IGCSE Physics Magnetism & EM effects notes

Every magnet has a north pole and a south pole. Like poles repel; unlike poles attract. Magnetic materials — iron, steel, cobalt, nickel — are attracted by magnets; non-magnetics (plastic, copper, wood) are not. Bring a magnet near an unmagnetised iron nail and the nail itself becomes a temporary magnet, attracting further pins: this is induced magnetism.

Magnet hardness matters. Soft iron magnetises and demagnetises easily — perfect for electromagnet cores, which must switch on and off. Steel holds its magnetism once gained — permanent magnets are steel (or similar hard alloys).

A magnetic field is a region in which a magnetic pole experiences a force. Field lines run (by convention) from the N pole to the S pole, and the direction of the field at a point is the direction of the force on a north pole placed there. Plot the field of a bar magnet by scattering iron filings on a sheet over the magnet, or by walking a small plotting compass around it and marking needle directions. Supplement: the closer the field lines are drawn together, the stronger the field — they crowd near the poles. And magnetic forces always come from interactions between magnetic fields (Supplement), whether between two magnets or between a magnet and a current’s own field.

Uses: permanent magnets hold compasses, fridge doors, loudspeaker magnets and electric motors’ fields; electromagnets — a coil around soft iron, switched by the current — power scrapyard cranes, electric bells, relays and magnetic locks, because they can be turned on and off.

A current in a straight wire makes circular magnetic field lines around it; a solenoid (a long coil) concentrates them into a pattern just like a bar magnet, with N and S ends. Plot either with a compass or iron filings, with the circuit switched on. Supplement: a larger current makes the field stronger everywhere; reversing the current reverses the field direction (poles swap ends). Adding an iron core into a solenoid hugely strengthens the field — that assembly is the electromagnet.

Two electromagnetic devices in the syllabus: the relay, a small-current circuit electromagnet that pulls a switch closed in a separate high-current circuit — letting a tiny signal turn on a motor or a heater safely; and the loudspeaker, where a changing current in a coil inside a magnet’s field pushes the cone in and out, vibrating the air to make sound.

Move a wire across magnetic field lines (or change the field linking a stationary coil) and an e.m.f. is induced in the wire; connect it into a circuit and a current flows. Demonstrate with a coil, a centre-zero meter and a bar magnet: push the magnet in — the meter kicks one way; hold still — no deflection; pull out — it kicks the other way. The e.m.f. grows with faster motion, a stronger magnet and more turns on the coil. Supplement: the induced e.m.f. always opposes the change causing it — pushing the magnet in creates a pole that repels it, which is why you always have to do work.

Supplement: an a.c. generator is induction made continuous. A coil rotates in a magnetic field; slip rings and brushes carry the induced current out without the wires tangling. Each time the coil passes through the plane where its sides move parallel to the field, the e.m.f. is zero; maximum e.m.f. occurs when the coil sides cut the field most directly — so the e.m.f.–time graph is a sine wave, one full cycle per revolution. Faster rotation raises both the frequency and the peak e.m.f.

Put a current-carrying wire inside a magnetic field and it feels a force at right angles to both the current and the field. Demonstrate with a wire resting between magnet poles on a supply: switching on makes the wire jump; reversing the current or reversing the field reverses the force; reversing both leaves it unchanged. Supplement: the three directions — force, field, current — are mutually perpendicular (the left-hand rule arrangement), and the same rule gives the direction of the force on a beam of charged particles moving through a field.

A d.c. motor is a coil in a magnetic field carrying current: one side is pushed up, the other down — a turning effect, made larger by more turns, a larger current or a stronger field. Supplement: the catch is that after half a turn the forces would push the coil backwards; the split-ring commutator swaps the current’s direction through the coil every half revolution (pressing against brushes), so the pushes keep the coil rotating the same way — that is how electric motors, fans and drills spin steadily.

A transformer is two coils — primary and secondary — wound on a shared soft-iron core. An alternating current in the primary makes a continuously changing magnetic field in the core; the field links the secondary and induces an alternating e.m.f. across it (Supplement: that changing field is exactly the induction condition from before — which is why transformers only work on a.c.). If the secondary has more turns than the primary it is a step-up transformer (voltage raised); fewer turns makes a step-down transformer.

Power stations step their output up to very high voltages for transmission. Supplement: cable heating losses follow P = I²R — at ten times the voltage the current for the same power is ten times smaller, cutting the lost power a hundredfold. That is why pylons run at hundreds of kilovolts, and step-down transformers near your home bring it back to safe levels.