A student moves a bar magnet into a coil of wire connected to a sensitive ammeter. The ammeter needle deflects, showing a current has been induced. Explain what happens to the induced current when the student moves the same magnet out of the coil at a faster speed than before.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A bar magnet is moved into and then out of a coil of wire. The coil is connected to a sensitive ammeter which detects any current flowing.
Model answer (4 marks)
The induced current reverses direction when the magnet is withdrawn, because the change in magnetic flux through the coil is now opposite to that during the approach.
The current is produced by the changing magnetic field through the coil – Faraday’s law states that a varying flux induces an emf.
When the magnet is pulled out more quickly the rate of change of flux is greater, so the induced emf is larger.
A larger emf drives a larger current through the coil, so the ammeter needle deflects more strongly.
The current is produced by the changing magnetic field through the coil – Faraday’s law states that a varying flux induces an emf.
When the magnet is pulled out more quickly the rate of change of flux is greater, so the induced emf is larger.
A larger emf drives a larger current through the coil, so the ammeter needle deflects more strongly.
Examiner tips
- State the direction change first, then explain flux change and speed effect, finally link to larger current.
- Use terms: induced emf, magnetic flux, Faraday’s law, Lenz’s law, rate of change of flux.
- Show the logical sequence: withdrawal → opposite flux change → larger dΦ/dt → larger emf → larger current.
Common mistakes
- Confusing the direction of the induced current with the direction of the magnet’s motion.
- Forgetting to mention that the current is larger because dΦ/dt is larger.
- Using vague phrases like ‘more current’ without linking to the speed of change of flux.
Mark scheme (4 marks)
- The direction of the induced current is reversed (compared to when the magnet was moving in)
- Moving the magnet out induces a current because the magnetic field through the coil is changing
- The faster speed means the magnetic field changes more quickly / rapidly
- The size / magnitude of the induced current is larger / greater
Key terms in this question
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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