Question

Derive the expression for net force and torque experienced by electric dipole placed in uniform external electric field such that angle b/w p and E is theta. Discuss the cases of stable and unstable equilibrium with diagram.

Answer 1

let the length of the dipole is s such that |r+| = s/2  |r-| = s/2
the forces on +q and -q is F+ = qE , F- = -qE
total force on the dipole is zero
torque = r+×F+  +  r-×F-
           = s/2×qE  +  -s/2×-qE
           =qs×E
           = p×E
           =pEsin(theta)