Chapter 8 Electrical Engineering 145
in proportion to its resistance, but the sum of all
drops equals the applied voltage. Use a string of
60 holiday lights wired in series as an example.
They would be plugged into a common 120 V
household electric outlet. To fi nd the voltage drop
across each light, divide 120 V by 60 lights. There
is a voltage drop of 2 V across each light.
This can be expressed as:
Et = E1 + E2 + E3 +…+ EN
Total resistance in a series circuit is equal
to the sum of the resistance of each individual
load. For example, picture three resistors wired
in series. Their values are 10 Ω, 20 Ω, and 30 Ω.
The total resistance in the circuit is the sum of all
resistances, or 60 Ω. This can be expressed as:
Rt = R1 + R2 + R3 +…+ RN
Current is constant throughout a series circuit.
Think of water fl owing through a garden hose.
Whatever fl ow goes into one end of the hose will
fl ow through and come out the other end. This is
the same in electricity. In the previous example,
the three resistors have a total resistance of 60 Ω. If
they are connected to a 120 V power source, there
will be 2 A (120 V / 60 Ω = 2 A) of current fl ow-
ing at every single point in the circuit. No matter
where the current is tested, it would read 2 A. This
can be expressed as:
It = I1 = I2 = I3 =…= IN
Parallel Circuits
Parallel circuits have more than one load
and have multiple paths for current fl ow. Each
path is a branch and contains one load. Current
is divided between the branches in proportion
to the resistance of the load in each branch. In
other words, the branch with the lowest resis-
tance will have the highest current fl ow. The
voltage across the load in each branch is equal to
the source voltage.
Many modern holiday lights are wired in
parallel. Each light is wired into its own branch
in the circuit. If one light burns out, only that one
light turns off and the rest stay lit. Figure 8-12
shows a typical parallel lighting circuit.
Voltage in a parallel circuit is the same across
each load as it is at the source. All loads get the
total source voltage. Think of your home as an
example. Numerous receptacles and lights might
be on the same circuit, but they all get 120 V. Volt-
age in a parallel circuit can be expressed as:
Et = ER1 = ER2 = ER3 =…= ERN
In a parallel circuit, the sum of all branch
currents equals the total current in the circuit. In
other words, to fi nd the total current in a circuit,
add all of the branch currents together. Current in
a parallel circuit can be expressed as:
It = IR1 + IR2 + IR3 +…+IRN
Resistance in a parallel circuit is a bit more
complicated. The total resistance in a parallel circuit
is always lower than the lowest branch resistance.
1.5 V
This is a typical series lighting circuit. Current must pass
through all seven incandescent lamps in this example.
Figure 8-11.
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3 V
This is an example of a typical parallel lighting circuit.
Figure 8-12.
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