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Homework Help: Sliding conductor on a metal rail, with perpendicular B-field

  1. Sep 24, 2011 #1
    1. The problem statement, all variables and given/known data

    In the following diagram, the brown metallic conductor rod is given a slight push to the left.
    The black lines are all conducting wires, with the entire setup placed in a perpendicular B-field. Ignore friction.

    What will happen?

    1) The rod slowly come to a stop?
    2) The rod continues to move at constant speed?
    3) The rod comes to a stop and reverses direction?

    [PLAIN]http://img52.imageshack.us/img52/9687/magqj.png [Broken]

    3. The attempt at a solution

    1) I think this should be the answer, as kinetic energy has to be converted to electrical current (heating of the wires). The motion has got to die down.
    However, I cannot identify the retardation force that acts on the rod which is necessary for the motion to stop.

    2) I don't think it's gonna move forever as argued above.

    3) I seriously doubt this man. Intuition, somehow.
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Sep 25, 2011 #2


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    Homework Helper

    As the conductor moves to the left, the area of the loop reduces. That means the amount of flux threading the loop reduces. That reduction in flux means a current is induced in the loop. Once that current is flowing through the moving wire, a force is induced on that wire [F = B.I.l ] that is the force that stops the wire.
    Last edited by a moderator: May 5, 2017
  4. Sep 25, 2011 #3
    Hmm, but,

    the direction of the induced current in this case is anti-clockwise.
    Therefore, the direction of the current in the rod, is upwards.
    Wont the force on the rod be to the left?! that's not retardation?
  5. Sep 25, 2011 #4


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    Induced Forces ALWAYS oppose change so one of those direction reckonings is backwards. See if you can work out which one.
  6. Sep 25, 2011 #5
    oh! sorry a slipped on my part.
    so it's clockwise induced current. B-force is therefore on the right.
    I see. thanks.
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