Magnetic fields

The field lines are imaginary  lines which represent the
magnetic field. The field is the strongest where the lines are
close together (at the poles of the magnet)

Play the animation

Electromagnets

An iron (or steel) bar can be made into a
magnet by wrapping a coil of wire around and
passing an electric current through the wire

Iron is the best material to us because it is easily
magnetised and quickly loses its magnetism when
the current is switched off.
  The strength of an electromagnet can be
increased by increasing the number of turns
in the coil or increasing the electric current.

Relay Switch

A relay switch is an example of the use of an
electromagnet. Play the animation and see how it
can be used to allow a small electric circuit to switch
on a larger circuit.