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Why is a magnet not an infinite source of energy?

  1. May 8, 2014 #1
    Hi Guys

    Could someone please point out where I'm going wrong with the following problem.

    If I place a magnet on a vertical surface such that the magnet must resit the force of gravity in order to hold itself in place, will this magnet stay in place forever? If it does why does this not mean the magnet is supplying infinite energy to resit the force of gravity. My thinking was that the magnet would naturally degrade over time thus getting out of the infinite energy problem, however, I'm not at all convinced this is correct.

    Any help you can give me on this problem would be hugely appreciated.

    Thanks
     
  2. jcsd
  3. May 8, 2014 #2

    NascentOxygen

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    Hi aspaceo! http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    Your kitchen table is holding itself in place, too, without the expenditure of energy.

    Practical magnets do degrade with time, but measured in years if you don't knock them about.
     
    Last edited by a moderator: May 6, 2017
  4. May 8, 2014 #3

    sophiecentaur

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    This is all about the difference between Force and Energy. To 'get' Energy out of a device, a Force has to be moved through a Distance. Once a magnet has attracted a piece of iron (or a ball has rolled down a hill, spring being stretched etc. etc.) then more Energy (in the form of Work) needs to be transferred to go back to the start situation. A Force, exerted for millions of years, even, is doing no work if there is no movement.

    In the case of an Electromagnet, you need to supply Energy (Electrical, this time) to generate the Magnetic Field and the field 'holds' energy, if you like. In an electromagnet, the (mechanical) Force times Distance is in a different (electromagnetic) form but you still have to put in Energy to set things up.

    Perpetual Motion is a verboten subject on PF but many of the cranky ideas and suggestions about PM are based on a lack of appreciation of the difference between Force and Work. It can be an interesting exercise to de-bunk some of those loopy ideas you can find elsewhere on the Internet.
     
  5. May 8, 2014 #4

    SteamKing

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    Who needs magnets? Why isn't a postage stamp or a piece of adhesive tape 'an infinite source of energy'? Both items stick to vertical surfaces.
     
  6. May 8, 2014 #5

    Drakkith

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    If you look up the basic definitions of energy, force, and work you will see why a magnet does not have infinite energy.
     
  7. May 8, 2014 #6

    CWatters

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    What Drakkith said.

    Work or Energy = force x distance.

    The magnet doesn't move so the distance is zero and the energy expended is zero.

    Consider what would happen if you put a thin bit of ice or oil between a fridge magnet and a fridge door. Thin enough that the magnet is still attracted to the door strongly. Would the magnet still hold itself up or slide to the floor?

    It's the friction between the magnet and the fridge door that holds it up not the magnetic field. Gluing something to the fridge door is just another way of increasing the friction. The paint on the fridge door doesn't need an energy source to stop itself falling off either :-)
     
  8. May 8, 2014 #7

    A.T.

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    That is only true in the rest frame of the magnet. There are inertial frames where the magnet does move. The key, to why the net work is zero, is not movement (which is relative) but the zero net force on the magnet.
     
    Last edited: May 9, 2014
  9. May 9, 2014 #8
    Not a very good example what if the door was horizontal,friction won't be holding it up then!
     
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