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Current and Electron Drift Velocity

EANDM-FJORPK

The current in a copper wire is caused by the drift velocity, $v_d$, of the electrons. A copper wire of radius, $r=0.5 \rm{mm}$, carries a current of $I = 200 \:\rm{mA}$.

What is the drift velocity of the electrons in this wire?

(The electron density in copper, i.e. the number of electrons per unit volume, is $n_{Cu}=8.5 \times 10^{28} \rm{m}^{\rm{-3}}$.)

A

$v_d = 0 \: \rm {ms}^{\rm{-1}}$

B

$v_d = 1.9 \: \rm{ mm s}^{\rm{-1}}$

C

$v_d = 37.4 \: \rm {\mu ms}^{\rm{-1}}$

D

$v_d = 4.7 \:\rm{nms}^{\rm{-1}}$

E

$v_d = 18.7 \: \rm{\mu ms}^{\rm{-1}}$