Magnetism and electromagnetism · Contrast

Magnets and poles

Every magnet has two ends that behave oppositely, and only one of the things a magnet does is proof that the other object is a magnet too.

Start here

Two bar magnets. Same two magnets, two different answers.

Slide two bar magnets towards each other along the bench and they snap together hard enough to click. Pick one up, turn it end for end, put it back and slide them together again — and now they fight you, and the harder you push the harder they push back.

Nothing about either magnet changed. What did?

• AOne of them lost its magnetism when it was picked up
• BTurning it round made it stronger, so it pushed instead
• CWhich pole of each magnet is facing the gap
• DThe bench pushed back the second time, because of friction

A magnet has two poles, one at each end. They are named for what a hanging magnet does: the end that swings round to point towards the Earth's north is the north-seeking pole, written N, and the other end is the south-seeking pole, written S. The names are about direction, not about the material — both ends are the same steel.

The rule between two magnets is one line. Like poles repel; unlike poles attract. N against N pushes apart, S against S pushes apart, N against S pulls together. The two forces are always the same size and in opposite directions, whichever magnet is bigger, and they get rapidly weaker as the gap grows.

Poles always come in pairs. Snap a bar magnet in half and you do not get a north piece and a south piece: you get two shorter magnets, each with its own N and S. Nobody has ever found a single pole on its own.

Only some materials feel a magnet at all. Iron, steel, nickel and cobalt are magnetic materials; aluminium, copper, brass, gold, wood and plastic are not, and a magnet does nothing to them. A magnet attracts a piece of unmagnetised steel, either way round, because being near a magnet turns the steel into a weak magnet itself for as long as it stays there. That is why attraction proves nothing about the other object, and repulsion is the only proof that both objects are magnets.

At the bench · two objects on a low-friction track

Put two things end to end.

Change a control to begin

Two objects sit on a track that lets them slide freely.

Commit first. A bar magnet is brought near an ordinary steel nail, then turned end for end and brought near again. What happens?

• AIt is pulled in once and pushed away once, like two magnets
• BIt is pulled in once and does nothing the second time
• CIt does nothing either time, because only a magnet can be pushed or pulled by another magnet
• DIt is pulled in both times, because the magnet magnetises the nail whichever way round it is

The figure

One test settles it. The other two do not.

You are handed a steel bar and told to find out whether it is a magnet. You have one magnet you trust. There are three things that can happen when you bring them together, and only one of them is an answer.

Proof

Only a magnet can be pushed away by a magnet. Nothing else does it, so the bar is a magnet.

No answer

A magnet pulls a magnet, and it pulls plain steel too. Both are still possible, so you have learnt nothing.

Different answer

The bar is not a magnetic material at all. It is not steel — aluminium and brass both look like this.

A magnet works on

iron · steel · nickel · cobalt

A magnet does nothing to

aluminium · copper · brass · gold · wood · plastic

!

Think again

“All metals are magnetic.”

Most are not. Take a magnet to a handful of metal objects and the aluminium drinks can, the copper pipe, the brass key and the gold ring all ignore it completely. What responds is the iron in things: the steel tin, the fridge door, the paper clip, the nail. That is why a recycling plant can pull steel cans out of a moving stream of rubbish with an electromagnet and leave the aluminium ones behind — the separation is free, and it works because “metal” and “magnetic” were never the same word.

“It stuck to the magnet, so it must be a magnet.”

A paper clip sticks to a magnet and a paper clip is not a magnet. Being close to a magnet lines up the iron inside the clip so that it becomes a weak magnet for as long as it stays there, with its near end always turning out opposite to the pole facing it — which is why it is pulled in whichever way round you hold the magnet. Turn the magnet end for end and the clip still comes. A real magnet would have pushed away one of those two times, and that push is the only thing that settles it.

Mastery ladder

Not started yet.

Rungs 3 and 4 you mark yourself.

