A pith ball having charge `-20` esu and mass 0.1 g remians suspended at rest in air 2 cm below an insulated charged sphere.What is the amount of charge on the sphere and what is the nature of that charge?
Given `g=980 cm.s^(-2)`

Explanation

Suppose the charge on the sphere `=q`
Since the pith ball is at rest
weight of the pith ball `=` upward attractive force on the pith ball
or `0.1x980=(20xxq)/((2)^(2))` or `q=19.6`
Since the pith ball is at rest in space, the charge on the sphere is of opposite nature to that pith ball. The nature of charge on the pith ball is negative. So the charge on the sphere is positive.
`:.` Charge on the sphere `=+19.6`esu