Find out energy stored in an imaginary cubical volume of side a in front of a infinitely large non-conducting sheet of uniform charge density `sigma`.

Explanation

Energy stored : `U = int 1/2 epsi_(0) E^(2) dV`, where dV is small volume
`:. U=1/2 epsi_(0) E^(2) int dV" ":' E` is constant
`:. U=1/2 epsi_(0) sigma^(2)/(4 epsi_(0)^(2)). A^(3). =(sigma^(2)a^(3))/(8 epsi_(0))`