Assume that each atom of copper contributes one free electron, If the current flowing through a copper wire of 1 mm diameter is 1.1 A, the drift velocity of electrons will be [density of Cu = 9gcm ^-3, At. wt. of Cu = 63)

The number of atoms in 63g of copper is equal to Avogadro's number = 6 .0 x 10²³
Volume of 63g copper  $\frac{63}{\text{ density }}=\frac{63}{9}=7{\mathrm{cm}}^3$
$\begin{array}{l}\therefore \mathrm{n}=\frac{6\cdot 02\times {10}^{23}}{7}\text{ per }{\mathrm{cm}}^3\\ =\left(6\cdot 02\times {10}^{29}/7\right)\text{ per }{\mathrm{m}}^3\end{array}$
$A=\pi r^2=\pi {\left(0\cdot 5\times {10}^{-3}\right)}^2{\mathrm{m}}^2$
The drift velocity, ${\mathrm{v}}_{\mathrm{d}}=\mathrm{i}/\left(\mathrm{neA}\right)$
$\begin{array}{l}\therefore {\mathrm{v}}_{\mathrm{d}}=\left[\frac{1\cdot 1}{\left(6\cdot 02\times {10}^{29}/7\right)\times \left(1\cdot 6\times {10}^{-19}\right)\times \mathrm{\pi }{\left(0\cdot 5\times {10}^{-3}\right)}^2}\right]\\ =0\cdot 1\times {10}^{-3}\mathrm{m}/\mathrm{s}=0\cdot 1\mathrm{mm}/\mathrm{s}\end{array}$