Electromagnetic induction is the process of generating an electromotive force (e.m.f.) by changing the magnetic flux through a conductor or coil.
How to induce an e.m.f.
An e.m.f. is induced when:
- A conductor moves through a magnetic field (cutting field lines).
- A magnet moves into or out of a coil.
- The magnetic field through a coil changes.
If the circuit is complete, the induced e.m.f. drives a current.
Factors that increase the induced e.m.f.
| Factor | Effect |
|---|---|
| Faster movement | Greater e.m.f. |
| Stronger magnet | Greater e.m.f. |
| More turns on the coil | Greater e.m.f. |
Direction of the induced current (Supplement)
Lenz’s law: the induced current flows in a direction that opposes the change producing it. If a north pole moves toward a coil, the coil creates a north pole at the approaching end (to repel the magnet). This is a consequence of energy conservation.
Fleming’s right-hand rule (for a conductor moving in a field): point the thumb in the direction of motion, the first finger in the direction of the field (N to S), and the second finger gives the direction of induced current.
Worked example
A bar magnet is pushed into a solenoid connected to a galvanometer. Describe what happens.
The galvanometer deflects, showing a current is induced. If the magnet is pushed faster, the deflection is larger (greater e.m.f.). If the magnet is pulled out, the deflection reverses (current flows in the opposite direction). When the magnet is stationary inside the coil, there is no deflection (no change in flux).
Common errors and how to correct them
Stating that a stationary magnet inside a coil induces an e.m.f. There must be relative motion or a changing field. A magnet sitting still inside a coil produces no e.m.f.
Confusing Fleming’s right-hand rule (generators) with the left-hand rule (motors). Right hand = generator (finding induced current). Left hand = motor (finding force on a current-carrying conductor).
How to apply this in an exam
State that a changing magnetic field or relative motion is needed. List the factors that increase the e.m.f. For direction, use Lenz’s law or Fleming’s right-hand rule and state which you are using.
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