Answer the following questions: (a) Define electric flux. Write its SI unit. (b) “Gauss’s law in electrostatics is true for any closed surface, no matter what  its shape or size is.” Justify this statement with the help of a suitable example.  [CBSE Allahabad 2015]

(a) Electric flux is a measure of the electric field passing through a given area. It is defined as the product of the electric field strength and the area perpendicular to the field. The SI unit of electric flux is volt-meter squared, written as V·m².

(b) According to the Gauss theorem, the electric flux through a closed surface depends only on the net charge enclosed by the surface and not upon the shape or size of the surface.

For any closed arbitrary shape of the surface enclosing a charge, the outward flux is the same as that due to a spherical Gaussian surface enclosing the same charge.

Justification: This is because

(i) The electric field is radial and

(ii) The electric field E \propto{1\over R^2}

Thus, the electric field at each point inside a charged thin spherical shell is zero.