Rung 1 · Read the result

You hold a bar magnet near an unlabelled steel bar and it is pulled towards you. You turn the magnet end for end and try again, and it is pulled towards you again. What do you now know about the bar?

• AIt is a magnet, because it was attracted both times
• BIt is a magnetic material, but whether it is a magnet is still open
• CIt is not a magnet, because a magnet would have been attracted only once
• DIt is not a magnetic material, because nothing was pushed away

Rung 2 · Apply the rule

Two identical bar magnets repel each other across a 4 cm gap. They are then moved to a 2 cm gap, still the same way round. What happens to the force pushing them apart?

• AIt doubles, because the gap halved
• BIt stays the same, because neither magnet has changed
• CIt grows a great deal — far more than doubling
• DIt falls, because they are being forced closer against the push

Rung 3 · Explain

A paper clip hangs from the north pole of a magnet. It hangs just as well from the south pole. Explain why, using the idea of poles.

Your explanationComplete

Rung 4 · Take it somewhere new

A scrapyard sorts crushed drinks cans. Steel cans and aluminium cans go past on the same belt, and a magnet lifts one kind out. Say which kind is lifted, and explain how you would check that a can that was left behind really is aluminium rather than a steel one the magnet missed.

Your answerComplete

Key note

A magnet has two poles, north-seeking and south-seeking, and they cannot be separated: break the magnet and each piece grows the pole it is missing. Like poles repel and unlike poles attract, always with equal and opposite forces, and the force falls away sharply as the gap opens. Only iron, steel, nickel and cobalt respond at all, and an unmagnetised piece of any of them is attracted whichever pole you offer it, because the magnet lines it up first. Attraction is therefore not evidence. Repulsion is.

Going further

Cut a magnet in half and you get two magnets. Cut those in half and you get four. Keep going and the pattern never breaks, because the magnetism is not stored in the ends — it comes from countless tiny magnetic regions inside the metal, all lined up the same way. In an unmagnetised piece of steel those regions point in every direction at once and cancel out; magnetising it is the act of lining them up, and dropping it hard or heating it in a flame knocks them out of line again. Nobody has ever isolated a single north pole, and physicists have looked hard: a lone pole would be a genuinely new object, and searches for one have been running for decades.

Magnets that are much stronger than the ones in a school lab are ordinary items now. The small silver discs in headphones, cordless-tool motors, wind turbines and fridge catches are usually neodymium, and a disc the size of a coin can pinch skin badly between two of them or shatter if two are allowed to snap together. The serious hazard is swallowing: two or more strong magnets that end up in different parts of the gut can pull towards each other through the wall between them, and that is a surgical emergency rather than something that passes. They are kept away from small children for exactly that reason.

If a magnet has been swallowed, or you are worried about someone Swallowing a strong magnet is treated urgently even when the person seems fine, so it is worth telling an adult straight away rather than waiting to see — a pharmacist, your GP or 111 will tell you what to do next, and at school the school nurse or any member of staff can start that. If you would rather talk to somebody outside all of that, Childline is free on 0800 1111, at any hour, and you do not have to give your name.

The bench is a teaching model. Strength is reported as a relative figure with the closest pair of magnets set to 100, and no force in newtons is given anywhere on this bench: the equation for the force between two magnets is well beyond this stage and any number in newtons here would be invented rather than measured. The relative figure falls as the fourth power of the gap, which is about right for two bar magnets end to end and is not the inverse-square law that applies to charges. The pull on unmagnetised steel is reported in relative words only, never as a figure, because how strongly a piece of steel magnetises depends on its shape, its carbon content and what has happened to it, and any coefficient chosen here would be a guess. The force arrows are clamped at both ends, so the closest setting and the widest are drawn shorter and longer than the figures alone would give. Both magnets are treated as identical and equally strong, the track as frictionless, and the objects as staying where you put them.

Lesson content © MrBadmusAI.