A wind turbine uses the generator effect to produce electricity. As the wind turns the turbine blades, a coil of wire rotates between two permanent magnets. Explain how a potential difference is induced in the coil and why the current produced is alternating.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Wind turbines generate electricity for thousands of homes. Inside each turbine, a coil of wire is made to rotate continuously between two permanent magnets by the spinning blades.
Model answer (5 marks)
The rotating coil moves through the magnetic field of the permanent magnets, creating relative motion between the coil and the field.
This relative motion changes the magnetic flux through the coil, and by Faraday’s law a potential difference is induced across the coil’s ends.
Because the coil is part of a closed circuit, the induced potential drives a current through the circuit.
As the coil turns, one side of the coil moves upward through the field while the other moves downward, so the direction of the induced force on the charges reverses every half‑turn.
Consequently the current changes direction continuously, giving an alternating current.
This relative motion changes the magnetic flux through the coil, and by Faraday’s law a potential difference is induced across the coil’s ends.
Because the coil is part of a closed circuit, the induced potential drives a current through the circuit.
As the coil turns, one side of the coil moves upward through the field while the other moves downward, so the direction of the induced force on the charges reverses every half‑turn.
Consequently the current changes direction continuously, giving an alternating current.
Examiner tips
- Use the phrase "relative motion between the coil and the magnetic field" to show understanding of the generator effect. Show the link between changing flux and induced EMF. Explain the reversal of force every half‑turn to justify AC. Use correct terminology: flux, EMF, Faraday’s law, alternating current.
Common mistakes
- Confusing the coil’s motion with the magnets moving; the magnets are stationary. Forgetting to mention that the coil is part of a complete circuit. Saying the current is alternating without explaining the reversal of the induced force or flux direction.
Mark scheme (5 marks)
- The rotating coil cuts through the magnetic field (lines) / there is relative motion between the coil and the magnetic field
- This change in magnetic field induces a potential difference across the ends of the coil
- Because the coil is part of a complete circuit, a current flows
- As the coil rotates, each side of the coil alternately moves upward then downward through the field / the direction of the force on the charges in the wire reverses every half turn
- This means the current continuously changes direction, producing alternating current
Key terms in this question
generator effect · potential difference · coil
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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