Susceptance and Admittance
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What is Conductance? 
In the study of DC circuits, the student of electricity comes across a term
meaning the opposite of resistance: conductance. It is a useful term when
exploring the mathematical formula for parallel resistances: Rparallel = 1 /
(1/R1 + 1/R2 + . . . 1/Rn).

Unlike resistance, which diminishes as more parallel components are included
in the circuit, conductance simply adds. Mathematically, conductance is the
reciprocal of resistance, and each 1/R term in the “parallel resistance
formula” is actually a conductance.

Whereas the term “resistance” denotes the amount of opposition to
flowing electrons in a circuit, “conductance” represents the ease of
which electrons may flow. Resistance is the measure of how much a circuit
resists current, while conductance is the measure of how much a circuit
conducts current.

Conductance used to be measured in the unit of mhos, or “ohms” spelled
backward. Now, the proper unit of measurement is Siemens. When symbolized in
a mathematical formula, the proper letter to use for conductance is
“G”.

Reactive components such as inductors and capacitors oppose the flow of
electrons with respect to time, rather than with constant, unchanging
friction as resistors do. We call this time-based opposition, reactance, and
like resistance, we also measure it in the unit of ohms.



What is Susceptance? 

As conductance is the complement of resistance, there is also a
complementary expression of reactance, called susceptance. Mathematically,
it is equal to 1/X, the reciprocal of reactance. Like conductance, it used
to be measured in the unit of mhos but now is measured in Siemens.

Its mathematical symbol is “B”, unfortunately, the same symbol used to
represent magnetic flux density.


Reactance vs. Susceptance

The terms “reactance” and “susceptance” have a certain linguistic
logic to them, just like resistance and conductance. While reactance is the
measure of how much a circuit reacts against change in current over time,
susceptance is the measure of how much a circuit is susceptible to
conducting a changing current.

If one were tasked with determining the total effect of several parallel
-connected, pure reactances, one could convert each reactance (X) to a
susceptance (B), then add susceptances rather than diminish reactances:
Xparallel = 1/(1/X1 + 1/X2 + . . . 1/Xn). Like conductances (G),
susceptances (B) add-in parallel and diminish in series.

Also like conductance, susceptance is a scalar quantity.

When resistive and reactive components are interconnected, their combined
effects can no longer be analyzed with scalar quantities of resistance (R)
and reactance (X).

Likewise, figures of conductance (G) and susceptance (B) are most useful in
circuits where the two types of opposition are not mixed, i.e. either a
purely resistive (conductive) circuit, or a purely reactive (susceptive)
circuit.

In order to express and quantify the effects of mixed resistive and reactive
components, we had to have a new term: impedance, measured in ohms and
symbolized by the letter “Z”.


What is Admittance? 

To be consistent, we need a complementary measure representing the
reciprocal of impedance. The name for this measure is admittance. Admittance
is measured in the unit of Siemens, and its symbol is “Y”. Like
impedance, admittance is a complex quantity rather than a scalar.

Again, we see a certain logic to the naming of this new term: while
impedance is a measure of how much alternating current is impeded in a
circuit, admittance is a measure of how much current is admitted.

Given a scientific calculator capable of handling complex number arithmetic
in both polar and rectangular forms, you may never have to work with figures
of susceptance (B) or admittance (Y). Be aware, though, of their existence
and their meanings.