Describe the properties of magnets and magnetic fields, explain how an electromagnet works, and state how its strength can be increased

What this dot point is asking

SEAB wants you to describe how magnets behave, the shape of a magnetic field, which materials are magnetic, and how an electric current can be used to make an electromagnet whose strength you can control. The central idea is that an electric current always produces a magnetic field, which links electricity and magnetism.

The answer

Magnetic poles

Every magnet has two poles, a north pole and a south pole. The rule for how they interact is:

• like poles repel (north and north, or south and south push apart),
• unlike poles attract (north and south pull together).

A freely hanging magnet always settles pointing north to south, which is how a compass works.

Magnetic materials

Only a few materials are magnetic (can be attracted to a magnet or made into one): iron, steel, nickel and cobalt. Most metals, such as copper and aluminium, are not magnetic. We split magnetic materials into:

• soft magnetic materials (such as soft iron), which magnetise easily but lose their magnetism easily, ideal for electromagnets,
• hard magnetic materials (such as steel), which are harder to magnetise but keep their magnetism, ideal for permanent magnets.

Magnetic fields

The region around a magnet where it can affect a magnetic material is its magnetic field. We draw it with field lines that:

• come out of the north pole and go into the south pole,
• are closer together where the field is stronger (near the poles).

Electromagnets

When a current flows through a coil of wire, it produces a magnetic field, turning the coil into a magnet called an electromagnet. Adding a soft iron core through the middle makes it much stronger. The big advantage of an electromagnet is that it can be switched on and off, and its strength can be changed.

You can make an electromagnet stronger by:

• increasing the current,
• increasing the number of turns on the coil,
• adding a soft iron core.

Examples in context

Example 1. A scrapyard crane. A crane lifts cars and scrap iron with a large electromagnet. Switching the current on creates a strong field that grips the metal; switching it off releases the load. A permanent magnet could not let go on command, which is why an electromagnet is used.

Example 2. An electric doorbell. When you press the button, current flows through an electromagnet that pulls a small hammer to strike the bell. The movement breaks the circuit, the electromagnet switches off, a spring pulls the hammer back, and the cycle repeats, ringing the bell.

Try this

• Cue. State what happens when a north pole is brought near a south pole. They attract each other.
• Cue. Name two ways to increase the strength of an electromagnet. Increase the current and increase the number of turns on the coil.
• Cue. Explain why soft iron, not steel, is used for an electromagnet core. Soft iron loses its magnetism when the current is switched off, so the magnet can be turned off.