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Q & A: magnetic energy

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Most recent answer: 09/20/2011
Q:
In the case of a permanent bar magnet. If iron filings are sprinkled around it after a piece of paper has been placed over the magnet, the filings will then align themselves and reveal the magnetic field. This shaping of the field using something with mass must therefore take a certain amount of energy from the magnet, no? So wouldn't the magnet eventually lose its strength after many repeats of same? And how can this seemingly 'inert'magnetic field be explained using photons and/or electrons etc?
- Vic Tovey (age 81)
Peterboro Ontario Canada
A:
Actually, as the filings align themselves along the field direction, they lower the magnetic field energy. The extra energy generally goes to heat up the filings, the paper, etc. If you then say pick up the filings and remove them, you supply the extra energy needed to raise the field energy back to its original value. So the magnet isn't supplying energy here, and it doesn't wear down in the process.

There is a way that magnets do in general wear down. The little domains inside can lower their energy (except in thin wire-like magnets) by getting out of alignment with each other. Over a long time time, especially if the magnet is warm or subject to mechanical blows, this will happen.

On the explanation of magnetism in terms of the interactions between the electron spins, you might search this site for various answers on magnetism. Follow up if there are still more questions.

Mike W.

(published on 09/20/2011)

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