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I just took a rare earth magnet out of an old hard drive. Lacking an appropriate screwdriver, force was used, and the magnet broke into two pieces; one about a quarter of the original size and one about 3/4 the original size.

Let's say this is the magnet:


and the arrows are pointing north, in the original magnet.

It broke into two pieces:

>>>>>}   }>

(where } represents the rough edge of the break).

I'd expect that the two magnets would keep their relative directions, so that the magnets would fit back together at the break, like this:


But instead, the broken edges repel, and the magnet wants to come together so that formerly "north-facing" sides are now connected, like so:


The obvious deduction here is that one of the pieces flipped polarities when the magnet was broken. I don't understand why this would be the case.

So why did one of the pieces flip polarities?

EDIT: To clarify, the actual magnet is a 120-degree arc of a circle. Thus, it's obvious what the original orientation of the pieces were. This isn't simply a case of me getting the pieces flipped over relative to each other.

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Are you sure that's what happened? – Mike Dunlavey Nov 8 '11 at 20:44
Nope! But I did forget to mention that the original magnet is a 120-degree arc of a circle, so it's pretty obvious what the original orientation was (i.e. this isn't a case of me accidentally flipping over the magnet) – Asmor Nov 8 '11 at 20:55
up vote 8 down vote accepted

No polarity reversal has occurred - you are dealing with a magnet that has an axial field (pointing out through the flat face.) When you break it, each half has similar field, pointing in the same direction, which is unstable. One piece will want to flip so that the fields line up antiparallel (lower energy situation).

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break into


and is put together

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protected by Qmechanic Feb 17 '13 at 19:31

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