Permanent Magnets

Magnets always have two poles called the north pole and the south pole. In a bar magnet the poles are close to the ends of the magnet.

A magnet can exert a force on another magnet (or a magnetic material) even when the two magnets are not in contact. The rule for the direction of the force is usually summarized as:

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

The space around the magnet where this force is exerted is called a magnetic field.

Magnetic fields are invisible, but one way show them is to place a sheet of card over a magnet and sprinkle iron filings on the card. If you then gently tap the card, the iron filings arrange themselves in the pattern show above.

We can represent a magnetic field as a set of lines called field lines, as shown in the animation. Choose different combinations of magnets and see how the pattern of field lines changes.

Field lines obey these rules:

• The spacing between the lines represents the strength of the field.
• The arrows on the lines show the direction of the field. This is the direction that a compass would point if placed in the field. If you drag the mouse on the left hand screen, you can see this.
• The field lines cannot cross each other.
• For magnets, the field lines start on a north pole and finish on a south pole.