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Residual electromagnet magnetism

  1. Jul 15, 2009 #1
    Have always wondered this so maybe I can get an answer here: when a large industrial electromagnet is switched off and the electricity-produced magnetism ceases, is there a small amount of residual magnetism in the uncharged electro-magnet? If so, does it dissipate according to a established formula? If it exists, it is essentially free energy at that point, so could it be used for an alternate and lesser purpose before it dissipates thus saving the energy needed to produce it for this alternate use?

    I imagine the real question here is: if it exists, does it have any practical use before it dissipates? Or, if it exists, is it too small an amount to be of any alternate benefit?

    Thanks for all informed answers.
  2. jcsd
  3. Jul 15, 2009 #2


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    Residual magnetism in an electromagnet is an undesirable side effect.
    It may mean that the magnet will still hold small particles like iron dust when it should let go of everything.

    I don't think you can call it free energy except that it may take slightly less power to magnetize the magnet next time. However, electromagnets usually get the same power every time they are turned on, so this slight advantage is not used.

    Mostly it is a very small effect because very low carbon iron is used to make the electromagnet.
  4. Jul 15, 2009 #3


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    You can't get energy out of a static magnetic field, so even with a high-remanence core (that's a fancy term that means a residual magnetic field is left after the coil is de-energized) you can't get any "free energy". As with all permanent magnets, you could move it through a coil to generate electricity--this is, after all, how all generators work--but that energy isn't free; the energy ultimately comes from whatever it is you use to move the magnet, be it coal, nuclear, gasoline or you.
  5. Jul 16, 2009 #4
    Thanks for the responses. In considering all forms of energy conservation, I sometimes ponder any possible way to save wasted energy no matter how small or inefficient the savings may be. One day I may think of one that is practical but not this time, I must assume.
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