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Cutting a Magnet

  1. Mar 18, 2010 #1

    FeDeX_LaTeX

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    Hello;

    When you cut a magnet that has north and south poles, you get two magnets with north and south poles. If I cut a million magnets out of it, each of those magnets will have north and south poles. However, what happens when I cut down to the last electron? Will that electron still have a north pole and a south pole, or just one pole?

    Thanks.
     
  2. jcsd
  3. Mar 18, 2010 #2
    Nice question.The way I imagine your thought experiment is at the limit we have an atom with all electrons stripped off except one the last electron then being removed.I'm not sure if the non ionised atom can be magnetic but if its ions are set in motion then each would set up a magnetic field there being two poles in that any magnetic field lines form closed loops.I am not at all confident with my first impression answer here but I am looking forward to any more illuminating discussion that may follow.
     
  4. Mar 18, 2010 #3
    Each atom has two poles in a way. For many poles together you can some sort of cancellation:
    +-, +-, +-, +-, +- = +(-+)(-+)(-+)(-+)- = +............-
    That why a big magnet can be though of as two poles.
    However they also strengthen each other. So if you start cutting the poles these atoms lose strength a bit and also lose their orientation, so that a small magnet might not have the nice order and therefore no two separate poles.

    Anyway, the final atom still have two poles.

    And also if you strip down the electrons they have two poles.

    No real magnetic monopole has been seen so far!
    (only some spin ordering effect that produced magnetic field reminiscent of magnetic monopoles)
     
  5. Mar 20, 2010 #4

    FeDeX_LaTeX

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    Is it possible to manufacture one?
     
  6. Mar 20, 2010 #5
    You don't really get to keep slicing a magnet down to electrons or even atoms. A few billion or a million atoms are the limit.

    It depends upon the material you are turning into dust. For any given ferromagnetic material there is a minimum volume that will support a magnetic domain. A magnetic domain consists of a bunch of atoms that all act like one uniform magnet. The domain creates the strongest possible magnetic field that material can ever have on its own. Less than this limit in size and it will no longer be ferromagnetic.

    By the same token there is a maximum size rule. If a piece of ferromagnetic material exceeds this size it can have more than one domain. It can contain two or more magnetic domains with their fields arranged at odd angles so that the combination of the fields is less than the sum of the two.

    Interestingly, there are bacterium that each contain a string of magnets. In examination it's found that each tiny magnet is large enough to have one domain, but too small to have two. Clever bacteria.
     
    Last edited: Mar 20, 2010
  7. Mar 20, 2010 #6

    FeDeX_LaTeX

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    Interesting...

    And do magnets last forever?
     
  8. Mar 20, 2010 #7
    As has been said, the magnetic field of a magnet is the sum of the magnetic fields of the atoms which ake up the magnet. Therefore, as long as the atoms stay oriented (polarized), the magnet will last. If you disturb the orientation, it may cancel. As an example, you can polarize iron by keeping it in a magnetic field (iron is ferromagnetic). After you remove the surrounding field, the atoms in the iron stay polarized, and you have a magnet. This is not a permanent magnet, and if you drop the iron piece to the floor or strike it with a hammer, the magnetic field will disappear. The now random orientation of the atoms in the iron produce no net magnetic field.
     
  9. Mar 20, 2010 #8

    FeDeX_LaTeX

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    So why can't we have an infinite amount of electricity, then? A current can be generated if you push a magnet into a coil repeatedly -- so why can't you simply attach a magnet to a string, put two magnets that repel the middle magnet back and forth into and out of the coil?
     
  10. Mar 20, 2010 #9
    Because of two fundamental principles; energy concervation and Newton's 3rd law.

    When you move the magnet into the coil, a force will act on the magnet opposite the direction of motion. This will damp the occilation and stop it.

    As for energy, your system would violate energy concervation since you could get infinite electrical energy from a finite energy system.
     
  11. Mar 20, 2010 #10
    No, but some people are hoping to find one.
    http://en.wikipedia.org/wiki/Magnetic_monopole
    Theoretically one could substitute the 0 in the Maxwell equations to allow for magnetic monopoles. Then the equation would even be more symmetric and moreover I heard magnetic monopoles would explain why charge is quantized. However, none has been found so far.

    And the only way we can produce magnetic fields, is by accelerating electrons or use the electron spin. They don't produce magnetic monopoles.
     
  12. Mar 20, 2010 #11

    DaveC426913

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    The middle magnet will simply reach equilibrium and stop moving. If you want it to continually move, you will have to push it out of equilibrium, which will require an input of mechanical energy. To get x electrical energy out of the system, guess how much mechanical energy you'd have to put in? :wink:
     
  13. Mar 25, 2010 #12
  14. Mar 25, 2010 #13

    DaveC426913

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  15. Mar 25, 2010 #14
    Hmm, I'm not a real expert and just can tell about undergrad level Maxwell stuff. Maybe FeDeX can ask a question? Maybe a theoretical physicist can step in?

    Basically everything we see in nature has an instrinsic magnetic dipole and many particles have a charge when at rest. But we have never found something pointlike which has a magnetic field not looping around, but instead diverging from one point to infinity like light rays from the sun. This would represent a magnetic monopole. In Maxwell's equation the place where the magnetic monopole charge should stand, we put a zero. It would be a great discovery to find particles which have magnetic monopoles. And I read it would explain the quantization of charge.

    The theoretical details behind that article I know only vaguely...
     
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