Question

Directions : The following question contains statement1 and statement-2. Of the four choices given, choose the one that best describes the two statements.
(a) Statement-1 is true, statement-2 is false.
(b) Statement-1 is false, statement-2 is true.
(c) Statement-1 is true, statement-2 is true; statement-2 is a correct explanation for statement-1.
(d) Statement-1 is true, statement-2 is true; statement-2 is not a correct explanation for statement-1.

Statement-1 : For a mass M kept at the centre of a cube of side a, the flux of gravitational field passing through its sides is 4πGM.
Statement-2 : If the direction of a field due to a point source is radial and its dependence on the distance r from the source is given as 1/r² , its flux through a closed surface depends only on the strength of the source enclosed by the surface and not on the size or shape of the surface.

Answer 1

(c) : Let A be the Gaussian surface enclosing a spherical charge Q.

Every line passing through A, has to pass through B, whether B is a cube or any surface. It is only for a Gaussian surface, the lines of a field should be normal. Assuming the mass is a point mass.

be the shape, all the lines passing through A are
passing through B, although all the lines are not
normal.
Statement-2 is correct because when the shape of the earth is spherical, an area of the Gaussian surface is 4πr². This ensures inverse square law.