AQA GCSE Physics 4.7 Magnetism and electromagnetism: a complete overview of magnets, electromagnets, the motor effect and transformers

What topic 4.7 actually demands

Magnetism and electromagnetism builds from simple bar magnets to the machines that generate and distribute electricity. AQA tests field-pattern reasoning, Fleming's left-hand rule, the force and transformer calculations, and clear explanations of the motor, the generator and the transformer.

This guide walks through all four dot points of the topic, then sets out the exam patterns AQA repeats. Each dot point has a matching page with practice questions; this overview ties them together.

Magnets and magnetic fields

A permanent magnet always produces its own field; an induced magnet is magnetic only in a field and loses it when removed (always causing attraction). Like poles repel, unlike poles attract, a non-contact force. Field lines run from north to south outside the magnet and are closest at the poles. The magnetic materials are iron, steel, cobalt and nickel, and a compass shows the Earth's magnetic field.

Electromagnetism

A current creates a circular field around a wire. A solenoid concentrates this into a strong, uniform field like a bar magnet, and adding an iron core makes an electromagnet. The field is strengthened by more current, more turns, or an iron core, and the electromagnet can be switched on and off.

The motor effect

A current-carrying conductor in a magnetic field feels a force, the motor effect, of size $F = BIl$. Fleming's left-hand rule (First finger Field, seCond finger Current, thuMb Motion) gives its direction. In an electric motor, opposite forces on the two sides of a coil make it spin, and a split-ring commutator reverses the current each half turn.

Induced potential and transformers

Electromagnetic induction (the generator effect) induces a potential difference when a conductor and field move relative to each other or the field changes. Generators (alternator for a.c., dynamo for d.c.) use this. A transformer changes an alternating voltage via two coils on an iron core, with $\frac{V_p}{V_s} = \frac{n_p}{n_s}$ and, for an ideal transformer, $V_p I_p = V_s I_s$.

How topic 4.7 is examined

A typical AQA profile for Magnetism and electromagnetism:

• Field patterns. Drawing and interpreting the fields of bar magnets, wires and solenoids.
• Rules. Applying Fleming's left-hand rule to find the direction of the force.
• Calculations. $F = BIl$ and the transformer equations.
• Extended answers. Explaining how the electric motor, the generator and the transformer work.

Check your knowledge

A mix of recall and calculation questions covering topic 4.7. Attempt them under timed conditions, then check against the solutions.

1. State what happens when two unlike magnetic poles are brought together. (1 mark)
2. Explain the difference between a permanent and an induced magnet. (2 marks)
3. Describe the magnetic field around a straight current-carrying wire. (1 mark)
4. Give two ways to increase the strength of an electromagnet. (2 marks)
5. State the equation for the force on a current-carrying conductor in a magnetic field. (1 mark)
6. A wire of length $0.4\,m$ carries $5\,A$ in a field of $0.2\,T$. Calculate the force on it. (2 marks)
7. Explain why a transformer only works with alternating current. (2 marks)
8. A transformer has $100$ primary turns and $500$ secondary turns, with $V_p = 20\,V$. Calculate $V_s$. (3 marks)