Two point masses, m each carrying charges `-q` and `+q` are attached to the ends of a massless rigid non - conducting wire of length L. When this arrangement is placed in a uniform electric field, then it deflects through an angle `theta`. The minimum time needed by the rod to align itself along the field is

Explanation

`impliestau=pEsintheta,("as "thetararr0,sinthetararrtheta)`
`implies tau=-(pE)theta` (If we assume angular displacement to be anti-clockwise, torque is clockwise)
`implies alpha=-((pE)/(l))theta=-omega^(2)theta`
As torque is proportional to `'theta'` and oppositely directed, the motion will be an angular S.H.M.
Here, p = q.L. and moment of inertia, l = M
`(L//2)^(2)+M(L//2)^(2)=ML^(2)//2`
As, time period or `T=2pisqrt((l)/(pE))`
The minimum time required to align itself is T/4 sec.