Derive an expression for energy stored in a parallel plate capacitor fo capacitane C with air as medium between the plates having charges `Q and -Q`. Show that this energy can be expressed in terms of electric field as `(1)/(2) in_(0) E^(2)` Ad, where A is area of each plate and d is the separation between the plates. How will the energy stored in a fully charged capacitor chanege when the separation between the plates is doubled and the dielectric medium of constant 4 is introduced between the plates ?