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Homework Help: Faraday's law problem

  1. Dec 9, 2007 #1
    [SOLVED] Faraday's law problem

    1. The problem statement, all variables and given/known data

    Charge Q is uniformly glued over a thin (insulating) ring of radius R and mass m.The ring can rotate freely in horizontal plane about z axis.A magnetic field B is switched on along +z axis.The ring rotates with angular velocity w.


    (B)w= -(QB/2m)

    (C)w= -(QB/m)


    2. Relevant equations
    3. The attempt at a solution

    I have rejected (A) and (D).Either (B) or (C) is correct...But,anyhow I cannot get the answer.Everytime I am ending up with dB/dt from Faraday's law...

    Can anyone say what is correct?
  2. jcsd
  3. Dec 9, 2007 #2


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    There'll be an electric field induced, so what's the torque due to the electric field on the ring? And so, what's the angular momentum imparted to the ring? The [tex]\frac{dB}{dt}[/tex] will come up, but remember you're going to integrate over time to find the net angular momentum imparted.
    Last edited: Dec 9, 2007
  4. Dec 9, 2007 #3
    If you do not mind,can you please only give me the answer?

    I wrote (B) in an exam today...And I am really tensed about it...
  5. Dec 9, 2007 #4
    OK,I am trying.
    Last edited: Dec 9, 2007
  6. Dec 9, 2007 #5


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    The answer is (B), cause
    [tex]\tau= \frac{Q}{2 \pi} \int E.dl = - \frac{QR^2}{2} \frac{dB}{dt}[/tex]
    and [tex]L=\int \tau dt = I \omega [/tex]
    Last edited: Dec 9, 2007
  7. Dec 9, 2007 #6
    Thank you very much...

    Oh...I was getting crazy over it...
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