Magnetism and Electromagnets

La méthode, étape par étape

1. 1

   Apply the pole rule

   like poles repel · unlike poles attract

   Two north poles push apart; a north and a south pull together. This is the basis of every magnetism question, and it holds regardless of the magnets' size or strength.

2. 2

   Draw field lines from north to south

   arrows leave the north pole and enter the south

   The direction is a convention and it is examined. Field lines never cross, and closer lines indicate a stronger field — which is why they bunch together at the poles.

3. 3

   Distinguish magnetic from magnetised

   iron, nickel, cobalt are magnetic materials

   Only a few metals are magnetic. Most metals, including aluminium and copper, are not attracted to magnets at all, which surprises students who expect 'metal' and 'magnetic' to mean the same thing.

4. 4

   Recognise that a current creates a field

   current through a wire → circular magnetic field around it

   This is the link between electricity and magnetism. Coiling the wire concentrates the field, and the coil then behaves like a bar magnet with a north and a south end.

5. 5

   Strengthen an electromagnet

   more current · more turns · an iron core

   All three increase the field strength, and questions usually expect at least two. The iron core is the most effective single change, because iron concentrates the field far more than air does.

Permanent magnets and electromagnets

A permanent magnet is always magnetic and cannot be switched off. An electromagnet is magnetic only while current flows, which makes it controllable — and that controllability is the whole reason electromagnets are useful.

Scrapyard cranes, electric bells, relays, loudspeakers and electric motors all depend on being able to turn a magnetic field on and off, or to reverse it. A permanent magnet cannot do any of those jobs.

• Like poles repel, unlike attract
• Field lines run north to south and never cross
• Magnetic materials: iron, nickel, cobalt
• A current creates a magnetic field
• Stronger electromagnet: more current, more turns, iron core

Most metals are not magnetic

Iron, nickel and cobalt are the common magnetic metals, along with steel because it is mostly iron. Aluminium, copper, gold and silver are not attracted to magnets at all, despite being metals and conducting electricity well.

This is a useful practical test and a frequent exam question. It also explains why aluminium drink cans are separated from steel ones by magnets during recycling.

The Earth as a magnet

The Earth has its own magnetic field, which is why a compass needle points north. The needle's north pole is attracted towards the Earth's geographic north, which means the Earth's magnetic pole up there is technically a south pole.

This inversion is a favourite exam question and follows directly from the rule that unlike poles attract. It is worth working through once rather than memorising as a curiosity.

How we teach magnetism

We connect magnetism to electricity as early as possible, because the two are usually taught apart and the interesting content is in the link. An electromagnet is the clearest demonstration that a current produces a field.

We also test the field-line conventions explicitly — direction, non-crossing, spacing indicating strength. These carry marks on diagram questions and are frequently lost by students whose physics is otherwise sound.