A 3 m long  copper wire carrying a current of 3 A.. How long does it take for an electron to drift from one end to other end ? The cross- sectional area of the wire is 2 * 10 ^ -6  m²  and the number of conduction electrons in copper is 8.5 * 10 ^ 28 m^-3 .

The drift velocity of the electrons is given by

v = I / (neA) 

here 

I = 3A

n = 8.5 X 10²⁸ m³

A = 2 X 10^-6 m²

and e = 1.6 X 10^-19 C

so,

v = 3 / (8.5 X 10²⁸ X 2 X 10^-6 X 1.6 X 10^-19)

or

by solving, we get

v = 1.1 X 10^-4 m/s

Now, to calculate the time taken by the electrons, we simple use the velocity-distance relation, thus,

time = distance / velocity

or t = length of the wire / drift speed

so,

t = 3 / 1.1 X 10^-4

thus, the time taken is, t = 27200 s = 7.55 hrs.