Poles of a broken magnet

[CompuChip]

Hi,

Recently I came across an annular permanent magnet (round magnet with a hole in it) which had fallen and broken in half, effectively forming two horse-shoe magnets. I noticed that the two halves had similar poles:

|---------- S     S--------
|                         |
|                         |
|                         |
|                         |
|                         |
|---- N         N---------|    

(top view)

Therefore it was impossible to put it back together because the two halves repulsed each other one way, and the way the attracted they didn't fit together anymore because it was - of course - not a perfect cut.

I was wondering whether this is a coincidence. Since these magnets are apparently quite expensive it was a bit unpractical to repeat the experiment, but something tells me that one would get this situation every time although I was unable to explain this theoretically.

Any ideas?

 

[mfb]

Are you sure you did not turn one magnet around? That looks line an unusual field configuration for your description of the magnet.

 

[CompuChip]

Yes, it broke into two uneven pieces. Each of them was about half of the size of the original but the pieces clearly only fitted together in one way.

 

[Nugatory]

What was the field direction (that is, where were the poles) of the ring-shaped magnet before it broke?

 

[CompuChip]

Based on symmetry (and a little intuition) I expect that the field looked like this

with a north pole at the top and the south pole at the bottom or vice versa - I don't think which option is relevant.

FWIW, I think it was a neodymium magnet like this one

I also tried imagining the magnet is first cut once, giving rise to something like this:

but if you then cut it at the top as well I would expect two halves that attract (ie in the left half a south pole is created at the top) - apparently in the case that I observed the poles of one part got flipped.

I'm also not sure whether such a reasoning would work for a violent event like dropping it from a table.

 

[physwizard]

It is likely it retained the same polarity after you broke it also viz North pole at the top and South pole at the bottom. So if you bring the broken ends together they would repel (the north at top repelling the north at top of the other one and south at bottom repelling the south at bottom of the other one) but if you flipped one of them over they would attract (north at top of one attracts south at top of other and south at bottom of one attracts north at bottom of the other).

 

[CompuChip]

Hmm, that is an interesting possibility which I didn't consider, physwizard. I would have thought that after breaking the ring in half you would effectively get two horse-shoe magnets.

I guess I could easily find out myself with a piece of paper and some iron filing... maybe I should have done that in the first place :)