Magnets and magnetic fields

The poles of a magnet are the places where its magnetic forces are strongest. Every magnet has a north pole and a south pole.

When two magnets are brought close together, they exert a non-contact force on each other:

• Like poles (north and north, or south and south) repel.
• Unlike poles (north and south) attract.

Magnetic materials are attracted to magnets. The main ones are iron, steel, cobalt and nickel.

A permanent magnet makes its own magnetic field all the time. A bar magnet is an example.

An induced magnet is a material that becomes a magnet only when it is placed in a magnetic field. A steel paperclip near a bar magnet becomes an induced magnet.

• The force between an induced magnet and the magnet causing it is always attraction.
• When it is taken out of the field, an induced magnet loses most or all of its magnetism quickly.

A magnetic field is the region around a magnet where a force acts on another magnet or a magnetic material. The field is strongest at the poles and gets weaker further away.

We show a field with field lines:

• They point from the north pole to the south pole. This is the direction of the force on a north pole placed there.
• The closer together the lines, the stronger the field.

Plotting a field: a plotting compass contains a tiny bar magnet, which points along the field.

1. Put the compass near one pole of the magnet and mark where its needle points.
2. Move the compass so its tail is on your mark, and mark again.
3. Repeat, then join the marks to draw a field line.

A compass needle points north even when there are no magnets nearby. It is lining up with the Earth's magnetic field.

This is evidence that the Earth's core is magnetic. The Earth's field has a similar shape to the field around a giant bar magnet.