What are "series" and
"parallel" circuits?
Circuits consisting of just one battery and
one load resistance are very simple to analyze, but they are
not often found in practical applications. Usually, we find
circuits where more than two components are connected
together.
There are two basic ways in which to connect
more than two circuit components: series and
parallel. First, an example of a series circuit:
Here, we have three resistors (labeled R1,
R2, and R3), connected in a long chain
from one terminal of the battery to the other. (It should be
noted that the subscript labeling -- those little numbers to
the lower-right of the letter "R" -- are unrelated to the
resistor values in ohms. They serve only to identify one
resistor from another.) The defining characteristic of a
series circuit is that there is only one path for electrons
to flow. In this circuit the electrons flow in a
counter-clockwise direction, from point 4 to point 3 to
point 2 to point 1 and back around to 4.
Now, let's look at the other type of
circuit, a parallel configuration:
Again, we have three resistors, but this
time they form more than one continuous path for electrons
to flow. There's one path from 8 to 7 to 2 to 1 and back to
8 again. There's another from 8 to 7 to 6 to 3 to 2 to 1 and
back to 8 again. And then there's a third path from 8 to 7
to 6 to 5 to 4 to 3 to 2 to 1 and back to 8 again. Each
individual path (through R1, R2, and R3)
is called a branch.
The defining characteristic of a parallel
circuit is that all components are connected between the
same set of electrically common points. Looking at the
schematic diagram, we see that points 1, 2, 3, and 4 are all
electrically common. So are points 8, 7, 6, and 5. Note that
all resistors as well as the battery are connected between
these two sets of points.
And, of course, the complexity doesn't stop
at simple series and parallel either! We can have circuits
that are a combination of series and parallel, too:
In this circuit, we have two loops for
electrons to flow through: one from 6 to 5 to 2 to 1 and
back to 6 again, and another from 6 to 5 to 4 to 3 to 2 to 1
and back to 6 again. Notice how both current paths go
through R1 (from point 2 to point 1). In this
configuration, we'd say that R2 and R3
are in parallel with each other, while R1 is in
series with the parallel combination of R2 and R3.
This is just a preview of things to come.
Don't worry! We'll explore all these circuit configurations
in detail, one at a time!
The basic idea of a "series" connection is
that components are connected end-to-end in a line to form a
single path for electrons to flow:
The basic idea of a "parallel" connection,
on the other hand, is that all components are connected
across each other's leads. In a purely parallel circuit,
there are never more than two sets of electrically common
points, no matter how many components are connected. There
are many paths for electrons to flow, but only one voltage
across all components:
Series and parallel resistor configurations
have very different electrical properties. We'll explore the
properties of each configuration in the sections to come.
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REVIEW:
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In a series circuit, all components are
connected end-to-end, forming a single path for electrons
to flow.
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In a parallel circuit, all components are
connected across each other, forming exactly two sets of
electrically common points.
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A "branch" in a parallel circuit is a path
for electric current formed by one of the load components
(such as a resistor).
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