Electromagnetic Induction

Electromagnetic induction — a voltage (e.m.f.) is induced in a conductor whenever the magnetic field through it changes. If the conductor is part of a complete circuit, that e.m.f. drives a current.

This is the motor effect run backwards. In a motor, current in a field produces motion; in induction, motion in a field produces current.

Two ways to produce the change:

1. Move a conductor through a magnetic field, cutting field lines. A wire moved across the poles of a magnet has an e.m.f. induced along it.
2. Move a magnet into or out of a coil, or switch an electromagnet on or off near it.

Change is essential. A magnet held stationary inside a coil induces nothing at all, however strong it is. A wire held still in a field likewise induces nothing. It is the changing field that matters, not the field itself — and this is the most frequently missed idea in the topic.

Direction of the induced current:

• Reversing the direction of motion reverses the induced current — pushing a magnet in gives one direction, pulling it out gives the other.
• Reversing the poles of the magnet also reverses it.

The induced current always opposes the change producing it. Push a north pole into a coil and the coil's near face becomes a north pole, repelling the magnet and resisting your push. This is why generating electricity takes effort: you are doing work against that opposition, and that work is where the electrical energy comes from. It is conservation of energy, not an inconvenience.

Four ways to increase the induced e.m.f.:

1. Move the magnet or wire faster.
2. Use a stronger magnet.
3. Use more turns on the coil.
4. Add a soft iron core to the coil.

Example: drop a magnet down a copper tube and it falls in slow motion. The changing field induces currents in the copper, those currents produce fields opposing the magnet's motion, and it is braked — even though copper is not magnetic at all.