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Can the force exerted by a perm magnet be equated to electro magnet watts?

  1. May 29, 2012 #1
    Hey guys, sorry if this is wrong place to post this. If it is let me and I will try and find the right place. It was the only category where I saw something about energy and physics. This is also probably a basic question but I hope you don't mind me asking.

    Let's say you have an electro magnet that takes 10 watts to hold a 10 pound weight for 1 hour and your power supply is a 20 watt hour battery. That means it could hold that weight for 2 hours.

    You also have a permanent magnet that can also hold a 10 pound weight for 1 year before its magnetic field decays and the weight drops. Does that mean that one could say that the permanent magnet is outputting the equivalent of 87650 Watt hours to hold that weight for 1 year? I came to 87650 by multiplying 10 watts by the hours in a year.
    I hope that makes sense and this isn't a stupid question. Thanks in advance.
  2. jcsd
  3. May 29, 2012 #2
    No, just as a table need not "output energy" to support the objects resting on it.

    Electromagnets consume energy because of the electrical resistance in the wires that compose their coils. As current flows through the wires this resistance causes them to heat up, and then this heat energy is radiated away into the surroundings. The energy supplied to the electromagnet goes not into the levitated object, but into this lost heat. No work is done (no energy is transferred) by the magnetic field. It's just that this particular method of maintaining a magnetic field requires constant energy input to maintain the current in the coils.

    In contrast, a permanent magnet maintains a magnetic field with no energy input at all--that's what makes it a permanent magnet. If you levitate something with a permanent magnet, no work is being done anywhere (no energy is being transferred).

    The key to this is understanding the difference between force and work/energy. Work = force x distance. If you apply a constant force to something that isn't moving, no work is done.
  4. May 30, 2012 #3
    As The Duck said, an electromagnet might use energy to levitate an object, but it doesn't need to. It's just an inefficient electromagnet. You could go out and get a better one, perhaps made with superconductors, and it wouldn't need any energy input at all.

    Likewise, a helicopter uses energy to hover in place, but it doesn't need to. You could easily replace the spinning rotor blade and thousands horsepower engine with a table, for example. Of course you'd then be stuck in place and the helicopter would be useless, but the point remains.
  5. May 30, 2012 #4
    Thanks Duck, and Lsos for the replies. They make a lot of sense. I am trying to get a good grasp on this stuff.
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