Chapter 4 — Magnetic Effects of Electric Current
A compass needle swings when a current flows in a nearby wire. Electricity makes magnetism: from the field lines of a bar magnet to electromagnets, the force that drives motors, and the safe wiring of your home.
One spring evening in 1820, during a lecture in Copenhagen, Hans Christian Ørsted switched on a current in a wire that happened to lie near a compass — and the needle swung round. Electricity and magnetism had been thought of as two separate subjects; that small twitch of a needle showed they are two faces of one. Within a few years people had built electromagnets that could lift a tonne, and within a lifetime the electric motor, the telegraph and the power station.
Today the same idea is inside almost every machine you use. A fan, a washing machine and an electric scooter all turn because a current-carrying wire in a magnetic field feels a force. A doorbell rings, a hospital scanner sees inside the body and a maglev train floats because a coil carrying a current becomes a magnet. In this chapter you first learn to picture a magnetic field with field lines, then find the field that a current makes round a wire, a loop and a solenoid, then the force that a magnetic field exerts on a current — and finally how the wiring of a house uses fuses, circuit breakers and an earth wire to keep that current safe.
| Everyday question | The idea that answers it | Section |
|---|---|---|
| Why does a compass point north, and how do you map a magnet's field? | magnetic field and field lines | 4.1 |
| Why does a compass near a cable swing when the current is switched on? | field of a current, right-hand thumb rule | 4.2 |
| How does a scrapyard crane let go of its load? | electromagnet: soft iron in a solenoid | 4.2 |
| What makes a loudspeaker cone or a motor move? | force on a current, Fleming's left-hand rule | 4.3 |
| Why does an MCB trip when too many heaters are switched on? | overloading, fuses and circuit breakers | 4.3 |
- state the rules of magnetic poles, and define a magnetic field and its direction using a compass;
- plot field lines with a compass, read their pattern and state their properties (closed curves, never crossing, crowded where the field is strong);
- describe the field round a straight current-carrying wire and find its direction with the right-hand thumb rule;
- describe the fields of a circular coil and a solenoid, name the poles of a coil, and explain how an electromagnet works;
- predict the direction of the force on a current-carrying conductor in a magnetic field with Fleming's left-hand rule, and say what the size of the force depends on;
- explain the live, neutral and earth wires of a house circuit, the role of the fuse and MCB, and the dangers of overloading and short-circuiting;
- distinguish AC from DC and state the advantage of AC over DC;
- 📘 (school assessment only) explain how an electric motor works, describe electromagnetic induction with Fleming's right-hand rule, and tell an AC generator from a DC generator.
Sections in this chapter
Magnets, Magnetic Fields and Field Lines
Poles and the rules they obey, the magnetic field and its direction, plotting field lines with a compass and with iron filings.
Magnetic Field due to a Current: Wire, Loop and Solenoid
Ørsted's discovery, the right-hand thumb rule, the fields of a loop and a solenoid, and electromagnets.
Force on a Current-Carrying Conductor and Domestic Circuits
The push on a wire in a magnetic field, Fleming's left-hand rule, AC and DC, and live, neutral and earth wires, fuses and MCBs at home — plus the motor and generator for school assessment.