Know that the direction of an induced e.m.f. opposes the change causing it

4.5.1 Electromagnetic Induction

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.

Lenz’s Law in Plain English

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!”

Analogy: The “Squeezing a Sponge” Example ⚡️

• 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.

Common Situations

• Moving a magnet into or out of a coil.
• Rotating a coil in a fixed magnetic field.
• Changing the current in a nearby coil (mutual induction).

Quick Practice Questions 📚

1. What happens to the induced e.m.f. if the magnetic field through a coil is increasing?
2. Describe the direction of the induced current when a magnet is pulled out of a coil.
3. Explain why a generator can produce electricity when its coil rotates in a magnetic field.

Summary Table of Induced e.m.f. Direction