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Homework Help: Eddy currents and torque

  1. Feb 20, 2008 #1
    1. The problem statement, all variables and given/known data

    An electromagnetic "eddy current" brake consists of a disc of conductivity [tex]\sigma[/tex] and thickness d rotating about an axis passing through its center and normal to the surface of the disc. A uniform B is applied perpendicular to the plane of the disc over a small area a^2 located a distance P from the axis. Show that the torque tending to slow down the disc at the instant its angular speed is W is given approximately by [tex]\sigma[/tex]*W*d*[B*P*a]^2

    3. The attempt at a solution

    I assume that you need to calculate the force on the disc at the small section a^2 and then from this a torque can be easily found. Can you say:

    [tex]F=q\cdot(E+v \times B)[/tex]

    And since the electric field is motional make this

    [tex]F=q\cdot(E+v \times B)=q\cdot(v \times B+v \times B)=q\cdot(2v \times B)[/tex]

    [tex]v \times B = BPW[/tex]


    If so, how do you find the charge enclosed inside of the little region of volume d*a^2? I don't quite see how the conductivity plays into all of this, or where the second factor of B comes from in the solution.

    Any help would be appreciated.
  2. jcsd
  3. Nov 27, 2014 #2
    I'm bumping this (I hope there's nothing wrong with me doing so). I'm having the exact same problem as well.
  4. Nov 29, 2014 #3
    Anyone? This problem is due soon, and I'm having trouble getting beyond that point.
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