Electrical Theory:
Outline:
- History of
Electricity
- Atoms
- Law of
Charges
- Centrifugal
Force
- Valence
Electrons
- Electron
Flow
- Insulators
- Semiconductors
- Molecules
This mysterious
force remained little more than a curious phenomenon until
about 2000 years later, when other people began to conduct
experiments. In the early 1600s, William Gilbert discovered
that amber was not the only material that could be charged to
attract other objects. He called materials that could be
charged elektriks and materials that could not be
charged noelektriks.
About 300 years
ago a few men began to study the behavior of various charged
objects. In 1773, a Frenchman named Charles DuFay found that a
piece of charged glass would repel some charged objects and
attract others. These men soon learned that the force of repulsion
was just as important as the force of attraction.
From these experiments, two lists were developed.
It was
determined that any material in list A would attract any
material in list B, and that all materials in list A would
repel each other and all material in list B would repel each
other. Various names were suggested for the materials in lists
A and B. Any opposite-sounding names could have been chosen,
such as east and west, north and south, male and female.
Benjamin Franklin named the materials in list A positive
and the materials in list B negative . The
first item in each list was used as a standard for determing
if a charged object was positive or negative. Any object
repelled by a piece of glass rubbed on silk would have a
positive charge and any item repelled by a hard rubber rod
rubbed on wool would have a negative charge.
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The proton has a
positive charge, the electron has a negative charge, and the
nuetron has no charge. The netron and proton combine to form
the nucleus of the atom. Since the neutron has no
charge, the nucleus will have a net positive charge. The
number of protons in the nucleus determines what kind of
element an atom is. Oxygen, for example, contains 8 protons in
its nucleus, and gold contain 79. The atomic number of
an element is the same as the number of protons in the
nucleus. The lines of force produced by the positive charge of
the proton extend outward in all directions. The nucleus may
or may not contain as many neutrons as protons. For example,
an atom of helium contains two protons and two neutrons in
itas nucleus, while an atom of copper contains 29 protons and
35 neutrons.
The electron
orbits the outside of the nucleus. An electron is about three
times as large as a proton. The estimated size of a proton is
0.07 trillionth of an inch in diameter, and the estimated size
of a proton is 0.22 trillionth of an inch in diameter.
Although the electron is larger in size, the proton weighs
about 1840 times more.
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Because the
nucleus of an atom is formed from the combination of protons
and neutrons, one might ask why the protons of the nucleus do
not repel each other since they all have the same charge. Two
theories attempt to explain this. The first asserts that the
force of gravity holds the protons and neutron together.
Neutrons, like protons, are extremely massive particles. Their
combined mass produces, the gravitational force necessary to
overcome the repelling force of the positive charges. The
second explanation involves a theoretical particle called gluon.
A gluon is a subatomic particle that acts as a bonding
agent that not only holds quarks together, but also holds the
protons and neutrons together.
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The law of
centrifugal force is the second law of physics. It states that
a spinning object will pull away from its center point and
that the faster it spins, the greater the centrifugal force
becomes. An example of this would be to tie an object to a
string and spin it around, it will try to pull away from you.
The faster the object spins, the greater the force that tries
to pull the object away. Centrifugal force prevents the
electron from falling into the nucleus of the atom. The faster
an electron spins, the farther away from the nucleus it will
be.
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The outer shell
of an atom is known as the valence shell. Any electrons
located in the outer shell of an atom are known as valence
electrons. The valence shell of an atom cannot hold more than
eight electrons. It is the valence electrons that are primary
concern in the study of electricity, because it is these that
explain much of electrical theory. A conductor for instance,
is generally made from a material that contains one or two
valence electrons. Atoms with one or two valence electrons are
unstable and can be made to give up these electrons with
little effort. Conductors are materials that permit electrons
to flow through them easily. When an atom has only one or two
valence electrons, these electrons are loosely held by the
atom and are easily given up for the current flow. Silver,
copper, gold, and aluminum all contain one valence electron
and are excellent conductors of electricity. Silver is the
best natural conductor of electricity, followed by copper,
gold, and aluminum.
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Electrical
current is the flow of electorns. It is produced when an
electron from one atom knocks electrons of another atom out of
orbit. When an atom contains only one valence electron, that
electron is easily given up when struck by another electron.
The striking electron gives its energy to the electron being
struck. The striking electron settles into orbit around the
atom, and the electron that was struck moves off to strike
another electron. This same effect in the game of pool. If the
moving cue ball strikes a stationary ball. The stationary ball
then moves off with the most of the cue ball's energy, and the
cue ball stops moving. The stationary ball did not move off
with all the energy of the cue ball. It moved off with most of
the energy of the cue ball. Some of the cue ball's energy was
lost to heat when it struck the stationary ball. Some energy
is also lost when one electron strikes another. That is why a
wire heats when current flows through it. If too much current
flows through a wire, overheating will damage the wire and
possibly become a fire hazard.
If an atom
containg two valence electrons is struck bya moving electron,
the energy of the striking electron will be divided between
the two valence electrons. If the valence electrons are
knocked out of orbit, they will contain only half the energy
of the striking electron. This effect can also be seen in the
game of pool. If a moving cue ball strikes two stationary
balls at the same time, the energy of the cue ball is divided
between the two stationary balls. Both stationary balls will
move, but with only half of the cue ball.
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Material
containg seven or eight valence elevtrons are known as
insulators. Insulators are materials that resist the flow of
electricity. When the valence shell of an atom is full, the
electrons are held tightly and are not given up easily. Some
good examples of insulator materials are rubber, plastic,
glass, and wood. The energy of the moving electron is divided
so many times that it has little effect on the atom. Any atom
that has seven or eight valence electrons is extremely stable
and does not easily give up an electron.
Semiconductors
are the materials that are neither good conductors nor good
insulators. Thy contain four valnce electrons and are
characterized by the fact that as they are heated, their
resistance decreases. Heat has the opposite effect on
conductors, whose resistance increases with an increase of
temperature. Semiconductors have become extremely important in
the elctrical industry since the invention of the transistor
in 1947. All solid state devices such as diodes, transistors,
and integrated circuits are made from combinations of
semiconductors materials. The two most common materials used
in the production of electronic components are silicon and
germanium. Of the two, silicon is used more often because of
its ability to withstand heat. Before and pure semiconductor
can be used to contractan electronic device, it must be mixed
or "doped" with an impurity.
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Although all
matter is made form atoms, atoms should not be confused with
molecules , which are the smallest part of a compund. Water,
for example, is a compund, not an element. The smallest
particle of wateris a molecule made of two atoms of hydrogen
and one atom of oxygen. If the molecule of water is broken
apart, it becomes two hydrogen atoms and one oxygen atom, and
is no longer water.
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