Key Stage 3 science · Physics – Electricity and electromagnetism

KS3 magnetism and electromagnets: a guide for parents

In Key Stage 3, magnetism moves beyond “magnets stick to fridges”. Pupils learn about poles and the forces between them, plot magnetic fields with a compass and draw them as field lines, and find out why a compass works by learning about Earth’s own magnetism. They then see how an electric current creates a magnetic field, how electromagnets are built and controlled, and the basic principle of a d.c. motor.

Magnetism and electromagnets, explained

Every magnet has two poles, a north-seeking pole and a south-seeking pole, where its pull is strongest. Like poles repel and unlike poles attract. Only a few materials are magnetic, mainly iron, steel, nickel and cobalt. Most other metals, such as copper, aluminium and gold, are not, and non-metals are not either. A magnet attracts a piece of iron by making it temporarily a magnet too, so the iron is attracted to either pole. This is why repulsion is the only certain test for whether something is a magnet.

A magnetic field is the region around a magnet where another magnet or magnetic material feels a force without touching it. Pupils plot a field with a small plotting compass: they place the compass near the magnet, mark the ends of the needle on paper, move the compass so its back end sits on the last mark, and repeat. Joining the marks gives a field line. Outside the magnet, the lines run from the north pole to the south pole, and they are drawn closer together where the field is stronger. The field exists everywhere around the magnet, not only along the lines drawn.

A compass needle is a small magnet free to turn. It lines up with Earth’s magnetic field, which behaves as though there were a giant bar magnet inside the planet, caused by the movement of molten metal in the core. The end of the needle that points north is a north-seeking pole, which is attracted to a magnetic south pole of Earth’s field that lies near the geographic North Pole. Compasses help walkers and sailors to navigate when they are used with a map, but they are thrown off by nearby magnets and iron. Magnetic north is also slightly different from true north and slowly shifts.

An electric current produces a magnetic field around the wire carrying it. Winding the wire into a coil concentrates the field, and putting an iron core inside the coil makes it much stronger. This is an electromagnet. Its strength increases with more current and more turns of wire. It can be switched on and off, and its poles reverse if the current is reversed. These features make electromagnets useful in cranes that lift scrap metal, electric bells, door locks and relays, as well as the loudspeakers in the sound topic.

Pupils meet the principle of a d.c. motor, but not the maths. A wire carrying a current in a magnetic field feels a force. In a motor a coil sits between the poles of a magnet, with the current flowing one way up one side and the other way down the opposite side, so the two sides are pushed in opposite directions and the coil turns. A part called a split-ring commutator swaps the current’s direction every half turn, so the coil keeps spinning the same way. A bigger current, a stronger magnet or more turns makes a stronger turning effect.

Words your child will hear

Pole: One of the two ends of a magnet where its magnetic effect is strongest: north-seeking and south-seeking.
Magnetic material: A material attracted to magnets, mainly iron, steel, nickel and cobalt.
Magnetic field: The region around a magnet where magnetic forces act.
Field lines: Lines with arrows showing the direction of a magnetic field, closer together where it is stronger.
Plotting compass: A small compass used to find the direction of a magnetic field at different points.
Electromagnet: A coil of wire, usually round an iron core, that is magnetic only while a current flows.
Commutator: The split ring in a d.c. motor that reverses the current every half turn so the coil keeps turning.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught about magnetic poles, attraction and repulsion
Pupils should be taught about magnetic fields by plotting with compass, representation by field lines
Pupils should be taught about Earth's magnetism, compass and navigation
Pupils should be taught about the magnetic effect of a current, electromagnets, D.C. motors (principles only)

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

Try it: planning a fair test on electromagnet strength

A pupil wants to find out how the number of turns of wire on an electromagnet affects how many paper clips it can pick up. How should she plan the test and what should she conclude?

Build the electromagnet by winding insulated wire round an iron nail and connecting it to a cell or a low-voltage supply through a switch.
Decide the variables. Change only the number of turns (10, 20, 30, 40 and 50). Measure the number of paper clips picked up. Keep the same nail, the same cell, the same length of wire (leaving any spare unwound), the same size of paper clips and the same way of dipping them.
Collect the results. Switch on only briefly, because the wire and the cell get warm. Repeat each setting three times and take the mean. These are sample results; real ones are usually less tidy.
```
Turns | Paper clips (mean)
 10   | 2
 20   | 5
 30   | 7
 40   | 10
 50   | 12
```
Describe the pattern. As the number of turns goes up, the number of paper clips goes up in a fairly regular way, roughly 2 or 3 more clips for each extra 10 turns.
Evaluate the test. If extra wire had been added to make more turns, the resistance would rise and the current would drop a little, which is why the length of wire is kept the same. A cell also runs down as it is used. Better control would use a low-voltage power supply with a fixed setting, an ammeter to check the current stays the same, and the same switch-on time for each test.

More turns make a stronger electromagnet, which picks up more paper clips. The claim is supported by the results, provided the current was kept the same throughout.

Common mistakes and how to help

Thinking magnets attract all metals

Fridge magnets and steel cans make it seem as if magnets stick to metal in general.

What helps: Test a coin, aluminium foil, a copper pipe, a steel fork and an iron nail. Only the magnetic materials are attracted. Many coins are made partly of steel, so results vary, which makes a good discussion.

Believing cutting a magnet in half gives one north-only and one south-only piece

It seems natural that the two ends are separate things that can be split.

What helps: Each piece becomes a smaller magnet with its own north and south pole. You can never isolate a single pole. Tiny regions inside the material line up with each other.

Thinking field lines are physical things and a magnet only works along them

Diagrams show a few neat lines, and children read them as the whole story.

What helps: The lines are a drawing tool. The field exists in the spaces in between, and a compass placed anywhere nearby will turn. Closer lines mean a stronger field.

Believing the end of Earth near the North Pole is a magnetic north pole

The compass points “north”, so Earth’s north is assumed to be the same kind of pole.

What helps: A compass’s north-seeking end is attracted to an unlike pole. Test it near a bar magnet: the needle’s north end points at the magnet’s south pole.

How to help at home

Magnet or not? Use a fridge magnet to test cutlery, coins, foil, paper clips, drink cans and keys. Sort them into magnetic and non-magnetic. Avoid very strong small magnets, which can pinch and are dangerous if swallowed, and keep magnets away from bank cards and phones.
Plot a field. Lay a bar or fridge magnet on a sheet of paper and move a small compass around it, marking where the needle points. Join the marks to draw field lines. Do it again with the magnet turned round and compare.
Find north. Take a compass to the garden or a park and work out which way is north. Then see how the needle behaves close to a metal gate or a parked car. In the UK the Sun is roughly south in the middle of the day, so you can check against it.
Make an electromagnet. With an adult, wind about a metre of insulated wire round a long iron nail and connect the ends to a single AA cell for a few seconds at a time. Pick up paper clips and try more turns. The wire and cell can get warm, so keep contact short.
A compass treasure hunt. Hide an object in the garden or park and write directions such as “20 paces north, then 10 paces east”. The searcher uses a compass to follow them. It is a good way to practise using Earth’s magnetism for navigation.

Builds on

Year 6: Circuit components and symbols

Leads on to

Key Stage 4: Current, resistance, charge and circuits
Key Stage 4: Mains electricity and power
Key Stage 4: Magnetic fields and transformers