Why does a broken magnet form two new ones?

We find out how broken magnets still 'know' they're magnets.....
29 November 2016


A horseshoe magnet



Why does a broken magnets form two new magnets?


Liam Messin set out to answer this attractive question with Tim Boyd, a Cambridge undergraduate...

Tim - Thinking of magnets having a north and south pole is slightly misleading. They have a thing known as a dipole. This means that they have one pole with a north and south end, but it is a little more complicated than that.

What we think of as a magnet - a bar magnet is made of lots of little magnets all pointing the same way as each other. These little magnets are called domains and each has it's own dipole, so a north and south end. Therefore the strength of the magnet is the total of all these little magnets. However, if all the little magnets point in a random direction, then the total would be zero; there would be no magnetisation.

Before you may have been told to be careful not to drop magnets. This is because, if we shake around all the little magnets, they will no longer point in the same direction and so the magnet will no longer be magnetised.

Liam - Noted - don't drop magnets. But wait a minute, how do we get our little magnets all lined up beforehand?

Tim - We do this by putting the magnet near a really strong magnet, which then pulls all the little magnets to point in the same direction. If the really strong magnet is then taken away, there is nothing to pull the little magnet away from pointing in this direction, so they stay happily pointing in the same direction and we have magnetisation.

Liam - To make a magnet, we need a magnet? Luckily there are both naturally occurring magnets and electromagnets for this task. However we make them we always get the situation of loads of little magnets all pointing in the same direction. So what happens when we break a magnet in half?

Tim - You split into two magnets made of lots of little magnets all pointing in the same way. So if the right hand of the original was north and the left hand south, then the right hand end of the two new magnets would be north and the left hand end south. The direction of the dipole hasn't changed - the north end and the south end are still in the same place. The strength of these new magnets is half that of the original as it is made from half of the little magnets. The magnet doesn't need to know it has been broken or change its magnetic structure, it already has the new poles.


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