An uncharged conducting sphere is brought in contact with an identical sphere having a charge of \(4 \times 10^{-8} \mat…

Let the distance between them is r.

According to the conservation of charge, the charge on each after contact is

\(\mathrm{q}=\frac{4\times {10}^{-8}}{2}=2\times {10}^{-8}\mathrm{C}\\ \mathrm{Now}\\ \mathrm{F}=\frac{\mathrm{k}{\mathrm{q}}^{2}}{{\mathrm{r}}^{2}}\\ 9\times {10}^{-3}=\frac{9\times {10}^{-3}\times 2\times {10}^{-8}\times 2\times {10}^{-8}}{{\mathrm{r}}^{2}}\\ \mathrm{r}=0.02\mathrm{m}\)