The number density of free electrons in copper is nearly $8 \times 10^{28} \mathrm{~m}^{-3}$. A copper wire has its area of cross section $=2 \times 10^{-6} \mathrm{~m}^2$ and is carrying a current of 3.2 A . The drift speed of the electrons is
$\_\_\_\_$ $\times 10^{-6} \mathrm{~ms}^{-1}$.
