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) epsilon_(0) E^(1) dV` where dV is small volume
`= (1)/(2) epsilon_(0) E^(2) int dV`
`because` E is constant `= (1)/(2)epsilon_(0)(sigma^(2))/(4 epsilon_(0)^(2)).a^(3) = (sigma^(2)a^(3))/(8 epsilon_(0))`