Magnetism

Magnets and poles

A magnet is an object that attracts certain metals and has a magnetic field around it.

• Every magnet has two ends called poles: a north (N) pole and a south (S) pole.
• Like poles repel: N pushes N away, S pushes S away.
• Unlike poles attract: N and S pull together.
• If you cut a magnet in half, you get two smaller magnets, each with its own N and S. You can never get a pole on its own.

Magnetic materials are attracted to magnets. The main ones are iron, nickel and cobalt, and steel (which is mostly iron). Copper, aluminium, gold, plastic and wood are not magnetic.

Magnetic fields

A magnetic field is the region around a magnet where a magnetic force acts. We draw it with field lines.

• Field lines go from N to S outside the magnet.
• Lines never cross.
• Where lines are closer together, the field is stronger. This is near the poles.

You can see a field by sprinkling iron filings around a magnet, or by moving a small plotting compass around it.

Earth is a giant magnet

Earth's core of molten iron makes Earth act like a huge bar magnet. A compass needle is a small magnet. Its north pole points towards Earth's geographic North Pole.

Since unlike poles attract, this means the magnetic pole near the geographic North Pole is actually a magnetic south pole. Earth's magnetic field also protects us by steering charged particles from the Sun away from the planet. Where some get through near the poles, they make the glowing auroras.

Electromagnets

An electric current in a wire makes a magnetic field around it. Coil the wire around an iron nail and you have an electromagnet.

You can make an electromagnet stronger by:

1. Increasing the current
2. Adding more turns of wire to the coil
3. Using a soft iron core inside the coil

The big advantage: you can switch it off. Scrapyard cranes use electromagnets to lift cars and drop them. Electromagnets are also in electric bells, speakers, MRI machines and door locks.

Quick summary

• Magnets have N and S poles. Like poles repel, unlike poles attract.
• Iron, nickel and cobalt are magnetic. Most other metals are not.
• Field lines go from N to S. Closer lines mean a stronger field.
• Earth behaves like a giant bar magnet, which is why compasses work.
• A current in a wire makes a magnetic field. A coil with an iron core makes a strong electromagnet that can be switched off.
• Electromagnets get stronger with more current, more turns and an iron core.

Quiz