An electric dipole $\mathrm{AB}$ consisting of charge $+\mathit{q}$ and $-\mathit{q}$ and of length $2\mathit{a}$ is placed in uniform electric field $\mathit{E}$ making an angle $\mathit{\theta }$ with the direction of electric field.

Force acting on $-\mathit{q}$ is $-\mathit{q}\mathit{E}$
Force acting on $+\mathit{q}$ is $\mathit{q}\mathrm{E}$
These two forces are equal and opposite to each other. Hence, a torque on the dipole is developed.

Torque $=$ Force $\times$ perpendicular distance between the forces




Dipole will attain stable equilibrium when it will be oriented along the direction of electric field.

OR
A capacitor is connected across the terminals of a d.c. battery.

The energy stored on a capacitor is equal to the work done by the battery.

Work done to move a small amount of charge dQ from the negative plate to the positive plate of the capacitor is equal to $\mathrm{V}\mathrm{dQ}$, where $\mathrm{V}$ is the voltage across the capacitor.
$\mathrm{dU}=\mathrm{VdQ}=\frac{\mathrm{Q}}{\mathrm{C}}\mathrm{dQ}$
$\therefore$ Energy stored $=$
$\mathrm{U}=\int \mathrm{VdQ}=\frac{1}{\mathrm{C}}\int \mathrm{QdQ}$ $=\frac{1}{2}\frac{{\mathrm{Q}}^2}{\mathrm{C}}=\frac{1}{2}{\mathrm{CV}}^2$ ......(i)
Energy density is defined as the total energy per unit volume of the capacitor.
For a parallel plate capacitor,
$\mathrm{C}=\frac{\mathit{A}{\mathit{\epsilon }}_0}{\mathit{d}}$
Putting in eqn (i)
$\mathrm{U}=\frac{1}{2}\frac{\mathit{A}{\mathit{\epsilon }}_0}{\mathit{d}}{\mathrm{V}}^2$ $=\frac{{\mathit{\epsilon }}_0}{2}\mathrm{Ad}{\left(\frac{\mathit{V}}{\mathit{d}}\right)}^2$

A $\times \mathrm{d}=$ Volume of space between plates
So, energy stored per unit volume