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Chapter 5: SERIES AND PARALLEL CIRCUITS

# Conductance

When students first see the parallel resistance equation, the natural question to ask is, "Where did *that* thing come from?" It is truly an odd piece of arithmetic, and its origin deserves a good explanation.

Resistance, by definition, is the measure of *friction* a component
presents to the flow of electrons through it. Resistance is symbolized
by the capital letter "R" and is measured in the unit of "ohm."
However, we can also think of this electrical property in terms of its
inverse: how *easy* it is for electrons to flow through a component, rather than how *difficult*. If *resistance*
is the word we use to symbolize the measure of how difficult it is for
electrons to flow, then a good word to express how easy it is for
electrons to flow would be *conductance*.

Mathematically, conductance is the reciprocal, or inverse, of resistance:

The greater the resistance, the less the conductance, and vice versa.
This should make intuitive sense, resistance and conductance being
opposite ways to denote the same essential electrical property. If two
components' resistances are compared and it is found that component "A"
has one-half the resistance of component "B," then we could
alternatively express this relationship by saying that component "A" is *twice* as conductive as component "B." If component "A" has but one-third the resistance of component "B," then we could say it is *three times* more conductive than component "B," and so on.

Carrying this idea further, a symbol and unit were created to represent
conductance. The symbol is the capital letter "G" and the unit is the *mho*,
which is "ohm" spelled backwards (and you didn't think electronics
engineers had any sense of humor!). Despite its appropriateness, the
unit of the mho was replaced in later years by the unit of *siemens*
(abbreviated by the capital letter "S"). This decision to change unit
names is reminiscent of the change from the temperature unit of degrees *Centigrade* to degrees *Celsius*, or the change from the unit of frequency *c.p.s.* (cycles per second) to *Hertz*.
If you're looking for a pattern here, Siemens, Celsius, and Hertz are
all surnames of famous scientists, the names of which, sadly, tell us
less about the nature of the units than the units' original
designations.

As a footnote, the unit of siemens is never expressed without the last letter "s." In other words, there is no such thing as a unit of "siemen" as there is in the case of the "ohm" or the "mho." The reason for this is the proper spelling of the respective scientists' surnames. The unit for electrical resistance was named after someone named "Ohm," whereas the unit for electrical conductance was named after someone named "Siemens," therefore it would be improper to "singularize" the latter unit as its final "s" does not denote plurality.

Back to our parallel circuit example, we should be able to see that
multiple paths (branches) for current reduces total resistance for the
whole circuit, as electrons are able to flow easier through the whole
network of multiple branches than through any one of those branch
resistances alone. In terms of *resistance*, additional branches result in a lesser total (current meets with less opposition). In terms of *conductance*, however, additional branches results in a greater total (electrons flow with greater conductance):

Total parallel resistance is *less* than any one of the individual branch resistances because parallel resistors resist less together than they would separately:

Total parallel conductance is *greater* than any of the individual branch conductances because parallel resistors conduct better together than they would separately:

To be more precise, the total conductance in a parallel circuit is equal to the sum of the individual conductances:

If we know that conductance is nothing more than the mathematical reciprocal (1/x) of resistance, we can translate each term of the above formula into resistance by substituting the reciprocal of each respective conductance:

Solving the above equation for total resistance (instead of the reciprocal of total resistance), we can invert (reciprocate) both sides of the equation:

So, we arrive at our cryptic resistance formula at last! Conductance (G) is seldom used as a practical measurement, and so the above formula is a common one to see in the analysis of parallel circuits.

**REVIEW:**- Conductance is the opposite of resistance: the measure of how
*easy*it is for electrons to flow through something. - Conductance is symbolized with the letter "G" and is measured in units of
*mhos*or*Siemens*. - Mathematically, conductance equals the reciprocal of resistance: G = 1/R