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Homework Help: Magnetostatics - Rotation of circular disc

  1. Feb 19, 2010 #1
    1. The problem statement, all variables and given/known data

    A disc has radius a and rotates with angular frequency w. Magnetic flux density is B. Such a disc of mass 10^4 kg and radius 3m is rotating freely at 3000 revs/min in a field of 0.5T. A load of 10^-3 ohms is connected suddenly between the rim and the axis of the disc. How long would it take the disc to slow to half its initial speed?

    2. Relevant equations

    3. The attempt at a solution

    I found an equation for the emf produced that is: w*a^2*B/2 . Using this I wanted to equate two equations -- kinetic energy of the rotating disc at half the intiial speed = (V^2/R)*T.

    The answer is meant to be T=6.16s, but thats not what I have.

    Thanks for any help.
  2. jcsd
  3. Feb 19, 2010 #2


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    Homework Helper

    No, the loss in kinetic energy should be equal to (V^2/R)*T.
  4. Feb 20, 2010 #3
    Ah I see now -- thanks for your help!
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