Electrical conductance is the reciprocal of electrical resistance. It is a measure of how easily electricity flows along a certain path through an electrical element. The SI derived unit of conductance is the siemens (also called the mho, because it is the inverse of an ohm). Oliver Heaviside coined the term in September 1885[citation needed].
Electrical conductance is related to but should not be confused with conduction, which is the mechanism by which charge flows, or with conductivity, which is a property of a material.
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As mentioned, conductance is related to resistance by:
for purely resistive circuits.
where:
(Note: this is not true where the impedance is complex)
Furthermore, conductance is related to susceptance and admittance by the equation:
or
where:
From Kirchhoff's circuit laws we can deduce the rules for combining conductances. For two conductances G1 and G2 in parallel the voltage across them is the same and from Kirchoff's Current Law the total current is
Substituting Ohm's law for conductances gives
and the equivalent conductance will be,
For two conductances G1 and G2 in series the current through them will be the same and Kirchhoff's Voltage Law tells us that the voltage across them is the sum of the voltages across each conductance, that is,
Substituting Ohm's law for conductance then gives,
which in turn gives the formula for the equivalent conductance,
This equation can be rearranged slightly,
The term conductance is applied to electronic devices such as transistors and diodes, where it usually refers to a small-signal model that is a linearization of the underlying device equations about a selected DC operating point or Q-point. This conductance is the reciprocal of the small-signal device resistance. See Early effect and channel length modulation.