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Q & A: Attraction or repulsion of bar magnets

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Most recent answer: 02/09/2014
Q:
I have an IGCSE exam past paper question on magnetism. 'Two bar magnets are held near to each other on a horizontal surface. The magnets are released. In which direction do they move?'
- Rianne (age 16)
London
A:

Hello Rianne,

Bar magnets usually have their direction of magnetization perpendicular to their flat surface.  It the bar magnets are resting face down on a table you can get either attraction or repulsion depending on whether or not the poles are anti-parallel or parallel.   I they are parallel then when you bring them closer the total magnetic energy increases and in order to do that you have to provide a force. They don't like that and will repel each other to reduce the total energy. In the other, when the poles are anti-parallel the total magnetic energy decreases.  They love that and want  more of of the same and tend to attract.

LeeH

And if there's not much friction on the table, even if the magnets initially repel, they'll end up rotating around so that they attract. That's a consequence of Earnshaw's theorem, which says that you can't get forces like magnetism and gravity to support some stable equilibrium. Things only stabilize when the magnets come in contact and short-range quantum forces become important. 

Mike W.


(published on 02/09/2014)

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