Permanent and induced magnetism, magnetic forces and fields

Key content

Poles of a magnet

• The poles of a magnet are the places where the magnetic forces are strongest.
• When two magnets are brought close together they exert a force on each other:
  • Like poles (N–N or S–S) repel.
  • Unlike poles (N–S) attract.
• Attraction and repulsion between magnetic poles are examples of non-contact forces.

Permanent and induced magnets

• A permanent magnet produces its own magnetic field.
• An induced magnet is a material that becomes a magnet when it is placed in a magnetic field. Induced magnetism always causes attraction. When removed from the field, an induced magnet loses most or all of its magnetism quickly.

Magnetic fields

• The magnetic field is the region around a magnet where a force acts on another magnet or on a magnetic material (iron, steel, cobalt and nickel).
• The force between a magnet and a magnetic material is always attraction.
• The field is strongest at the poles, and its strength decreases with distance from the magnet.
• The direction of the field at any point is the direction of the force on a north pole placed there. Field lines go from the north (seeking) pole to the south (seeking) pole.
• The closer together the field lines, the stronger the field.
• Bar magnet field pattern: the lines leave the N pole, curve round the magnet and enter the S pole, with arrows pointing from N to S. The lines never cross; they are closest together at the poles (strongest field) and spread out further away (weaker field).

Plotting a field with a compass: a magnetic compass contains a small bar magnet. Place the compass near the N pole of the magnet and mark the point the north end of the needle points to. Move the compass so its tail is on that mark, and repeat until you reach the S pole. Join the dots to draw a field line; repeat from different starting points.

The Earth has a magnetic field: a compass needle points in the direction of the Earth's magnetic field. Away from any magnet, a freely suspended compass lines up in the same direction, roughly north–south, all over the Earth. The Earth's field has the same shape as the field of a giant bar magnet inside it, so this is evidence that the Earth's core is magnetic.

Common misconceptions