Magnets, Magnetic Materials and the Properties of Magnets

1Why this matters

Magnets are hidden almost everywhere around you: in the loudspeaker of a phone and a radio, in the motor of an electric moto, in the rubber seal that keeps a fridge door shut, in the needle of a compass, and in the huge cranes that sort iron from other scrap metal. Before you can understand any of these, you need the basic rules that every magnet obeys.

2What is a magnet?

A magnet is an object that attracts iron and some other metals, and that sets itself pointing north–south when it is free to turn. The first magnets people knew were natural: lodestone, a rock made of the iron mineral magnetite. Today's magnets are artificial, made from steel, alnico (an alloy of aluminium, nickel and cobalt with iron), ceramic ferrites, or very strong alloys of neodymium.

3Magnetic and non-magnetic materials

Start with an example. Ishimwe holds a magnet near a pile of objects: an iron nail, a steel paper clip, a steel food tin, an aluminium soda can, a copper wire, a brass key, a plastic pen and a wooden ruler. Only the nail, the paper clip and the tin jump to the magnet.

• A magnetic material is strongly attracted by a magnet and can itself be made into a magnet. The main ones are the metals iron, nickel and cobalt, and alloys containing them, such as steel. (Physicists call these materials ferromagnetic, from the Latin ferrum, iron.)
• A non-magnetic material is not noticeably attracted. Examples: copper, aluminium, brass, gold, silver, zinc, wood, plastic, glass, rubber and paper.

A "tin" can is attracted because it is really steel with a thin coating of tin. Being a metal is not enough: copper and aluminium are metals but are non-magnetic.

4Properties of magnets

1. Poles. Every magnet has two ends where its effect is strongest, called poles. Dip a bar magnet into iron filings and the filings cluster at the two ends, with hardly any in the middle.
2. A freely suspended magnet points north–south. Hang a bar magnet from a thread and it turns until one end points north. That end is called the north-seeking pole, or simply the north pole (N). The other is the south pole (S). A compass needle is just a small magnet free to turn.
3. The law of magnetic poles: like poles repel; unlike poles attract. N pushes N away; N pulls S.
4. Magnets attract magnetic materials, and either pole attracts an unmagnetised piece of iron.
5. Poles always come in pairs. Cut a bar magnet in half and you do not get a separate N and S: you get two complete, smaller magnets, each with its own N and S pole.
6. Magnetic force acts through non-magnetic materials, such as paper, wood, glass, water and your hand.

5The only sure test for a magnet

Worked example. Mutesi has three iron bars, A, B and C, that look identical. She finds that one end of A repels one end of B, and that C is attracted to both ends of A.

Repulsion can only happen between two magnets (like poles), so A and B are both magnets. C is attracted by both poles of A, which is what an unmagnetised magnetic material does, so C is not a magnet.

Attraction proves nothing on its own, because a magnet also attracts plain iron. Repulsion is the only sure test for a magnet.

6Permanent and temporary magnets

An iron nail touching a magnet becomes a temporary magnet itself: it can pick up a chain of paper clips. Pull the magnet away and the chain falls apart. This is induced magnetism.

7Why magnets behave like this: domains

Inside a magnetic material, each atom behaves like a tiny magnet. The atoms line up with their neighbours in small regions called magnetic domains. In an unmagnetised bar, the domains point in random directions and cancel out. When the bar is magnetised, the domains line up in the same direction and their effects add together. This picture explains why cutting a magnet gives two magnets (the domains in each half are still lined up), and why there is a limit to how strong a magnet can become (once every domain is lined up, the magnet is saturated).

Practice