A point particle of mass M is attached to one end of massless rigid non-conducting rod of length L. Another point particle of the same mass is attached to other end of the rod. The two particles carry charges +q and –q respectively. This arrangement is held in a region of a uniform electric field E, such that the rod makes a small angle θ (say of about 5 degree) with the field direction. Find an expression for the maximum time needed for the rod to become parallel to the field after it is set free-
Text Solution
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As shown in Fig, 
Tonque on the rod AB, 
When θ is small, 
(i)
Moment of inertia of the rod about O.
Clearly, α is directly proportional to θ, and torque is trying to decreses θ.
The motion of the rod si S.H.M., with
The rod will become parallel to 
from a position, 
in a time
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