Two insulating small spheres are rubbed against each other and placed 1 cm apart. Calculate the number of electrons transferred from one sphere to another if they attract each other with a force of 0. 1N.

Let the number of electrons transfered be n.

According to coulomb's law force between two charges is given by,

$\mathrm{F}=\frac{{\mathrm{kq}}_1{\mathrm{q}}_2}{{\mathrm{r}}^2}$

hence, the magnitude of force between the given two spheres will be

$\begin{array}{l}\Rightarrow \mathrm{F}=\frac{{\mathrm{kq}}^2}{{\mathrm{r}}^2}=0.1\\ \Rightarrow \frac{9\times {10}^9\times {\left(\mathrm{n}\times 1.6\times {10}^{-19}\right)}^2}{0{.01}^2}=0.1\\ \Rightarrow \left(\mathrm{n}\times 1.6\times {10}^{-19}\right)=\frac{1}{3\times {10}^7}\\ \Rightarrow \mathrm{n}=\frac{1}{4.8}\times {10}^{12}\approx 2\times {10}^{11}\end{array}$