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Q.
The number of free electrons per $ 100 \,mm $ of ordinary copper wire is $ 2 × 10^{21} $ . Average drift speed of electrons is $ 0.25\, mm/s $ . The current flowing is :
Drift velocity is defined as the average velocity that a particle attains due to an electric field.
$v_{d}=\frac{i}{ne \,A}$
where $i$ is current, $n$ the number of electrons, $e$ the electron charge and $A$ the area of crosssection of wire.
Number of electrons per unit volume
$n=\frac{2 \times 10^{21}}{A \times 100}$
Hence, current in the wire
$ i=ne \,A v_{d} $
$i=\frac{2 \times 10^{21}}{A \times 100} \times 1.6 \times 10^{-19} \times A \times 0.25$
$i=0.8\, A$