Problem 3.209

A straight copper wire of length $l=1000 \mathrm{~m}$ and crosssectional area $S=1.0 \mathrm{~mm}^{2}$ carries a current $I=4.5$ A. Assuming that one free electron corresponds to each copper atom, find: (a) the time it takes an electron to displace from one end of the wire to the other; (b) the sum of electric forces acting on all free electrons in the given wire.