Current density and surface current
density
It is possible to establish a relationship between
the surface resistance and surface resistivity for
any electrode configuration. An idea of the current
density is very helpful in understanding of that
relationship. Consider two samples of a material
as shown in Figure 2. With a constant voltage U
and both samples made of the same material the
amount of current flowing through the material will
be different. The thicker bar (sample #1) conducts
"more easily" than the thin bar (sample #2). One
may use a water pipe analogy - given a constant
water pressure, there will be more water per unit
time coming through the pipe with a larger diameter.
The flow density, be it water or electric current,
is the amount of flow passing through a unit area
of the pipe or the sample of the material. The surface
area is perpendicular to the direction of the
flowing current (or water).