A Question About Magnets

  • #1
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I have a feeling that this might sound like a question I should already know the answer to, but here goes.

If a man holds a heavy weight up, he's expending energy to work against gravity and keep it there. However, he is not doing work if he doesn't move the weight.

A magnet can stick to the side of a fridge, but it does no work because nothing moves. However, doesn't it have to expend some energy to defy gravity like it does?

Am I wrong? If so, how does it work? If not, where does the energy come from? I assume that I am wrong, because I can't think of any place the energy could come from.

Thanks!
 

Answers and Replies

  • #2
Work and energy are related but not the same thing. You can expend energy without doing work. It is by definition of the word Work. Something must move in order to do work.

The magnet is attracted to the fridge by the electrostatic force, though the energy is conserved in the system.
 
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  • #3
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Could somebody explain how the energy is conserved?
 
  • #4
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Magnets do wear out. If you wait long enough the magnet will fall off the fridge. The domains get randomised again and the magnetism slowly weakens. If you pit magnets against each other, like these "free energy" magnet motors, youll see they wear out really quickly. Which is why they never seem to make it off youtube :biggrin:
 
  • #5
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Are you saying that the domains losing alignment releases energy in some way?
 
  • #6
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Are you saying that the domains losing alignment releases energy in some way?

They basically start to cancel each other out when alignments lose coherence, if I understand it correctly ( don't quote me on that ). My metallurgy book doesn't go too much into it, so I don't really know for sure.
 
  • #7
alxm
Science Advisor
1,848
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You shouldn't think about objects as if they were humans. Mechanical work is not analogous to physical exertion. Obviously you expend energy to hold something up against gravity, even if that means you don't perform any mechanical work. You expend energy standing up too, it doesn't mean a table does so.

A magnet stuck to a metal surface doesn't expend any energy to remain in that state. Now, it's true that a magnetized object has a little bit more energy than a non-magnetized one, which is why a magnet will eventually become de-magnetized. But that energy isn't being expended to remain stuck to a surface; that doesn't take any energy. It's this tendency to think about stuff as if they were human that causes magnets to baffle us. From our perspective, it requires energy to stand up against gravity. It takes energy to grab onto a tree-branch and hang from it. Etc. But it doesn't take energy for a simple physical object to remain stationary (or for humans, it's just that 'standing still' to us really isn't).
 
  • #8
It appears what Restless is asking can be put in a bit simpler model.

If you have 2 opposite charges at some displacement at rest. They begin to move. They gain energy, momentum. Where does this energy come from.

To answer the question about how energy is conserved. Both charges gain exactly the same momentum in exactly opposite directions. Add these vectors and you get null.
 
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  • #9
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Ah, that makes sense.
 

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