# 3.13 The number density of free electrons in a copper conductor estimated in Example 3.1 is $8.5\times 10^{28}m^{-3}$. How long does an electron take to drift from one end of a wire 3.0 m long to its other end? The area of cross-section of the wire is $2.0\times 10^{-6} m^2$and it is carrying a current of 3.0 A.

We know,

$I = neAv_d$

$\dpi{90} V_d$:Drift Velocity = Length of wire(l) / time taken to cover

$I = neA \frac{l}{t}$

by substituting the given values

$\dpi{90} \implies$ t = 2.7 x $\dpi{90} 10^4$ s

Therefore, the time required by an electron to drift from one end of a wire to its other end is  $2.7\times 10^4$ s.

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