Question

Current is flowing with a current density $J=480\, A\, cm ^{-2}$ in a copper wire. Assuming that each copper atom contributes one free electron and given that
Avogadro number $=6.0 \times 10^{23}$ atoms / mole
Density of copper $=9.0\, g / cm ^{3}$
Atomic weight of copper $=64\, g /$ mole
Electronic charge $=1.6 \times 10^{-19}$ coulomb
The drift velocity of electrons is

• A. $1 \,mm / s$
• B. $2\, mm / s$
• C. $0.5 \,mm / s$
• D. $0.36 \,mm / s$

Answer 1

Answer: (D)

Drift velocity, $v_{d}=\frac{\text { Current density }}{n e}$
$n=$ number of electrons per unit volume
or $n=\frac{(\text { Avogadro number }) \times \text { Density }}{\text { Atomic weight of copper }}$
or $n=\frac{6 \times 10^{25} \times 9}{64}$
$v_{d}=480 \times \frac{64}{6 \times 10^{23} \times 9 \times 1.6 \times 10^{-19}}$
$=\frac{480 \times 64}{6 \times 9 \times 1.6 \times 10000} cm / s$
or $v_{d}=\frac{32}{900} cm / s =\frac{32 \times 10}{900} mm / s =0.36\, mm / s$