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Q & A: heating bar magnet

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Most recent answer: 04/24/2014
Q:
After experimenting to see if heat effected the magnetism of a magnet I discovered that when the bar magnet reached a temperature of 75oC, only the north end managed to pick up 6 steel washers, the south end appeared to have no magnetic charge. This was a very dramatic change after the whole magnet at room temperature picked up 99 washers. Why is this?
- Lois (age 26)
UK
A:

It sounds like the bar magnet lost a lot of its magnetism. That's probably not because the individual domains ceased to be magnetic but because they lost their systematic alignment and started to point more randomly. That's all perfectly normal behavior for a hot magnet.

This old thread provides some background: .

Now the North-South asymmetry is a little odder. There is no such thing as "magnetic charge", at least in any material we've ever found. There are only magnetic dipoles, like bar magnets with North and South poles. So the field lines coming out of the North end of your magnet still must wrap around and re-enter the magnet. It seems that they're doing that over a spread-out region, rather than bundled up into the South end. That means that the Southish field is weaker than the more concentrated Northish field, because it's more spread out. 

How did that asymmetry develop? Maybe the South end got a little hotter. Maybe when the magnet was made the composition wasn't quite uniform from one end to the other.

Anyway, you should be able to check what the field lines look like by the standard trick of using some iron filings on a sheet of paper to visualize them. If I'm wrong about how they look, please write back.

Mike W.

 


(published on 04/24/2014)

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