Two copper balls, each weighing 10g are kept in air 10 cm apart. If one electron from every 10⁶ atoms is transferred from one ball to the other, the coulomb force between them is (atomic weight of copper is 63.5)

Number of atoms in given mass $=\frac{10}{63.5}\times 6.02\times {10}^{23}$

$=9.48\times {10}^{22}$

Transfer of electron between balls $=\frac{9.48\times {10}^{22}}{{10}^6}$

$=9.48\times {10}^{16}$

Hence magnitude of charge gained by each ball. 

$\mathrm{Q}=9.48\times {10}^{16}\times 1.6\times {10}^{-19}=0.015\mathrm{C}$

Force of attraction between the balls 

$\mathrm{F}=9\times {10}^9\times \frac{(0.015{)}^2}{(0.1{)}^2}=2\times {10}^8\mathrm{N}$