What is Electromagnetic Induction?
Electromagnetic induction is the process by which a changing magnetic field generates an electromotive force (emf) — and if the circuit is closed, a current — in a conductor. This is not just a lab curiosity: it is the principle behind every electrical generator, transformer, and wireless charger in the modern world.
The key insight, discovered by Michael Faraday in 1831, is that it is change that matters. A static magnetic field through a loop does nothing. But as soon as that field changes — whether by moving the magnet, changing the field strength, or rotating the loop — electricity is produced.
Think of magnetic flux like water filling a bucket. A full, still bucket does nothing interesting. But tip the bucket — change how much water is inside — and you get a flow. Similarly, it's the change in magnetic flux, not the flux itself, that drives induction.
Magnetic Flux
Magnetic flux is a measure of the total magnetic field passing through a given area. It is defined as:
where is the magnetic flux density (T), is the area of the loop (m²), and is the angle between the magnetic field direction and the normal (perpendicular) to the loop surface.
The SI unit of magnetic flux is the weber (Wb), where .
Understanding the factor is crucial:
- When the field is perpendicular to the loop (): — maximum flux.
- When the field is parallel to the loop (): — no flux threads through the loop.
Students frequently confuse the angle . It is measured between the magnetic field vector B and the normal to the loop — NOT between B and the plane of the loop. If the field is parallel to the plane of the loop, (not ), giving zero flux.