Key Stage 4 science · Physics – Magnetism and electromagnetism

GCSE magnetic fields, solenoids and transformers: a guide

Magnetism and electric current are closely linked, and this topic is where they meet. At GCSE, students map the magnetic fields of permanent and induced magnets and of the Earth using a compass, learn that a current in a wire creates a magnetic field that a coil called a solenoid strengthens, and see how transformers use this to change voltage so electricity can be sent across the country without wasting most of it as heat.

Magnetic fields and transformers, explained

A magnetic field is the region around a magnet where another magnet or a piece of magnetic material feels a force. It is drawn with field lines that run from the north pole to the south pole outside the magnet. Where the lines are close together the field is strong, which for a bar magnet is at the poles. Students explore these fields with a plotting compass: the needle is a tiny magnet that lines up with the field, so moving the compass step by step and marking where it points traces out a field line. The same method shows that the Earth has a magnetic field, as if a huge bar magnet were inside it, which is why a compass needle settles to point north.

A permanent magnet keeps its magnetism. An induced magnet is a piece of magnetic material, such as iron, that becomes a magnet only while it is in another magnet’s field, and loses most of that magnetism when the field is taken away. This is why a magnet can pick up a paper clip and the clip can then pick up another. It also explains why a magnet always attracts an unmagnetised piece of iron but can either attract or repel another magnet. The magnetic materials students meet are iron, steel, cobalt and nickel.

In 1820 it was discovered that an electric current makes a magnetic field. Around a straight wire the field lines are circles, and the field gets stronger with a bigger current and weaker as you move away from the wire. If the wire is wound into a coil called a solenoid, the fields from each turn add together. Inside the coil the field is strong and nearly uniform, and outside it looks like the field of a bar magnet. The effect is made stronger by increasing the current, adding more turns, or putting a soft iron core inside. Unlike a permanent magnet, an electromagnet can be switched on and off.

A transformer is two coils wound on the same iron core. An alternating current in the primary coil produces a constantly changing magnetic field in the core, and this induces an alternating p.d. in the secondary coil. The ratio of the p.d.s matches the ratio of the turns: more turns on the secondary makes a step-up transformer, fewer makes a step-down. In the national grid, step-up transformers raise the p.d. to hundreds of thousands of volts for transmission and step-down transformers lower it again near homes. A high p.d. means a low current for the same power, and a low current means much less energy is wasted heating the cables. Transformers only work with a.c., because a steady current gives a steady field and induces nothing. Depending on the course, students may also use the equation for turns and p.d.s.

Words your child will hear

Magnetic field: The region around a magnet where magnetic materials and other magnets feel a force.
Induced magnet: A magnetic material that acts as a magnet only while it is in another magnet’s field.
Solenoid: A long coil of wire that makes a strong magnetic field when a current passes through it.
Electromagnet: A solenoid, usually with an iron core, that is magnetic only while the current is on.
Transformer: Two coils on an iron core that change the size of an alternating p.d.
Step-up and step-down: A step-up transformer raises the p.d.; a step-down transformer lowers it.
Primary and secondary coil: The input coil and the output coil of a transformer.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about exploring the magnetic fields of permanent and induced magnets, and the Earth's magnetic field, using a compass
Students should be taught about magnetic effects of currents, and how solenoids enhance the effect
Students should be taught about how transformers are used in the national grid and the reasons for their use

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

A worked example

A power station generates at 1000 V and sends 100 kW of power through cables with a total resistance of 5 Ω. A step-up transformer with 500 turns on its primary coil and 50 000 turns on its secondary coil is used. Compare the power wasted in the cables with and without the transformer. Assume the transformer is 100% efficient.

Find the p.d. after the transformer. The p.d.s are in the same ratio as the turns.
```
Vs ÷ Vp = Ns ÷ Np
Vs = 1000 × 50 000 ÷ 500
   = 100 000 V
```
Find the current without the transformer, at 1000 V, using P = I × V.
```
I = P ÷ V
  = 100 000 ÷ 1000
  = 100 A
```
Find the power wasted in the cables, using P = I² × R.
```
P = 100 × 100 × 5
  = 50 000 W
```
Repeat at 100 000 V. The current is much smaller, and because it is squared the waste falls dramatically.
```
I = 100 000 ÷ 100 000
  = 1 A
P = 1 × 1 × 5
  = 5 W
```

Without the transformer 50 kW (half the power) is wasted heating the cables. With it, only 5 W is wasted. Raising the p.d. 100 times cuts the current 100 times and the heating loss 10 000 times.

Common mistakes and how to help

Magnets attract all metals

Everyday experience of fridge magnets and steel tins makes it seem as if any metal sticks.

What helps: Only a few metals are magnetic: iron, steel, cobalt and nickel. Test coins, foil and a copper pipe with a magnet and ask what they have in common.

The Earth’s magnetic pole in the far north must be a magnetic north pole

The compass needle’s north-seeking end points to the north, so it seems the pole there must also be a north pole.

What helps: Opposite poles attract. The needle’s north end is attracted towards a magnetic south pole of the Earth’s field, which happens to lie near the geographic north. Drawing the Earth as a bar magnet with the labels the right way round settles it.

A transformer creates more power

“Step-up” sounds like a gain, and the p.d. does increase, so students assume something is multiplied for free.

What helps: The p.d. rises but the current falls by the same factor. Power out can never be more than power in, and real transformers are very efficient but not perfect. The benefit is lower current, so less heating loss.

Transformers work with batteries

Students know a battery supplies a p.d. and don’t connect the working to the changing field.

What helps: A transformer needs a changing magnetic field, which only an alternating current gives. A battery makes a steady field that induces nothing in the secondary. Ask what happens at the moment a switch is closed, and what happens after.

How to help at home

Plot a field with a compass. Place a bar magnet or fridge magnet on a sheet of paper and set a small compass beside it. Mark the needle’s two ends, move the compass along, and join the dots to draw a field line. Repeat from several starting points, then remove the magnet and see which way the compass points on its own.
Test which things are magnetic. Collect around ten household objects, predict which a magnet will pick up, then check. Try to make a paper clip temporarily magnetic by stroking or touching it with the magnet, and see if it picks up a second clip.
Spot the transformers. A phone or laptop charger contains a small transformer, and many appliances hum when running because of the changing magnetic field. When you pass a line of pylons, ask your teenager why the cables carry such a high p.d., and when you pass a local electricity substation, ask what change it makes before the supply reaches homes.
Make a simple electromagnet with an adult. Wind insulated wire around a large iron nail and connect it briefly to a single AA battery. It should pick up small paper clips. Never use mains electricity, and stop at once if the wire or the battery becomes warm, because a short circuit can heat quickly.

Builds on

Key Stage 3: Current, voltage and resistance
Key Stage 3: Static electricity and electric fields
Key Stage 3: Magnetism and electromagnets

Everything in Key Stage 4 science

Updated 20 October 2026.