The electrostatic force on a small sphere of charge `0.4 muC` due to another small sphere of charge -0.8 uC in air is 0.2 N.
(a) What is the distance between the two spheres ?
(b) What is the force on the second sphere due to the first ?

Explanation

(a) Coulombian force between two point as charges is,
`F = k(q_(1)q_(2))/r^(2)`
`therefore -0.2 = 9 xx 10^(9) xx (0.4 xx 10^(-6))(-0.8 xx 10^(-6))/r^(2)`
(Here, F is attractive and so its value is taken as negative)
`therefore r^(2) = (9 xx 10^(9))(4 xx 10^(-7))(8 xx 10^(-7))/(2 xx 10^(-1))`
`therefore r^(2) = 144 xx 10^(-4) m^(2)`
`therefore r = 12 xx 10^(-2) m = 0.12 m`
(b) Here Coulombian force on second sphere due to first sphere is also 0.2 N and attractive in nature because electric force is two body force which is always exerted mutually with equal magnitude and opposite directions.