Electromagnetism

When a current flows through a wire a magnetic field is produced around the wire. This field is called and electromagnetic field and the device produced is called an electromagnet. If the wire is wrapped round into a coil the magnetic field is stronger.

It is very easy to detect the magnetic field by putting a compass needle near a wire. When a current is passed through the wire the compass needle will turn. The greater the current in the wire the more the compass needle moved. This experiment was first done by a man called Oersted in 1819 who made the discovery by accident.

You can find the direction of the magnetic field produced by a wire by putting a small compass near the wire (Figure 1).

The direction of the magnetic field produced depends on the direction of the current and the shape of the field depends on the arrangement of the wires.

A long straight coil of wire - called a solenoid – produces a magnetic field very much like that of a bar magnet. The shape of some magnetic fields are shown in the diagrams below.

The core of an electromagnet is made of iron because this is easy to magnetise and demagnetise. Steel would not be suitable because it stays magnetised for much longer and so the electromagnet could not be switched on and off easily.

You can use a small compass to test the polarity of an electromagnet. The following diagrams show the result of doing this for different current directions, different shaped cores and different ways of winding the wire round the core.