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Something Vexing me about Magnetism and Conservation of Energy

  1. Jan 25, 2010 #1
    Something Vexing me about Magnetism and Conservation of Energy

    I have been thinking about an issue at work and it has been bugging me so I thought I would post something on here to see if anyone can explain where my thought process is going wrong.

    Say you have a super conducting magnet. It is a big one. The current is switched on and so the magnet becomes active. As it is super conductive the current is not impeded and continues to flow. The magnet is used to lift a large weight off the ground. At somepoint the current is channeled off somewhere else, the magnet stops working and the weight falls. The energy of the weight falling under gravity is captured somehow. The current is channelled back to the magnet, weight is lifted again and so on....

    So, my question is, does this system not break the conservation of energy principle? Are we not getting something for nothing here? I am sure I am missing something in my understanding of magnetism, but I thought it was just a field and so no actual mass is lost when it is active?
     
  2. jcsd
  3. Mar 30, 2011 #2
    Since the superconducting magnet is lifting a load, it is coupled to it like a transformer. That coupling will limit the current even though you are pushing electrons into a superconducting material. Once the weight stops moving, the superconductor will reach full current flow.
    That's my understanding.
     
  4. Mar 30, 2011 #3

    russ_watters

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    Staff: Mentor

    Welcome to PF.

    You seem to be under the misconception that a superconducting magnet will not create any restriction to the flow of current. This is not correct: a superconductor has no resistive loss, but it does still obey the laws of inductance.
     
  5. Mar 30, 2011 #4
    I would also offer to keep in mind the large energy EXPENDITURE necessary to keep a superconducting magnet cool enough.
    Thus, no matter what one does, the output will never meet or exceed the energy required to keep the system running.
    What about hypothetical room temp. superconductors? Ok, we removed the cooling factor but Russ addressed that other factors come into play.
     
  6. Mar 31, 2011 #5
    In other words, the energy conservation still applies.
    As the weight potential energy will increase, the magnetic energy will decrease, and so should the magnetic field and the current.
    Pulling the weight back will restore the magnetic field.

    Losses due to currents induced in the weight should be added to this picture.
     
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