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Can Magnets Do Work?

  1. Nov 26, 2008 #1
    Before I get flamed for proposing a perpetual motion machine, I would like to point out that I understand there is something fundamentally flawed in my logic. That is why I am posting this, I want someone to point out to me what I am missing to explain why this won't work.

    Suppose you have two magnets positioned so that their opposite poles are facing toward each other. There is a magnetically shielding material in between the two magnets so that they do not move into contact with one another. Now suppose one magnet is held in place by a stop and the other magnet is free to move and located underneath the first with the magnetically shielding material somewhere in between the two but free to be slid in and out of the device. When the magnetically shielding material is removed, the top magnet pulls with enough force to move the bottom magnet up and into contact with the stop. Now the shielding material is slid back into the system and the bottom magnet falls back down. This can be done repeatedly. We now have devised an engine that can do work.

    In my little thought experiment this seems completely valid to me, however i know something must be wrong. I would love to test this out physically but i don't have any magnetically shielding material. Can anyone point out to me why this will not work?

    Thank you.
  2. jcsd
  3. Nov 26, 2008 #2


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    Science Advisor
    Gold Member

    And how much energy is used to slide the shield in and out of position?
  4. Nov 26, 2008 #3
    i don't know, are you suggesting that perhaps sliding the shielding material would take the same amount of energy as is being used by the bottom magnet moving upward. I have no idea how magnetically shielding material works, but i can only assume that it involves some sort of interaction with the magnetic field. I suppose that could explain it.
  5. Nov 26, 2008 #4
    I think that may be it. the "Magnetic shielding" material would itself act like a magnet. Magnetic dipoles within the material flip to cancel the magnetic field of the lower magnet. It would take work to pull this material out of the way.
  6. Nov 26, 2008 #5


    Staff: Mentor

    Hi MadMike, the bottom line is that magnets can do work but your scenario won't work for the reason alluded to by f95toli.

    The magnetic field itself has energy, if you have a magnetic shielding material then that material reduces the magnetic field which means that a shielded configuration is a lower energy state than an unshielded configuration. The magnet will therefore do work on the shield as you put it in and it will take an equal amount of work to pull the shield out.
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