Chapter 4: Electricity and magnetism — Part 1

01 Poles and magnetic materials

A magnet has a north pole (N) and a south pole (S).

• Unlike poles attract (N and S).
• Like poles repel (N and N, or S and S).

Magnetic materials (iron, steel, nickel, cobalt and some alloys) are attracted to a magnet. Non-magnetic materials (for example copper, aluminium, plastic, wood) are not.

A magnetic material can be magnetised (it behaves as a magnet) or unmagnetised (it is still attracted to a magnet but has no poles of its own).

02 Induced, temporary and permanent magnets

Induced magnetism: a magnetic material placed in a magnetic field becomes a magnet. The induced pole nearest the inducing pole is the opposite pole, so the material is attracted.

Temporary magnets (soft iron)

Easy to magnetise and easy to demagnetise. Used as cores in electromagnets and transformers.

Permanent magnets (steel)

Harder to magnetise, but they keep their magnetism. Used as compass needles, fridge magnets and the magnets in motors and loudspeakers.

03 Magnetic fields

A magnetic field is a region in which a magnetic pole experiences a force.

The direction of the field at a point is the direction of the force on the N pole of a magnet at that point (the direction a compass needle points).

Draw field lines around a bar magnet: they leave N and enter S, never cross, and are closer together where the field is stronger.

Plot field lines with a compass (or iron filings to show the pattern). A compass gives the direction.

Uses: permanent magnets (compass, holding, motor/loudspeaker magnets); electromagnets (lifting scrap, relays, electric bells) because they can be switched on and off.