Q. The number of free electrons per 100 mm of ordinary copper wire is $2 \times 10^{21}.$ Average drift speed of electrons is 0.25mm$s^{- 1}$. The current flowing is
A) 8 A
B) 0.8 A
C) 80 A
D) 5 A
Explanation
(b) Drift velocity,
$v_{d} = \frac{i}{neA}$
Where I is current, n the number of electrons, e the electron charge and A the area of cross-section of wire.
Number of electrons per unit volume
$n = \frac{2 \times 10^{21}}{A \times 100\ }$
So, current in the wire
$i = neAv_{d}$
$i = \frac{2 \times 10^{21}}{A \times 100} \times 1.6 \times 10^{- 19} \times A \times 0.25$
=0.8A
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.