Solved Faraday's Law Problem: Find Correct Answer

[neelakash]

[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?

 

[siddharth]

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.

 

[neelakash]

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...

 

[neelakash]

OK,I am trying.

 

[siddharth]

The answer is (B), cause
$$\tau= \frac{Q}{2 \pi} \int E.dl = - \frac{QR^2}{2} \frac{dB}{dt}$$
and $$L=\int \tau dt = I \omega$$

 

[neelakash]

Thank you very much...

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