144 Engineering Fundamentals
Use the following formula:
R = E / I
= 3 V / 0.1 A
= 30 Ω of resistance
The handy reminder in Figure 8-10 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 find any one
of the three values when two are known. Let’s
say you wanted to know how much current
was flowing 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
kilowatt-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. They
are series, parallel, and series-parallel. Electri-
cal engineers must understand these circuits
in order to understand, design, build, and
troubleshoot electrical devices. Each circuit
has unique benefits 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 8-11 shows a schematic drawing of
lights wired in series.
Voltage in a series circuit is equal to the
sum of the voltage drop across each load in the
circuit. The voltage drop across each load varies
E
I
R
This diagram can help you remember the three formulas
related to Ohm’s law.
Figure 8-10.
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