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.
- REVIEW:
- In a series circuit, all components are connected end-to-end,
forming a single path for electrons to flow.
- In a parallel circuit, all components are connected across
each other, forming exactly two sets of electrically common
points.
- 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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