138 Engineering Fundamentals
Static Electricity
Static electricity is the excess of charge on
the surface of an object. Have you ever dragged
your feet on the carpet or gone down a plastic
slide and then gotten a bit of a shock when you
touch something metal, such as a doorknob?
If so, you have seen static electricity in action.
As you rub against the carpet or slide, you
develop a charge. The charge is neutralized
when you touch a material capable of conduct-
ing electricity.
Static electricity has many industrial appli-
cations. One of the most popular applications is
the electrostatic precipitator, which can be used
to remove particles from the air. See Figure 8-4.
Particles are charged on their way in, and the
collection plates are given an opposite charge.
The particles then stick to the plates.
Electricity through a
Conductor
Electricity usually flows through a solid
piece of material called a conductor. Copper, in
the form of copper wire, is the most commonly
used conductor. There are two theories of elec-
trical current flow: conventional current flow
theory and electron flow theory. The conven-
tional current flow theory states that electrical
current flows from positive to negative. Elec-
tron flow theory states that electrons flow from
negative to positive because the negatively
charged electrons are attracted to the posi-
tively charged protons. Individual electrons
in the conductor move almost as slowly as the
minute hand on a clock. The effect of electric-
ity (rather than actual electricity) moves at the
speed of light, which is roughly 186,000 miles
per second. It is the effect of electricity flow-
ing because the copper wire is full of electrons.
When one electron is added to one end, it pushes
on all of the electrons in the wire until one pops
out the other end. Each electron only moves a
tiny bit inside the wire, but the effect is as if an
electron flowed all the way through from one
end to the other. Think of a long tube full of
balls. If you push a ball in one end of the tube,
a ball immediately comes out of the other end.
Your ball does not travel the entire length of the
tube. Each ball moves a little bit, which makes
it seem like your ball instantly comes out the
other end. See Figure 8-5.
Sources of Electricity
You may have wondered where the electric-
ity comes from to power your lights at night or
to operate the International Space Station. Sources
of electricity include magnetism, chemical action,
and solar cells. In each case some form of energy
is converted into electrical energy. The law of
conservation of energy states that energy cannot
be created or destroyed. It can only be converted
from one form to another.
Magnetism
Most electricity is produced using electric
generators. Generators produce electricity by
changing mechanical energy to electrical energy.
When a magnet passes by a wire, a small amount
of voltage is induced in that wire. Voltage is the
electrical force that causes electrons to move. The
action of producing voltage in a wire this way is
called induction. As a generator turns, a conductor
is passed across invisible lines of magnetic force
that surround magnets called magnetic lines of
fl ux, and a current is induced in the conductor.
Particles
from air
Plates
Charged particles
stick to plates
Clean
air
This schematic of an electrostatic precipitator shows how
particles are filtered from the air using static electricity.
Figure 8-4.
Goodheart-Willcox Publisher