Electromagnetism

Key points

• An electromagnet uses an electrical current to generate a magnetic field.

• There are three ways to strengthen an electromagnet:

  • increasing current

  • increasing the number of turns of the coil

  • adding an iron core

• An electromagnet with a permanent magnet can be used to make motors and loudspeakers.

Play an Atomic Labs experiment exploring electromagnets.

You can also play the full game

Any wire with current flowing through it has a magnetic field. Most of the time the magnetic field is too small to be important or have any noticeable effect.

The magnetic field lines around a current-carrying wire are circular. The field lines are closer together near the wire. This shows that the magnetic field is stronger when close to the wire.

A coil of wire with many turns is called a solenoid.

The shape of the magnetic field around a current-carrying solenoid is like the magnetic field pattern of a bar magnet.

If the magnetic field becomes strong enough to be useful, it is called an electromagnet.

A typical electromagnet consists of a wire coiled around an iron core.

The iron core is often called a soft iron core. This is because iron is magnetically soft. This means the iron core is easy to magnetise and easy to demagnetise.

Electromagnets are used in automatic door locks, headphones, scrap yard cranes and even to make magnetic, levitating trains.

Electromagnets can be switched on and off. They are only magnets while current is flowing though the coil. Electromagnets can be made stronger or weaker.

There are three main ways to increase the strength of an electromagnet.

1. Using more turns on the coil of wire will produce a stronger magnetic field.

Coiling up a longer piece of wire adds up the magnetic field of each turn.

2. A greater current will produce a stronger magnetic field.

3. Adding a soft iron core increases the strength of an electromagnet.

Iron is a magnetic material. A coil of wire is wrapped around an iron core. The iron core becomes magnetised when the electromagnet is switched on. This increases the overall strength of the electromagnet.

Try out this experiment in Atomic Labs. Go to the Physics lab and try the Electromagnets experiment.

If two of the same pole (either north with north or south with south) are brought near each other, they push each other away. Like poles repel.

If two opposite poles (north with south) are brought near to each other, they pull towards each other. Un-like poles attract.

Find out more about magnetism in the Magnets and magnetic fields guide.

Knowing that two magnets can attract or repel helps us to explain how an electric motor works. In the diagram, Both the permanent magnet and the current-carrying wire have a magnetic field around them. The permanent magnet and the wire exert a force on each other.

The force on a current-carrying wire near a permanent magnet causes the wire to move. This effect is called the motor effect. This is used in a DC motor.

A device supplies an electrical signal to a coil of wire that is attached to a cone. When current flows one way through the coil, the cone is pushed forward. When current flows in the opposite direction, the cone is pulled backwards. As the current in the coil repeatedly changes direction it makes the cone vibrate. The air vibrates and sound waves travel to our ears.

Microphones have a very similar structure to speakers. Both have a coil of wire that is able to move near a permanent magnet.

In the microphone however, incoming sound causes the diaphragm to vibrate. This moves the attached coil of wire.

The effect is the opposite of the motor effect. The wire moving near the magnet causes an electrical output.