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Q & A: Lined-up repelling magnets

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Most recent answer: 08/29/2012
Q:
take a non ferrous, strong round pipe with x id. force a set of bar magnets (greater than 3) with a diameter just a % smaller than x, force N to N and S to S, lock them in place what will eventually happen to the fields in the short term,in the long term and the very long term, also will the number effect the fields say even number of magnets as opposed to odd numbered?
- lance bon (age 66)
san juan, puerto rico
A:
First, a minor point. You mean for that tube to be non-magnetic, so that it only holds things in place. There are plenty of non-ferrous materials that are magnetic.

If there are an even number of magnets, then the total magnetic moment will cancel.  We say that the magnetic dipole moment is zero, leaving only a quadrupole moment. That means that at a distance the field will fall off as distance to the fourth power rather than to the third power. If there's an odd number of magnets the net dipole moment is just the same as from one magnet.

In the very long term, the fields from magnets decay because the energy can be lowered by breaking up into side-by-side domains pointing opposite ways. Since your set-up forces the magnets together, that raises their field energy more than when they were separate. That should cause the field to decay a bit faster, but it could still take an enormously long time, depending on the material and the temperature.

I'm not sure to what the long-term, as opposed to very-long-term, behavior is intended to refer.

Mike W.

Why is the field energy larger? The field energy goes as the integral of the square of the field. Although the fields outside the magnets tend to cancel, the fields inside tend to add up in the same direction. That's where the biggest part of the energy resides, so the net energy goes up.



(published on 08/29/2012)

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