Electromagnetic induction

A current from a magnet

We have seen that a current makes a magnetic field. Can a magnetic field make a current? In 1831, the English scientist Michael Faraday found that it can.

A coil, a magnet and a galvanometer

Join the ends of a coil of wire to a galvanometer. There is no battery in the circuit. Now move a bar magnet.

• When the magnet moves into the coil, the needle of the galvanometer moves to one side. A current flows in the coil.
• When the magnet is held still in the coil, the needle is at zero. There is no current, however strong the magnet is.
• When the magnet moves out of the coil, the needle moves to the other side. The current is in the opposite direction.

The same happens when we keep the magnet still and move the coil. What matters is that the magnet and the coil move relative to each other, so the magnetic field through the coil changes.

You should find that the needle swings only while the magnet moves, the other way when it comes out, and farther for a faster magnet or more turns.

The current in the coil is called an induced current. The process is called electromagnetic induction. The voltage that drives the induced current is the induced emf.

What makes the induced current larger?

• A faster movement of the magnet.
• A stronger magnet.
• A coil with more turns.

A current is also induced in one coil when the current in a nearby coil is changed, for example when it is switched on or off.

The direction of the induced current

We find the direction with Fleming's right-hand rule. It is used when a conductor moves in a magnetic field.

Stretch the thumb, the forefinger and the middle finger of your right hand at right angles to one another.

- The thumb shows the direction of the motion of the conductor.

- The forefinger shows the direction of the magnetic field.

- Then the middle finger shows the direction of the induced current.

A short way to remember: the left hand is for the motor (current gives motion), and the right hand is for the generator (motion gives current).

Why must we do work?

When you push the magnet into the coil, you feel a small opposite force. The induced current is in such a direction that it opposes the change that makes it. This is Lenz's law. So you must do work to move the magnet, and this work becomes the electrical energy of the induced current. Energy is not made out of nothing.

Uses of electromagnetic induction

• The electric generator, which makes the electricity of power stations.
• The microphone, the transformer and the dynamo of a bicycle.
• Cards and tags that are read by moving them past a coil.

Practise with new numbers

Try it in Python

The program finds the induced voltage for coils of different turns and magnets of different speeds.

Remember these

• A change of the magnetic field through a coil induces a current in the coil. This is electromagnetic induction (Faraday).
• A magnet that is still gives no current. The current changes direction when the magnet moves out instead of in.
• The induced current is larger for a faster motion, a stronger field and more turns.
• Fleming's right-hand rule: thumb = motion, forefinger = field, middle finger = induced current.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. A bar magnet is held still inside a coil. What does the galvanometer show?

2. What happens to the direction of the induced current when the magnet is pulled out instead of pushed in?

3. How can you make a larger induced current without changing the magnet?

4. A coil is moved toward a magnet that is held still. Is a current induced?