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)


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?

Well, without telling you the answer, use the force...

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...