Electromagnetism

When a current flows through a wire, a circular magnetic field is created around it. This field can be seen using either iron filings or compasses.
The field only exists when the current is flowing. If it is turned off, the field disappears if the current flows in the opposite direction, the field changes direction.
Field strength
The field created when current flows in a single piece of wire can be quite weak, but its strength can be increased by
• Increasing the current through the wire,
• Increasing the number of pieces of wire, i.e. making the wire into a coil.

Field shape
The magnetic field around a long coil of current-carrying wire is the same shape as that around a bar magnet.
Electromagnets
If a piece of iron is inserted down the centre of a long coil of wire, this increases the strength of its magnetic field. This combination of iron core and coil is known as an electromagnet. When current flows around the coil, the iron becomes magnetized. When current stops flowing, the iron loses its magnetism
Electromagnets are very useful because
• They can be turned on and off,
• They can be made stronger and weaker.