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Q & A: Hot Magnets?

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Most recent answer: 10/22/2007
Q:
Hi I was wondering how the temperature of the magnet affects its "strength"?
- Jessica Wehr (age 11)
W. Lafayette, IN , USA
A:
The first pert is a little tricky. Some atoms have an interesting property called "spin". These atoms act like tiny magnets, each of which can have its North pole pointing in some direction.

Often the spins in a large clump of atoms are so random that they have equal numbers pointing in different directions. Then they cancel out. But in some materials, nearby spins tend to settle down pointing in the same direction. If these clumps of nearby spins (called domains) are also lined up mostly the same way, you end up with a real magnet.

Now comes another tricky part. The hotter things are, the more energy can be found in them. The reason the spins settled into the lined-up state was that they had lower energy that way, the same as the reason water will settle down to the bottom of a bowl. But if it's hot, there's enough energy around for many of them to get out of line, like when some of the water in a bowl leaves the bottom by evaporating. So as the temperature gets warmer, the magnet gets weaker. When it's hot enough, it loses ALL of it's alignment and stops being a magnet at all. (that's a little like the way that water completely boils away from the bottom of a bowl when it gets too hot.) Depending on the material, you may need to get things really hot in order to see this effect. Even before the magnet completely loses its alignment in every domain, the domains themselves can shake lose and start pointing different directions. If you cool the magnet down again, the spins inside each domain will line up again with each other, but the domains will still point every which way and your magnet will remain weakened.

Adam

(published on 10/22/2007)

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