A parallel plate condenser has a uniform electric field $$E\left( {\frac{V}{m}} \right)$$  in the space between the plates. If the distance between the plates is $$d\left( m \right)$$  and area of each plate is $$A\left( {{m^2}} \right),$$  the energy (joule) stored in the condenser is

Solution :
As we know that the energy stored in the capacitor is given by,
$$\eqalign{ & U = \frac{1}{2}C{V^2}\left[ {_{V = \,\,{\text{voltage across the plate}}}^{C = \,\,{\text{capacitance of capacitor}}}} \right] \cr & U = \frac{1}{2}\left( {\frac{{A{\varepsilon _0}}}{d}} \right){\left( {Ed} \right)^2}\,\,\,\,\left( {\because C = \frac{{{\varepsilon _0}A}}{d}\,\,{\text{and}}\,\,V = Ed} \right) \cr & U = \frac{1}{2}\frac{{{\varepsilon _0}A}}{d}{\left( {Ed} \right)^2},\,\,U = \frac{1}{2}{\varepsilon _0}{E^2}Ad \cr} $$