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Does conservation of energy apply to magnetism?

  1. Apr 8, 2008 #1
    Does conservation of energy apply to magnetism?


    Do magnets ever "wear" out. Do they ever lose their magnetism?
  2. jcsd
  3. Apr 8, 2008 #2
    Energy Conservation / Magnetism

    The answer to the first question is yes, conservation of energy does apply to magnetism. There is energy in the magnetic field. There must be a source of energy to establish the field, and if the field "goes away", the energy in the field is transferred or transformed into some other form of energy.

    Assuming you are speaking of something like a bar magnet, the answer is yes and no. At the atomic level, each, say, iron atom has a magnetic field by virtue of electrons orbiting the nucleus, and due to the spin of each electon about its own axis. The magnetic field in an iron bar magnet is induced by applying an external magnetic field which causes small magnetized regions (magnetic domains) within the bar to align in such a way that the bar now has a noticable magnet field. I have read that it is possible to detect the sound emitted when the domains align and the magnet is "formed".

    This process can be reversed by a sharp mechanical impact to the magnet, or by heating the magnet. The magnetic domains reorient in random directions, and the bar magnet "loses" its overall magnetism.
  4. Apr 8, 2008 #3


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    Staff: Mentor

    Just to make sure we're clear, a magnet in a static situation (such as a refrigerator magnet) is not consuming or producing any energy. So there isn't anything to "wear out" or use up.
  5. Apr 8, 2008 #4
    That is correct.
  6. Apr 8, 2008 #5

    Shooting Star

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    The so called permanent magnets eventually do wear out, i.e., their strength slowly lessens. How fast this happens depends on how it has been treated. If you heat it, or drop it, or keep it near a strong electrical current, the faster it wears out. That is why you have "keeper" of a magnet, which is nothing but a piece of ferromagnetic material placed so as to connect the poles of the magnet, and keeping the "magnetic lines of force" closed.

    Random thermal motion of the molecules tend to destroy the alignment of the domains. A powerful magnet is in a highly ordered state, and the thermal motion tends to destroy that. As with every other process in nature, the entropy increases. In fact, above the Curie point of a metal, a magnet cannot be formed at all due to thermal motion of the molecules.

    But, as russ_watters has mentioned, it does not wear out simply because it's pulling on a piece of metal or such phenomenon. In fact, if you stick a magnet on the refrigerator, it'll retain its magnetism longer, because the metal will act as a keeper.
  7. Apr 8, 2008 #6
    Huh? How does thermal motion destroy the alignment of the domains?

  8. Apr 9, 2008 #7
    Because if the domains change orientation, they're no longer as perfectly aligned...? Think of it as brownian motion for unit vectors at the origin.
  9. Mar 10, 2009 #8


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    So, I've been thinking about this one (high school student mind you). Based on the above comments, I gather that the law of conservation does apply to magnets. Permanent magnets do wear out if there is no 'keeper'.
    what is the source of energy that put energy towards giving a magnetic material (say natural neodymium magnet) each and every one of it's domains?
    was there kinetic energy involved in creating the magnet? Does this mean kinetic energy captured in the material's formation is stored in the form of domains within a magnet and then is released when those domains lose their orientation?

    thought experiment:
    one neodymium magnet and one hunk of iron in space. because a magnetic field propagates to eternity (naturally losing intensity with distance) the hunk of iron will at any distance be drawn towards the magnet (to some degree). In this version of the experiment, there was no force to place the hunk of iron at its original distance. so the energy could not come from there.
    but the other form of the experiment does include the energy used to displace the magnet from the iron. perhaps the energy used to displace the two was stored in the magnet in the form of domains. then when the iron is attracted to the magnet, those domains are disoriented again. this differs somewhat from my experiences with magnets; but it is a start to understanding this phenomenon.
  10. Mar 11, 2009 #9
    The superconducting magnets used for MRIs in hospitals have a lot of stored magnetic energy, even though they are not connected to any power supply. the stored energy is

    E = (u0/2) Integral [B2 dv]

    integrated over all volume, including inside the superconducting coil itself.

    Electrically, the energy is expressed as E = (1/2)L I2,

    where L is the inductance of the coil.
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