## 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)

## 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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siddharth
Homework Helper
Gold Member
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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siddharth
Homework Helper
Gold Member
$$\tau= \frac{Q}{2 \pi} \int E.dl = - \frac{QR^2}{2} \frac{dB}{dt}$$
and $$L=\int \tau dt = I \omega$$