According to Gauss’ law in electrostatics, the surface integral of electrostatic field E produced by any source over any closed surface S enclosing a volume V in vacuum, i.e. total electric flux over the closed surface 5 in vacuum, is
Gauss’ law in electrostatics is true for a closed surface, no matter what its shape or size is.
So, in order to justify the above statement, suppose in isolated positive charge q is situated at the centre O of a sphere of radius r.
According to Coulomb’s law, electric field intensity at any point P on the surface of the sphere is