Question

A long, thin rod (length=4.0 m) lies along the x axis, with its midpoint at the origin. In a vacuum, a +8.0 point charge is fixed to one end of the rod, and a -8.0 point charge is fixed to the other end. Everywhere in the x, y plane there is a constant external electric field (magnitude=5.0x10^3 N/C) that is perpendicular to the rod. With respect to the z axis, find the magnitude of the net torque applied to the rod.