If ' $n$ ' electrons are placed on each of the two small spheres with their centres spaced $3 \mathrm{~cm}$…
If ' $n$ ' electrons are placed on each of the two small spheres with their centres spaced $3 \mathrm{~cm}$ apart so that they repel each other by a force of $10^{-19} \mathrm{~N}$ then ' $\mathrm{n}$ ' will be
$\left[\frac{1}{4 \pi \varepsilon_0}=9 \times 10^9 \mathrm{Nm}^2 \mathrm{C}^{-2}\right]$