TABLE OF CONTENTS A material medium generally contains a considerable number of charge carriers located at microscopic distances from one another, usually on the order of a nanometer or less. However, it is not possible to look upon the properties and trajectories of every carrier. Therefore, using the statistical physical description of material properties, the average distance used to characterize the distribution of the carriers is of macroscopic order. Conductors have an essential role in electronic technology. They have the properties that some charge carriers are free to move to macroscopic distances when exposed to the action of an electric field. The average distances used to characterize the distribution of these carriers are, therefore, macroscopic.
We can examine (Fig. 1) the case of a volume ?? in which there is a distribution of n charge carriers in which each carrier q i has mass m i and velocity 
with respect to the laboratory reference point.
We characterize the transport of matter associated with a carrier by its momentum 
. We can also characterize the charge transport by introducing the quantity 
. We can then define, at each point P of the charge distribution, the volume current vector 
by:
is...
