Electrical conductance is the real part of the reciprocal of electrical impedance. 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 (formerly referred to as the reciprocal ohm or mho). Oliver Heaviside coined the term in September 1885. 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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Relation to other quantities
As mentioned, conductance is related to resistance by:
- <math>G = \frac{1}{R} = \frac{I}{V} \, </math>
for purely resistive circuits where:
- G is the electrical conductance,
- R is the electrical resistance,
- I is the electric current,
- V is the voltage.
(Note: this is not true where the impedance is complex) Furthermore, conductance is related to susceptance and admittance by the equation:
- <math>Y = G + j B \,</math>
or
- <math>G = Re(Y) \,</math>
where:
- Y is the admittance,
- <math>j</math> is the imaginary unit,
- B is the susceptance.
Combining conductances
The conductance an object of cross-sectional area A and length <math>\ell</math> can be determined from the material's conductivity σ by the formula,
- <math>G=\frac{\sigma \, A}{\ell}</math>
From Kirchhoff's circuit laws we can deduce the rules for combining conductances. For two conductances <math>G_1</math> and <math>G_2</math> in parallel the voltage across them is the same and from Kirchoff's Current Law the total current is
- <math>I_{Eq} = I_1 + I_2\ \,</math>.
Substituting Ohm's law for conductances gives
- <math>G_{Eq} V = G_1 V + G_2 V\ \,</math>
and the equivalent conductance will be,
- <math>G_{Eq} = G_1 + G_2\ \,</math>.
For two conductances <math>G_1</math> and <math>G_2</math> 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,
- <math>V_{Eq} = V_1 + V_2\ \,</math>.
Substituting Ohm's law for conductance then gives,
- <math>\frac {I}{G_{Eq}} = \frac {I}{G_1} + \frac {I}{G_2}</math>
which in turn gives the formula for the equivalent conductance,
- <math>\frac {1}{G_{Eq}} = \frac {1}{G_1} + \frac {1}{G_2}</math>
This equation can be rearranged slightly,
- <math>G_{Eq} = \frac{G_1 G_2}{G_1+G_2}</math>.
Small-signal device conductances
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.
References
- Halliday, David (1960). Physics Part II. John Wiley and Sons.
