Magnets in Magnets in Magnets

barMagnetLet’s think about our trusty bar magnet, with one end a north pole and the other end a south pole. Suppose you stick two of them together, a north pole to a south pole, so that we have a long stick with NSNS in a row as its poles. Then at one end we have a north pole and at the other we have a south pole – just like for the original magnet. We could add on two more magnets and get a string of NSNSNSNS – and of course we could keep going with this for as long as we liked, getting bigger and bigger chains of magnets – or effectively getting bigger and bigger magnets.

What would happen if we broke this chain in half? We’d have NSNS and NSNS – we’d effectively have two separate, whole magnets. And we could do the same again with one of these and get NS NS – again, two whole magnets from breaking a magnet in half.

This is exactly the problem we considered in the last post, where we puzzled over the fact that a magnet cut in half produces two entire magnets, where we might have expected to get two separate poles.

atomicMagnet

The trick is to realise that the magnetism of a large magnet arises because of the magnetism of the atoms it is made of. Each atom is effectively a tiny atomic magnet pointing in a certain direction. In a good magnet, all of the atomic magnets will be lined up, pointing in the same direction. By lining them up, we add up all their magnetism (and similarly if they were all pointing in different directions, we wouldn’t get much of a magnetic effect). So we get one magnetic field overall, with one north pole and one south pole, like the N and S we had at the end of our long string of magnets.

If we cut the magnet in half, each half is still made up of atomic magnets, so the same thing happens overall – we get one magnetic field with one north pole and one south pole. This means you can – in theory! – keep cutting your magnet in half and getting two magnets out until you get down to the atomic level. In practice, it becomes very difficult because of the importance of keeping the atomic magnets lined up. The cutting-in-half has to be done very gently, as physical force, and especially shocks, can cause the atoms to lose their alignment. That’s why dropping magnets makes them weaker and weaker – the atoms are shaken out of alignment. Conversely, you can use a magnet to drag the atoms in something like a steel rod into alignment – that’s how you can magnetise something like a paperclip or a pair of scissors with a magnet. The iron atoms in the steel are naturally little atomic magnets, but most of the time they aren’t going to have any overall preferred direction. A strong enough magnet, however, can be used to tell them all which way to point, lining them all up and effectively creating your very own magnet!

ferromagnet

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