Explain the force acting on an electric dipole when it is placed parallel or anti-parallel to a non-uniform electric field.

(N/A) When an electric dipole with dipole moment $\vec{p}$ is placed in a non-uniform electric field $\vec{E}$,it experiences a net force.
$1$. When $\vec{p}$ is parallel to $\vec{E}$: The positive charge $q$ experiences a force $q\vec{E}$ in the direction of the field,and the negative charge $-q$ experiences a force $-q\vec{E}$ in the opposite direction. Since the field is non-uniform and stronger at the position of the positive charge,the net force on the dipole is in the direction of the increasing electric field.
$2$. When $\vec{p}$ is anti-parallel to $\vec{E}$: The positive charge $q$ experiences a force $q\vec{E}$ in the direction of the field,and the negative charge $-q$ experiences a force $-q\vec{E}$ in the opposite direction. Since the field is stronger at the position of the negative charge,the net force on the dipole is in the direction of the decreasing electric field.
In both cases,the net torque on the dipole is zero because the forces are collinear with the dipole axis,but a net translational force exists due to the non-uniformity of the field.