Question

The inverse square of law of intensity (i.e. the intensity $\propto$ $\dfrac{1}{{{r^2}}}$) is valid for a
(A) Point source
(B) Line source
(C) Plane source
(D) Cylindrical source

Answer 1

Hint The intensity of the influence at any given radius r is the source strength divided by the area of the sphere. Being so strictly geometric in its origin, the inverse square law can be applied in the inverse way.

Complete step by step answer:
Firstly the inverse square law: Any point source which spreads its influence equally in all directions without limits to its range will obey the inverse square law .
Actually this comes from the strictly geometrical consideration. The intensity of the influence at any given radius r is the source strength divided by the area of the sphere. Being strictly geometric in its origin; the inverse square law is used in a diverse way.
Point source of the gravitational force, electrostatic field, light, sound or radiation obeys the inverse square law.
Line or plane or cylindrical source, it varies with the distance i.e.
Intensity $\propto$ $\dfrac{1}{{{r^{}}}}$

Note
The value of the gravitational force is smaller than the electrostatic force.
In the gravitational field there is the involvement of the mass but in case of the electrostatic field there is the involvement of the charge.
Electrostatic fields can be defined as the force applied per unit charge.
Gravitational force having SI unit Newton.
In case of the electrostatic force Its SI unit is also Newton.