Magnets and magnetic fields

Magnetic materials

A magnet attracts some metals but not others. The magnetic materials are iron, nickel and cobalt, and many mixtures of metals (alloys) that contain them, especially steel, which is mostly iron. A magnet won't pick up aluminium cans, copper wire, gold, plastic, wood or paper. A quick way to sort recycling: steel cans stick to a magnet, aluminium drink cans don't.

Different kinds of magnets suit different jobs:

• Alnico magnets (aluminium, nickel and cobalt with iron) are the red-and-silver bar magnets in school labs. They can lose some magnetism if dropped or heated.
• Ferrite magnets (iron oxide mixed with other elements) are cheap and light: fridge magnets and the magnets in speakers.
• Rare earth magnets, made with neodymium, are the strongest permanent magnets. A tiny one can hold a stack of paper to a fridge. They're used in earbuds, phone speakers, cordless drills, electric car motors and wind turbine generators. They are brittle and are coated to protect them.

Poles: attract and repel

A magnet's force is strongest at its ends, called poles. Every magnet has a north pole and a south pole. If you hang a bar magnet from a thread, it swings until its north pole points north, which is how the poles got their names.

• Unlike poles attract: N and S pull together.
• Like poles repel: N and N, or S and S, push apart.

Only a magnet can repel another magnet. A piece of plain iron is always attracted to either pole, so the only sure test for a magnet is repulsion. And you can never get a magnet with only one pole: snap a bar magnet in half, and you get two smaller magnets, each with a north and a south pole.

Magnetic fields

Around every magnet is a magnetic field, the region where its force acts. You can't see or feel it, but you can map it:

• Iron filings sprinkled on paper over a magnet line up like tiny compass needles and show the pattern of the field.
• A small plotting compass points along the field wherever you put it. Mark the direction, move the compass along, and you can trace a field line.

Field lines are drawn from the north pole to the south pole. Where the lines are close together, near the poles, the field is strongest. Magnetic fields are three-dimensional: they spread out in all directions around a magnet, not just across the page.

The Earth is a giant magnet

The Earth has a magnetic field, made by electric currents in its liquid iron outer core. It stretches tens of thousands of kilometres into space, where it's called the magnetosphere.

• A compass is a small magnet that lines up with the Earth's field, so its north end points to the Earth's magnetic north pole. The first compasses, used in China more than 2000 years ago, were spoons carved from a naturally magnetic rock called lodestone. By about 1000 years ago, sailors were navigating with them.
• The magnetic north pole is not at the geographic North Pole, and it wanders. Since the 1990s it has moved from northern Canada towards Siberia, at times more than 50 km a year, so maps and phone compasses must be updated regularly.
• The magnetosphere shields us from the solar wind, a stream of charged particles from the Sun. Some of these particles are steered down near the poles, where they make the upper atmosphere glow: the aurora australis (southern lights) and the aurora borealis (northern lights).
• Every few hundred thousand years, the Earth's field flips, so north and south swap. The last flip was about 780 000 years ago. Scientists know this from the direction of magnetism frozen into ancient volcanic rocks.

Many animals use the Earth's field to find their way, including sea turtles, migrating birds and, it seems, Australia's Bogong moths, which fly up to 1000 km each spring from Queensland and New South Wales to caves in the Australian Alps, using the stars and possibly the magnetic field to steer.