Question

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

Answer 1

a) Using Coulomb's Law,

$F=\frac{1}{4\pi {\epsilon }_0}\frac{q_1{q}_2}{r^2}$

$r^2=9\times {10}^9\times \frac{0.4\times 0.8\times {10}^{-12}}{0.2}$

$r^2=14.4\times {10}^{-3}$

$=144\times {10}^{-4}$

$r=12\times {10}^{-2}=0.12m$

b) Force on second sphere due to first is $0.2N$ but in opposite direction, because the charges due to same and distance is same; so force is also same.