(Note: this is not true where the impedance is complex)
The conductance G of an object of cross-sectional area A and length can be determined from the material's conductivity σ by the formula,
From Kirchhoff's circuit laws we can deduce the rules for combining conductances. For two conductances and 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 and 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,
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