Electromagnetic induction is the process where a changing magnetic field creates an electric current in a conductor. The key idea you need to remember for the IGCSE is that the induced electromotive force (e.m.f.) always acts in a direction that opposes the change that produced it. This is known as Lenz’s Law and is mathematically expressed by Faraday’s law:
\$E = -\dfrac{d\Phi}{dt}\$
The negative sign is the clue: it tells us that the induced e.m.f. works against the change in magnetic flux, Φ, that created it.
Imagine you’re pushing a door open. The door resists because of the friction and the hinge’s tension. Similarly, when a magnetic field changes, the induced e.m.f. creates a magnetic field that resists the change. Think of it as the system’s way of saying, “I don’t want this to happen!”
• If you squeeze a sponge, it pushes back against the squeeze.
• In induction, when the magnetic flux through a coil increases (like squeezing the sponge), the induced current flows in a direction that creates a magnetic field opposing that increase.
• If the flux decreases (like releasing the squeeze), the induced current flows to try to keep the flux the same, again opposing the change.
| Situation | Change in Flux | Induced e.m.f. Direction | Resulting Current Flow |
|---|---|---|---|
| Magnet moving into coil | Increasing Φ | Opposes increase (creates field outwards) | Current flows to produce outward field |
| Magnet moving out of coil | Decreasing Φ | Opposes decrease (creates field inwards) | Current flows to produce inward field |
| Coil rotating in fixed field | Φ varies sinusoidally | Alternating e.m.f. (Lenz’s Law each half‑cycle) | Alternating current (AC) |