torque that rotates the coil. Fleming’s left‑hand rule (thumb = force, first finger = field, second finger = current) shows the direction of this torque. A split‑ring commutator reverses the current every half turn, so that the forces always act in the same rotational sense, giving continuous rotation.
F = Iℓ×B acts on each side of the coil that is perpendicular to the field. The two sides experience forces in opposite directions, creating a torque that rotates the coil. Fleming’s left‑hand rule (thumb = force, first finger = field, second finger = current) shows the direction of this torque. A split‑ring commutator reverses the current every half turn, so that the forces always act in the same rotational sense, giving continuous rotation.
Examiner tips
- Use the motor effect formula to show the force on the coil.
- Explain the opposite forces on the two sides and how they create torque.
- Mention Fleming’s left‑hand rule to justify the direction of rotation.
- State the role of the commutator in reversing current for continuous rotation.
Common mistakes
- Confusing the motor effect with the generator effect.
- Failing to explain why the commutator is needed for continuous rotation.
- Using the wrong hand rule (right‑hand instead of left‑hand).
Mark scheme (5 marks)
- The 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, with thumb showing force, first finger showing magnetic field direction, and second finger showing current direction
- A split-ring commutator reverses the current direction every half turn so that the forces always act in the same rotational direction, allowing continuous 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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