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Induced current

  1. Mar 31, 2008 #1
    There is something on Lenz's law about induced current and the magnetic field that I don't quite understand. "Apply Lenz's law that when the magnet is brought near the coil, the magnet's magnetic field through the coil increases, and therefore the flux increases."
    Here, why the magnet's magnetic field would increase?? No matter if it goes near the coil, I think it keeps the same, instead, the magnetic field created by the current should increase or coming to existence?
    "The magnetic field of the magnet points upward. To oppose the upward increase, the magnetic field inside the coil produced by the induced current needs to point downward."
    Here, why the magnetic field in the coil needs to oppose the upward increase??!!
    "When the magnet is brought far away from th coil, the flux decreases, so th induced current in the coil produces an upward magnetic field through the coil that is 'trying' to maintain the status quo."
    Again, why it needs to main the status quo? I don't get the purpose of all these. Hope you can explain them to me. Thank you very much.
     
    Last edited: Mar 31, 2008
  2. jcsd
  3. Apr 1, 2008 #2

    Doc Al

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    Staff: Mentor

    Conservation of energy

    The field from the magnet is not uniform--it's stronger the closer you get to the end of the magnet. So when you bring the magnet closer to the loop, the field through the loop is stronger.
    Lenz's law states that current is induced in such a way as to oppose the the change in flux. This is a consequence of conservation of energy. Having the current oppose the change means that it takes work (a force pushing through a distance) to create that induced current. If it didn't, we could get free current! (That's not going to happen.)
     
  4. Apr 6, 2008 #3
    Do you mean that when the flux seems to increase,the magnetic field inside the coil will point in the opposite direction of the magnetic field of the magnet, just because of the conservation of energy; when the flux seems to decrease, B in the coil will point in the same direction of B in the magnet? But by the way, flux=BA. here Is B the magnetic field of the coil or the magnet?
     
  5. Apr 8, 2008 #4
    I mean if it is only either magnetic field of the coil or of the magnet, then it doesn't matter if the other one's magnetic field's direction as I stated before. If you don't understand what I meant, tell me. thank you.
     
  6. Apr 8, 2008 #5

    Doc Al

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    Staff: Mentor

    Think of the induced current as a response to the changing flux within the loop due to the movement of the magnet. The induced current will be in such a direction as to create a secondary field that opposes the change in flux.

    You might find this discussion of Lenz's law helpful: Lenz's Law

    If I missed your point, please ask again.
     
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