A dipole is placed in an electric field as shown. In which direction will it move ?
Answer: (C) towards the right as its potential energy will decrease.
The field lines spread out to the right, so the field is non-uniform: stronger on the left and weaker on the right.
$+q$ is on the left, where the field is stronger, and is pushed right with force $qE_{\text{left}}$. $-q$ is on the right, where the field is weaker, and is pulled left with the smaller force $qE_{\text{right}}$. The net force is to the right.
Equivalently, $\vec{p}$ (from $-q$ to $+q$) points to the left, opposite to $\vec{E}$, so $U = -\vec{p}\cdot\vec{E} = +pE$. Moving right into the weaker field lowers $U$. The dipole moves to the right and its potential energy decreases.
Solution by Sreeraj P, M.Sc Physics