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Chapter 5: REACTANCE AND IMPEDANCE - R, L, AND C

# Susceptance and Admittance

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: R_{parallel} = 1 / (1/R_{1} + 1/R_{2} + . . . 1/R_{n}).
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 a 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*.

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.

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: X_{parallel} = 1/(1/X_{1} + 1/X_{2} + . . . 1/X_{n}).
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".

To be consistent, we need a complementary measure representing the reciprocal of impedance. The name for this measure is *admittance*.
Admittance is measured in (guess what?) the unit of Siemens, and its
symbol is "Y". Like impedance, admittance is a complex quantity rather
than 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.