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Electromagnet energy ?

  1. Jun 28, 2012 #1
    Electromagnet energy ???

    Hi i have very little knowledge of electrodynamics so this is probably a no brainer but:

    Given an electromagnet holding a ferrous mass suspended in gravity is the electric energy required exactly equivalent to the basis draw with no mass load??

    Thanks
     
  2. jcsd
  3. Jun 29, 2012 #2

    Simon Bridge

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    Re: Electromagnet energy ???

    The energy needed to do something is the applied force multiplied by the distance moved (in the direction applied). Since the suspended mass does not move... the electromagnetic energy needed to hold it there is zero.
     
  4. Jun 29, 2012 #3
    Re: Electromagnet energy ???

    Thanks
    Does this imply that if the magnet picks something up off the ground through the air that there would be a current increase during the acceleration because work was being done?
     
  5. Jun 29, 2012 #4
    Re: Electromagnet energy ???

    If you will take a magnet and a small piece of iron,for example. Then you may put the piece of iron on a table and try to pick it up by your magnet. Eventually, you will be able to change the position of the piece by lifting in up. At first glance, it seems that you magnetic field did some work. But, there is an energy stored in the magnetic field of your magnet u = B*B/2μ. where, u - energy density, B - magnetic field , μ - magnetic permiability in μ0 units. So, If you will add a piece of iron, it will alter the total magnetic field, so increasing of the potential energy of the piece of iron will be canceled by the decreasing of the magnetic field of the system (magnet + iron).
     
  6. Jun 29, 2012 #5

    Simon Bridge

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    Re: Electromagnet energy ???

    As a conductor moves towards a magnet, there will be an induced current - sure.
    I believe your question has been answered - you can imagine that there is some caution about answering questions about "energy of magnets" since they usually come from free-energy advocates.
     
  7. Jun 30, 2012 #6

    Dale

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    Re: Electromagnet energy ???

    Typically it would be a current decrease, but yes. This is actually an important effect for MRI machines. Say you have a MRI scanner installed next to an elevator shaft. As the elevator comes near the magnet work is done on the elevator, the current in the coil drops, and the field is reduced. This causes visible artifacts in the images. To correct it, the MRI system has coils wound the opposite way so that as the field of one goes up the field of the other goes down.
     
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