A student moves a bar magnet into a coil of wire connected to a sensitive ammeter. The ammeter needle deflects, showing that a current has been induced. Explain why a current is induced in the coil when the magnet moves into it, and describe TWO ways the student could increase the size of the induced current.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A bar magnet is pushed into a coil of wire. The coil is connected to a sensitive ammeter, which shows a reading when the magnet moves.
Model answer (5 marks)
A changing magnetic flux through the coil induces an e.m.f. in the coil, which drives a current around the complete circuit.
1. When the bar magnet is moved into the coil the number of magnetic field lines passing through the coil increases, so the magnetic flux through the coil changes.
2. A changing flux induces an e.m.f. in the coil (Faraday’s law).
3. The induced e.m.f. drives a current through the coil because the circuit is complete.
Ways to increase the induced current:
4. Move the magnet more rapidly – a greater rate of change of flux gives a larger e.m.f.
5. Use a stronger magnet or increase the number of turns in the coil – both raise the flux change per turn, giving a larger e.m.f. and hence a larger current.
1. When the bar magnet is moved into the coil the number of magnetic field lines passing through the coil increases, so the magnetic flux through the coil changes.
2. A changing flux induces an e.m.f. in the coil (Faraday’s law).
3. The induced e.m.f. drives a current through the coil because the circuit is complete.
Ways to increase the induced current:
4. Move the magnet more rapidly – a greater rate of change of flux gives a larger e.m.f.
5. Use a stronger magnet or increase the number of turns in the coil – both raise the flux change per turn, giving a larger e.m.f. and hence a larger current.
Examiner tips
- Use the sequence: change in flux → e.m.f. → current; mention the circuit is closed.
- Mention the two specific methods: faster motion and stronger magnet or more turns.
- Keep each point brief and use exact terms like ‘magnetic flux’, ‘e.m.f.’, ‘Faraday’s law’.
- Show the link between e.m.f. and current (Ohm’s law not required for 5 marks).
Common mistakes
- Confusing ‘moving the magnet’ with ‘moving the coil’ – only the change in flux matters.
- Failing to state that the circuit must be complete to allow current.
- Mentioning only one method to increase current or giving an irrelevant method such as changing the wire resistance.
Mark scheme (5 marks)
- Moving the magnet into the coil causes the magnetic field through the coil to change (the magnetic flux through the coil changes / the number of magnetic field lines through the coil increases)
- A changing magnetic field induces a voltage (e.m.f.) in the coil
- The induced voltage drives a current around the circuit (because the circuit is complete)
- First valid method to increase the induced current: move the magnet faster / increase the speed of the magnet
- Second valid method to increase the induced current: use a stronger magnet OR increase the number of turns on the coil
Key terms in this question
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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