An e.m.f. is induced in a conductor when the magnetic flux linked with it changes. No change means no induced e.m.f., however strong the magnet.
This lesson is part of SPM Physics electromagnetism. It is the reverse of determining magnetic-force direction.
What happens in the magnet and coil experiment?
Connect a coil to a sensitive centre-zero galvanometer. Move a bar magnet relative to the coil and watch the meter.
| Situation | Meter |
|---|---|
| Magnet pushed in | deflects one way |
| Magnet held still inside | zero |
| Magnet withdrawn | deflects the opposite way |
| Magnet pushed in faster | larger deflection |
| Stronger magnet or more turns | larger deflection |
Worked example: applying Lenz’s law
A north pole is pushed towards the coil. The flux through the coil is increasing, so the induced current opposes this.
To oppose the approach, the coil end nearest the magnet must become a north pole, which repels it. Use this to decide the current direction in the coil by the grip rule your course uses.
When the north pole is withdrawn, the flux decreases. The nearest end becomes south, which attracts the magnet and opposes the withdrawal.
The mistake that loses marks
A common slip is to write “the magnet inside the coil produces a current”. The magnet alone does not. Only a change of flux does.
The fix is to state the change. Say “as the magnet moves into the coil, the flux linked with the coil increases, so an e.m.f. is induced”.
Check yourself
A student holds a coil still and a magnet still, and the meter reads zero. She then pulls the magnet away quickly. Describe what the meter shows, and give the reason.
Answer
The meter deflects while the magnet is moving away, and the flux linked with the coil decreases. A quicker pull gives a larger deflection because the flux changes faster.
When the magnet stops moving again, the meter returns to zero because the flux is no longer changing.
What to study next
See induction used in machines in comparing motors and generators, and between two coils in calculating transformer relationships.
Practise wording in the practice set. For a teacher to run new magnet-and-coil predictions with you, see online one-to-one Physics tuition or the one-hour trial class (from RM50).