Magnetism

A magnet has two poles, north and south, and the rule is the same as for charge: like poles repel, unlike poles attract. Repulsion is the only conclusive test for a magnet, because a magnet will attract an unmagnetised piece of iron as readily as it attracts another magnet.

A magnetic field is the region around a magnet in which another magnet or a magnetic material experiences a force. It is drawn with field lines, and the conventions matter: lines run from north to south outside the magnet, they never cross, and where they are closer together the field is stronger.

Only a few materials are magnetic at all — iron, steel, nickel, cobalt. Aluminium, copper and brass are not, despite being metals, which is a useful thing to know when a question asks how to separate scrap.

Field lines leave the north pole and return to the south. Bring two like poles together and watch the lines crowd and push apart; bring unlike poles together and they link up, pulling the magnets in. The small dial is a compass — a probe you can raise and lower, whose needle always turns to lie along the field line running through it, with its red N end pointing the way the field points.

Two magnetic materials behave quite differently, and choosing the right one is a standard exam question.

Soft magnetic materials, of which soft iron is the example, magnetise easily and lose their magnetism just as easily. That makes them right for anything where the magnetism must be switched off — the core of an electromagnet, or the core of a transformer, where the field reverses fifty times a second.

Hard magnetic materials, such as steel, are harder to magnetise but keep their magnetism once they have it. They are used for permanent magnets: compass needles, loudspeaker magnets, fridge magnets.

The word "soft" here has nothing to do with physical hardness. Soft iron is a perfectly solid metal; the softness refers to how readily it gives up its magnetism.

Every electric current produces a magnetic field around it. This is the link between electricity and magnetism, and everything from a doorbell to a power station generator depends on it.

Around a straight wire the field lines form concentric circles. Their direction is given by the right-hand grip rule: point the thumb of your right hand along the conventional current and your curled fingers show the direction of the field.

Coil the wire into a solenoid and the fields from each turn add together, producing a field very like that of a bar magnet — uniform and strong inside, spreading out from the ends. Which end is north is again given by a grip rule: curl your right fingers the way the current goes round the coil, and your thumb points to the north pole.

The strength of the field can be increased in three ways: increase the current, increase the number of turns on the coil, or insert a soft iron core. All three are standard answers to "how would you make this electromagnet stronger?".

A coil in a circuit that measures it: the ammeter reads the current going through the coil, the voltmeter the p.d. across it, and the variable resistor sets how much current flows. The number of field lines is the strength of the field, so the three ways of strengthening an electromagnet are the three controls — turn the current up, add turns, or put soft iron inside instead of air. Reverse the current and the poles swap ends, which is the grip rule: the arrows on the winding show the way the current goes round. Take the current to zero and the field disappears entirely — that is the difference between this and a permanent magnet.

A relay uses a small current to switch a large one. Current through the electromagnet attracts an iron armature, which closes the contacts of a separate, high-power circuit. This keeps the dangerous current away from the switch the operator touches — which is how a car starter motor is controlled from a key.

An electric bell uses the same effect to make itself repeat. The electromagnet attracts an armature that strikes the gong, and in moving it breaks its own circuit. The magnet releases, a spring pulls the armature back, the circuit remakes, and the cycle starts again — several times a second.

A circuit breaker is a safety device built the same way. Excessive current makes the electromagnet strong enough to pull a catch, which springs the contacts apart and cuts the supply. Unlike a fuse it can simply be reset.

A loudspeaker combines a permanent magnet with a coil carrying the varying current of an audio signal. The coil experiences a varying force, which moves the cone back and forth, pushing on the air and producing sound.