A student moves a bar magnet into a coil of wire connected to a sensitive ammeter. The ammeter needle deflects, showing a current in the circuit. Explain why a current is produced when the magnet moves into the coil, and describe TWO ways the student could increase the size of the current produced.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
A change in the magnetic flux through the coil occurs when the magnet is moved, so the flux increases.
According to Faraday’s law, this changing flux induces an electromotive force (e.m.f.) in the coil.
The induced e.m.f. drives a current around the closed circuit, which the ammeter detects.
To increase the current:
1. Move the magnet more quickly – a faster change in flux gives a larger e.m.f.
2. Use a stronger magnet or increase the number of turns in the coil – both raise the induced e.m.f. and thus the current.
According to Faraday’s law, this changing flux induces an electromotive force (e.m.f.) in the coil.
The induced e.m.f. drives a current around the closed circuit, which the ammeter detects.
To increase the current:
1. Move the magnet more quickly – a faster change in flux gives a larger e.m.f.
2. Use a stronger magnet or increase the number of turns in the coil – both raise the induced e.m.f. and thus the current.
Examiner tips
- Use the phrase ‘changing magnetic flux’ to show understanding of Faraday’s law.
- Show the chain: flux change → e.m.f. → current. Mention that the circuit must be closed.
- Give two specific methods: speed of magnet, stronger magnet or more turns.
Common mistakes
- Confusing magnetic field lines with magnetic flux. Forgetting that the circuit must be closed for current to flow. Listing unrelated ways such as changing the wire resistance without justification.
Mark scheme (5 marks)
- Moving the magnet causes the magnetic field through the coil to change (the flux / field lines through the coil increase)
- The changing magnetic field induces a voltage / e.m.f. in the coil
- The induced voltage drives a current around the circuit (because it is a complete circuit)
- First valid method to increase the current: move the magnet faster / increase the speed of the magnet
- Second valid method to increase the current: use a stronger magnet OR increase the number of turns on the coil
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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