A coil of wire is connected to a sensitive galvanometer. A bar magnet is pushed quickly into the centre of the coil and then held stationary inside it. Describe what happens to the galvanometer reading during each stage, and explain why the reading changes between the two stages.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
When the bar magnet is pushed into the coil the galvanometer deflects, showing a current (or voltage) flowing.
When the magnet is held stationary inside the coil the galvanometer returns to zero, showing no deflection.
During the motion the magnetic field through the coil changes because the magnet is moving relative to the coil, so an emf is induced (generator effect) and a current flows.
When the magnet is stationary the magnetic field through the coil is no longer changing, so no emf is induced and no current flows.
When the magnet is held stationary inside the coil the galvanometer returns to zero, showing no deflection.
During the motion the magnetic field through the coil changes because the magnet is moving relative to the coil, so an emf is induced (generator effect) and a current flows.
When the magnet is stationary the magnetic field through the coil is no longer changing, so no emf is induced and no current flows.
Examiner tips
- Use the words ‘deflects’ and ‘returns to zero’ to show understanding of the reading. Explain the change in terms of relative motion and changing magnetic flux. Mention the generator effect and that no emf is produced when the field is constant.
Common mistakes
- Saying the galvanometer always shows a current, even when the magnet is stationary. Confusing the direction of deflection with the direction of the induced current. Forgetting to mention that the change is due to changing magnetic flux, not the magnet’s position alone.
Mark scheme (4 marks)
- When the magnet is pushed into the coil, the galvanometer shows a deflection / reading (indicating an induced current or voltage).
- When the magnet is held stationary inside the coil, the galvanometer reading returns to zero / shows no deflection.
- Whilst the magnet is moving, there is relative motion between the magnet and the coil, so the magnetic field through the coil is changing, inducing a voltage (generator effect).
- When the magnet is stationary, there is no relative motion / the magnetic field through the coil is no longer changing, so no voltage is induced and no current flows.
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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