The electrostatic force on a metal sphere of charge `0.4muC` due to another identical metal sphere of charge `-0.8muC` in air is 0.2N. Find the distance between the two spheres and also the force between the same two spheres when they are brought into contact and then replaced in their initial positions.

Explanation

(a) We know that
`|vecF|=(1/(4piepsi_(0)))((q_(1)q_(2))/x^(2))`
i.e., `0.2=(9xx10^(9)xx0.4xx0.8xx10^(-12))/x^(2)`
`therefore" "x^(2)=14.4xx10^(-12+9)=14.4xx10^(-3)=0.0144m^(2)`
`therefore" "x=0.12mor12cm`.
(b) `|vecF_(21)|=|vecF_(12)|=0.2N` repulsive.