An electric dipole $AB$ consisting of charge $+q$ and $-q$ and of length $2a$ is placed in uniform electric field $E$ making an angle $\theta$ with the direction of electric field. Force acting on $-q$ is $-q E$Force acting on $+q$ is $q E$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 $\;\text{Or,}\;\; \tau = q E \times 2a \sin \theta$ $\;\text{Or,}\;\; \tau = (q \times 2a) E \sin \theta$ $\therefore \;\tau = \;\text{PE}\; \sin \theta$ $\therefore \;\; \tau = PE \sin \theta \;\; \;\text{(where}\; P \;\text{is dipole moment)}\;$Dipole will attain stable equilibrium when it will be oriented along the direction of electric field. ORA 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 $V dQ$, where $V$ is the voltage across the capacitor. $dU = VdQ = \frac{Q}{C} dQ$ $\therefore$ Energy stored $=$ $U = \int VdQ = \frac{1}{C} \int QdQ$ $= \frac{1}{2} \frac{Q^2}{C} = \frac{1}{2} CV^2$ ......(i) Energy density is defined as the total energy per unit volume of the capacitor. For a parallel plate capacitor, $C = \frac{A \varepsilon_0}{d}$ Putting in eqn (i) $U = \frac{1}{2} \frac{A \varepsilon_0}{d} V^2$ $= \frac{\varepsilon_0}{2} Ad \left( \frac{V}{d} \right)^2$ $U = \frac{\varepsilon_0}{2} Ad^2 \left( \;\text{putting}\; \; \frac{V}{d} = E \right)$A $\times d =$ Volume of space between platesSo, energy stored per unit volume