In many materials there is a simple relation between the applied potential difference V across two points and the resulting current I between those points. Such materials are called Ohmic materials, and obey what is called Ohm's law:
R is a constant called the resistance
of the material, which has units of V/A, or Ohms ( 
).
The resistance depends on the type of
material - materials with low resistance are called good
conductors, while those with high resistance are good insulators.
It also depends on the shape of the material. It is convenient in
some circumstances to introduce a quantity called the resistivity,
, which depends only on the type of
material. If we consider a cylindrical wire of cross-sectional
area A and length L, the resistivity is defined as
The units of resistivity are thus 
m.
Suppose in a section of a circuit we encounter a combination of two resistors as in Fig. 17.4:
Figure 17.4: Two resistors in series
These resistors are said to be in series,
and as indicated, it is possible to consider them as one single
equivalent resistor 
. To find this equivalent resistor, we exploit the
fact from energy conservation that 
, which using Ohm's law becomes
This is readily extended to the case of multiple resistors in series:
Suppose now in a section of a circuit we encounter a combination of two resistors as in Fig. 17.5:
Figure 17.5: Two resistors in parallel
These resistors are said to be in parallel,
and as before, it is possible to consider them as one single
equivalent resistor . To find this equivalent resistor, we exploit the
fact from charge conservation that I=I1+I2. Using again Ohm's law
, as well as the point that the potential difference across 
and 
is the same, we find this becomes
This also is readily extended to the case of multiple resistors in parallel:
