An electric dipole is placed in a uniform electric field in with its axis inclined at a certain angle with the field. Does any force or torque act on the dipole?

Explanation

When and electric dipole along a uniform electric field, force acting on +q of the dipole is qE along the direction of `vec(E)` and force acting on -q is qE in the opposite direction of `vec(E)` [Fig.2.75]. So a torque acts on the dipole which tends to bring the dipole along the direction of the field. In this case net force acting on the dipole `=qvec(E)-avec(E)=0`
Therefore, a torque acts on the dipole but no resultant force acts on it. Because if the dipole is deflected through an angle `theta`, the torque `pEsintheta` acting on it brings it back along the field. When `vec(p)` and `vec(E)` are in opposite directions `(theta=180^(@))`, the torque acting the dipole is zero and the dipole is aging in equilibrium. But equilibrium is unstable, because if the dipole is deflected force its present position, the torque acting on it now will rotate the dipole to set it in the direction of the field.