Chapter 8 Electrical Engineering 141
This energy knocks electrons loose, and they begin
to fl ow in the form of electrical current. Many
small cells can be connected to power devices like
calculators, satellites, and even whole houses. See
Figure 8-8.
Characteristics and
Measurements
Once electricity is generated, it has certain
characteristics that electrical engineers use in a
number of different ways. It is important for elec-
trical engineers to understand these characteris-
tics and how to measure them.
Voltage
Voltage is the amount of pressure causing
the fl ow of electrons, which is expressed as elec-
tromotive force (EMF). Voltage is also known
as potential difference because it describes the
difference in charge from one place to the other. A
higher voltage will cause more electrons to fl ow.
The letter E is often used as an abbreviation for
voltage and stands for electromotive force. Volt-
age is measured in volts. The abbreviation for
volts is the letter V. The typical wall outlets in
a house provide 120 V, while a battery used to
power a fl ashlight might provide 1.5 V.
Current
Current is a measure of the fl ow of electrons
per unit time. To understand current, you must
fi rst understand coulombs, which is a measure of
the amount of electricity. One coulomb is equal
to 6.24 × 1018 electrons. The term amperage is also
used to describe current.
When discussing the fl ow of electrons, it
is important to include time. As an example,
let’s say 100 students walked down the hall
in front of your classroom. That doesn’t mean
much to you unless you know the time frame.
Did it take all day for 100 students to pass by?
Would it mean more if it were 100 students in
two minutes?
One ampere, or amp, is the measurement
of one coulomb of charge passing a point in one
second. The letter I is used as an abbreviation for
current because it stands for intensity. Current
is measured in amps. The letter A is used as an
abbreviation for amps. For example, a common
100-watt lightbulb draws about 0.83 amps.
Polarity refers to the positive or negative
condition at the power supply terminal. Sometimes
polarity is constant and current fl ows in only one
direction. This is called direct current (dc). Some-
times the polarity changes back and forth from
positive to negative, causing current to go back
and forth. This is called alternating current (ac).
Batteries supply direct current. A battery-oper-
ated fl ashlight uses dc. The wall outlets in your
school provide ac. Figure 8-9 shows the difference
between ac and dc.
Solar cells can be used to power several different types
of devices, such as this navigational buoy.
Figure 8-8.
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