Describe the properties of magnets and magnetic materials, the behaviour of poles, and how an electromagnet is made and used

What this dot point is asking

This dot point wants you to describe how magnets behave, which materials are magnetic, and how electricity can be used to make a magnet called an electromagnet. The big idea is that magnets attract certain metals and have two poles that attract or repel each other, and that an electric current flowing through a coil of wire makes a magnet that can be switched on and off. You should be able to explain how to make an electromagnet stronger and give everyday uses.

The answer

What a magnet does

A magnet attracts (pulls toward it) certain materials. Only a few materials are magnetic, meaning they are attracted to a magnet: iron, steel, nickel and cobalt. Most other materials, such as plastic, wood, copper, aluminium and glass, are not magnetic and are not attracted.

The pulling effect of a magnet works through a small distance, even without touching, and is strongest at the ends of the magnet.

Poles of a magnet

Every magnet has two ends called poles: a north pole and a south pole. The poles are where the magnet is strongest.

The rule for poles is important:

• Opposite poles attract: a north pole and a south pole pull toward each other.
• Like poles repel: two north poles (or two south poles) push apart.

This is why one magnet can pull another toward it or push it away, depending on which poles face each other. A magnet that is free to turn, like a compass needle, lines up pointing north and south, which is how a compass works.

What an electromagnet is

An electromagnet is a magnet made using electricity. You make one by winding a coil of wire around a piece of iron (such as an iron nail) and connecting the coil to a cell so a current flows.

When the current flows, the iron becomes a magnet and can pick up magnetic objects like pins. The big advantage is that an electromagnet can be switched on and off: switch the current off and the iron loses its magnetism and drops whatever it was holding. This is something an ordinary (permanent) magnet cannot do.

Making an electromagnet stronger

You can make an electromagnet stronger in three ways:

• Increase the current (for example, by using more cells).
• Add more turns of wire to the coil.
• Use a larger or better iron core inside the coil.

Uses of electromagnets

Because they can be switched on and off, electromagnets are very useful:

• A scrapyard crane uses a huge electromagnet to pick up iron and steel scrap, then switches off to drop it where it is needed.
• An electric bell uses an electromagnet to make the hammer hit the bell over and over.
• Sorting machines use electromagnets to separate magnetic metals from other materials in recycling.

Examples in context

Example 1. A scrapyard crane. At a scrapyard, a crane fitted with a giant electromagnet swings over a pile of mixed scrap. Switch the current on and the electromagnet lifts tonnes of iron and steel; swing over a railway truck and switch off, and the metal drops in. The on-off control is exactly why an electromagnet is used instead of a permanent magnet.

Example 2. A fridge door and fridge magnets. The fridge door seals shut because a permanent magnet in the seal is attracted to the steel of the door frame. The fun magnets that hold notes onto the door work the same way: they are permanent magnets sticking to the magnetic steel. They cannot be switched off, unlike an electromagnet.

Try this

• Cue. State what happens when the south pole of one magnet is brought close to the south pole of another. They repel (push apart), because like poles repel.

• Cue. Name two magnetic materials and one material that is not magnetic. Magnetic: iron and steel (also nickel, cobalt). Not magnetic: plastic (also wood, copper, aluminium, glass).

• Cue. Explain why an electromagnet is used in a scrapyard crane instead of an ordinary magnet. An electromagnet can be switched on to pick up the metal and switched off to drop it exactly where wanted, which an ordinary magnet cannot do.