Copyright Goodheart-Willcox Co., Inc.
228 Engineering Fundamentals
The handy reminder in Figure 11-10 can be
used to fi nd all three formulas. If you place your
fi nger over the quantity you are looking for, you
will see the necessary formula. For instance, cover
the R with your fi nger and you will see E / I.
Watt’s law states that power equals effort
multiplied by rate. This can be expressed using
the following formula:
P = I × E
Using this formula, you can fi nd any one of
the three values when two are known. Let’s say
you wanted to know how much current was fl ow-
ing through a 100-watt incandescent lightbulb in
your home. You know that the supply voltage is
120 V and the bulb is rated for 100 W.
You would use the following formula:
I = P / E
= 100 W/120 V
= 0.84 A of current flowing
through the lightbulb
Your electric bill at home is calculated in kilo-
watt-hours, or thousands of watt-hours. If you
take the wattage used times the number of hours
it was used, you can fi nd the watt-hours. Let’s say
you turn on a 100-watt lightbulb for ten hours.
100 watts × 10 hours = 1,000 watt-hours =
1 kilowatt-hour
As an example, if you are being charged
15 cents per kilowatt-hour by your local power
company, it costs 15 cents to operate that 100-watt
lightbulb for 10 hours.
Applications
Electrical engineers use their knowledge of
the characteristics and laws of electricity to design
applications for its use. The applications utilize
circuits and electrical components.
Basic Circuits
The most basic circuit consists of a power
source, a load (device that uses the electricity), and
conductors to connect them. Electrical circuits can
be designed in three ways: series, parallel, or series-
parallel. Electrical engineers must understand these
circuits in order to understand, design, build, and
troubleshoot electrical devices. Each circuit has
unique benefi ts and drawbacks for given situations.
Series Circuits
Series circuits have only one path for current
to fl ow from the power source through the circuit
and back to the power source. Current leaves
the power source, fl ows through all loads in the
circuit, and goes back to the power source.
Holiday light strings are a good example of
series circuits. The current in these lights runs
through each individual light and back to the
power source. If one light burns out, every light
in the string turns off due to that one open in the
circuit. Figure 11-11 shows a schematic drawing
of lights wired in series.
E
I
R
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Figure 11-10. 
This diagram can help you remember the three formulas
related to Ohm’s law.
1.5 V
Goodheart-Willcox Publisher
Figure 11-11. 
This is a typical series lighting circuit. Current must pass
through all seven incandescent lamps in this example.