A Gaussian surface is chosen to obtain an expression for the electric field at a point due to a given charge configuration. It is a closed surface. State one essential property of such a chosen Gaussian surface.

A point-charge is placed at the centre of a closed Gaussian spherical surface. How is electric flux ${\Phi }_E$, through the surface affected when

$\left(\mathrm{i}\right)$ The sphere is replaced by a cylinder of the different volume?

A point-charge is placed at the centre of a closed Gaussian spherical surface. How is electric flux ${\Phi }_E$, through the surface affected when

(ii) The charge is shifted from the centre to some other place inside the sphere?

A point-charge is placed at the centre of a closed Gaussian spherical surface. How is electric flux ${\Phi }_E$,  through the surface affected when

 (iii) A second charge is placed inside the sphere?