What
is the Unit of Resistance?
The resistance is a quotient
of voltage divided by current, which is
V/I.
It is measured in ohms and abbreviated by
the Greek letter omega ( ).
The ohm is named after the German scientist
GEORGE
S. OHM who published in 1827
his experimental results that described
the results of one of the first efforts
to measure currents and voltages, and to
relate them mathematically.
Geometrical Parameters
to Compute Resistance
The resistance of a conductor
has an effect on the current-carrying capacity
of the conductor. A conductor with high
resistance will have higher power loss than
a conductor with a lower resistance when
carrying the same current. In addition to
increased power loss, higher resistance
will result in larger voltage drop. The
resistance of a conductor is a function
of the conductor length, cross-sectional
area, and material resistivity, as given
by the following equation
where 
= conductor resistivity in ohms-meters (-m),
l
= conductor length in meters (m),
= conductor conductivity in mhos (1/),
and S
= cross-sectional area in square meters
(m2).
Note that the resistance is directly related
to conductor length and inversely related
to area. Figure 4.2 shows cylindrical shaped
conductors with various dimensions.
Figure4.2 Resistance
for three different conductors.
Conductance
A useful quantity in electric
circuit analysis is the reciprocal of resistance,
known as conductance and
denoted by G:
Conductance is the ability
of an element to conduct electric current;
it is measured in mhos or siemens (S).
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