Electromagnetic induction and the generator
Making electricity from movement
- The motor effect turns electricity into movement. Run it backwards — movement into electricity — and you have a generator.
- This is electromagnetic induction, and it powers almost every power station on Earth.
- Let's see how movement makes a current.
Electromagnetic induction
- When a wire moves across a magnetic field — or the field through a coil changes — an e.m.f. is induced.
- If the wire is part of a complete circuit, that e.m.f. drives a current.
- Move a magnet in and out of a coil joined to a meter, and the needle flicks.
How can you induce a current in a coil with a magnet?
A current is induced only when the field through the coil is changing — for example by moving the magnet.
A magnet held still inside a coil induces a steady current.
There must be movement or a changing field. A still magnet causes no change, so no e.m.f. is induced.
Making the induced e.m.f. bigger
- The induced e.m.f. is bigger when you use a stronger magnet, move faster, or use more turns on the coil.
- There must be movement or change — a magnet sitting still inside a coil induces nothing.
- The induced e.m.f. always acts to oppose the change that causes it.
Which change makes the induced e.m.f. bigger?
A stronger magnet, faster movement, or more turns all increase the induced e.m.f.
The a.c. generator
- An a.c. generator spins a coil in a magnetic field. As it turns, the field through the coil keeps changing, so an alternating e.m.f. is induced.
- Slip rings and carbon brushes carry the current out while the coil keeps turning.
- The e.m.f. is largest when the coil is flat (cutting field lines fastest) and zero when it is upright — giving a wave-shaped (sine) output.
The output of a simple a.c. generator is:
As the coil spins, the field through it changes back and forth, giving an alternating (sine-wave) e.m.f.
What do the slip rings and brushes do in an a.c. generator?
Slip rings and brushes keep a sliding contact, carrying the a.c. out without the wires twisting. (A commutator, by contrast, reverses the current — that is for a motor or d.c. generator.)
You've got it
- moving a wire across a field (or changing the field through a coil) induces an e.m.f.
- bigger e.m.f.: stronger magnet, faster movement, more turns — and there must be change
- an a.c. generator spins a coil → alternating e.m.f.; slip rings + brushes carry it out
- output is a sine wave: max when the coil is flat, zero when upright