Magnets, Fields & Electromagnets

Explained

Poles, fields, and making a magnet with a current

Every magnet has two poles, a north and a south, and you cannot have one without the other. Break a bar magnet in half and you get two shorter magnets, each with both poles, rather than a separate north and south.

The rule for the force between them is short: like poles repel, opposite poles attract.

Magnetic fields

A magnetic field is the region where a magnetic material, another magnet or a current carrying conductor experiences a force. It is invisible, so it is drawn as field lines.

Four rules govern the drawing. Lines run from north to south outside the magnet. They never cross. They are closer together where the field is stronger, which is at the poles. And every line needs an arrow showing its direction.

A plotting compass reveals the direction at any point, because the needle lines up with the field. Iron filings reveal the shape quickly but show no direction, since a filing has no north end of its own.

Magnetic materials

Only iron, cobalt, nickel and steel are magnetic. Most metals, including copper and aluminium, are not.

A magnetic material is attracted to either pole of a magnet, because the magnet induces magnetism in it first. A magnet, by contrast, is repelled by one pole and attracted by the other. That is the test for whether something is a magnet or merely magnetic: repulsion is the only proof, because attraction happens either way.

Electromagnets

A current in a wire produces a magnetic field around it, in circles centred on the wire. Wind the wire into a coil, called a solenoid, and those circular fields combine to give a field like a bar magnet's: strong and uniform inside, with a north and a south pole at the ends.

Three things make it stronger: more turns on the coil, a larger current, and an iron core inside it.

The core is soft iron rather than steel, deliberately. Soft iron magnetises and demagnetises easily, so the electromagnet switches off cleanly when the current stops. Steel would keep its magnetism and the device would not release.

That switchability is the whole point. It is why electromagnets are used in scrapyard cranes, electric bells, relays and circuit breakers, and why a permanent magnet would be useless in all four.

Reversing the poles

Reverse the current and the field direction reverses, so the north and south ends of a solenoid swap over. Nothing physical has to move.

The right hand grip rule gives the direction: point the thumb of your right hand along the current in the wire, and your fingers curl the way the field goes.