Question

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 :

• A. $5\,A$
• B. $80\,A$
• C. $8\,A$
• D. $0.8\,A$

Answer 1

Answer: (D)

$0.8\,A$

Answer 2

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
$i=ne \,A v_{d}$
$i=\frac{2 \times 10^{21}}{A \times 100} \times 1.6 \times 10^{-19} \times A \times 0.25$
$i=0.8\, A$