Electric charge q is uniformly distributed in a spherical space from which a cavity of diameter equal to the radius R of the spherical space has been removed. Calculate the potential and field at a point P lying on the diameter of the cavity at a distance `rgtr//2` but lt R from the centre of the sphere.

Answer

`(1)/(4piepsilon_(0))(4q)/(qR^(2))((5)/(2)R-r),(q)/(7piepsilon_(0))`