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Re: [Phys-L] breaking magnets



On 04/28/2014 10:27 AM, Larry Smith wrote:
If you break a simple dipole bar magnet in two unequal pieces, how
does the strength of the pieces relate to the strength of the
original?

Well, for a /long/ straight bar magnet made of two pieces
stuck together, ideally, the pole strength for each piece
is the same as what you started with. It's equivalent to
a long straight solenoid. Think about the Amperian
microscopic circulating currents.

In reality, the separate pieces will be slightly less
strong. The connected state has a lower energy than
the disconnected state, and is more able to resist
thermal fluctuations and other disturbances.

In the limit where you slice the thing like a salami,
so that each piece is thinner than it is long, then
asymptotically the pole strength goes to zero. The
thin slab will find it energetically favorable to
break up into alternating domains. The energetics
of the short fat configuration is wildly different
from the long thin configuration, as you can easily
verify.

Last but not least, if you actually "break" a metal
or ceramic magnet, you will reduce the field strength
even more, because the trauma of breakage disturbs the
domains. (That's why my ideal case spoke of pulling
apart two parts that were already separate pieces.)