Solved Faraday's Law Problem: Find Correct Answer

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[SOLVED] Faraday's law problem

Homework Statement



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.

(A)w=0

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

(C)w= -(QB/m)

(D)w=(3/5)(QB/m)

Homework Equations


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?
 
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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 \frac{dB}{dt} will come up, but remember you're going to integrate over time to find the net angular momentum imparted.
 
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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...
 
OK,I am trying.
 
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The answer is (B), cause
\tau= \frac{Q}{2 \pi} \int E.dl = - \frac{QR^2}{2} \frac{dB}{dt}
and L=\int \tau dt = I \omega
 
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Thank you very much...

Oh...I was getting crazy over it...
 

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