Electromagnetic induction and the a.c. generator

Last time

Before we start: last time, in Electrical power, cost and safety, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Making electricity from motion

A bicycle dynamo lights the lamp, and there is no battery. A wind turbine feeds the national grid. Both use the same effect, discovered by Michael Faraday in 1831: move a wire through a magnetic field, and a voltage appears.

Electromagnetic induction

When a conductor cuts through magnetic field lines, an e.m.f. is induced across its ends. This is electromagnetic induction. If the conductor is part of a complete circuit, a current flows. It is called an induced current. It works with the wire moving, the magnet moving, or the field changing. What matters is the relative movement.

Magnet and coil

Connect a coil to a sensitive meter. Push a bar magnet into the coil, and the meter flicks to one side. Hold the magnet still inside the coil, and the reading goes back to zero. Pull the magnet out, and the meter flicks the other way. The e.m.f. exists only while the magnet is moving.

Making a bigger e.m.f.

Move the magnet or the wire faster. The faster it cuts the field lines, the bigger the e.m.f. Use a stronger magnet. Use more turns of wire in the coil. To reverse the e.m.f., reverse the direction of the movement, or swap the poles of the magnet.