Two point charges, +Q and -Q of mass m, are placed on the ends of a massless rod of length L, which is fixed to a table by a pin through its center.

Question

If the apparatus is then subjected to a uniform electric field E parallel to the table and perpendicular to the rod, find the net torque on the system of rod plus charges.
Express your answer in terms of some or all of the variables Q, m, L, and E.