Calculate the drift speed of the electrons when 1 A of current exists in a copper wire of cross-section $2m{m}^2$. The number of free electrons in $1c{m}^3$ of copper is $8.5\times {10}^{22}$.

$n=$ free electron density,

$=8.5\times {10}^{22}\text{ per }c{m}^3$

$=\left(8.5\times {10}^{22}\right)\left({10}^6\right)\text{ per }{m}^3$

$=8.5\times {10}^{28}\text{ per }{m}^3$  (Given)

From $i=neA{v}_d$, we get

$v_d=\frac{i}{neA}$

Substituting the values in SI units we have,

$v_d=\frac{1}{\left(8.5\times {10}^{28}\right)\left(1.6\times {10}^{-19}\right)\left(2\times {10}^{-6}\right)}$

$=3.6\times {10}^{-5}m/s$