Electrical Theory:

Outline:

  1. History of Electricity
  2. Atoms
  3. Law of Charges
  4. Centrifugal Force
  5. Valence Electrons
  6. Electron Flow
  7. Insulators
  8. Semiconductors
  9. Molecules

History of Electricity:

The Greeks were the first to discover electricity about 2500 years ago. They noticed that when an amber was rubbed with other materials it became charged with an unknown force that had the power to attract objects such as dried leaves, feathers, bits of cloth, or other lightweight materials. The Greeks called amber electron. The word electric was derived from it and meant "to be like amber," or to have the ability to attract other objects.

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 electriks and materials that could not be charged noelectriks.

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.

List A List B
Glass (rubbed on silk) Hard rubber (rubbed on wool)
Glass (rubbed on wool or cotton) Block or sulfur(rubbed on wool or fur)
Mica (rubbed on cloth) Most kinds of rubber (rubbed on cloth)
Asbestos (rubbed on cloth or paper) Sealing wax (rubbed on silk, wool, or fur)
Stick of sealing wax (rubbed on wool) Glass or mica (rubbed on dry wool)
  Amber (rubbed on cloth)

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 determining 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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Atoms:

The atom is the basic building block of the universe. All mater is made from a combination of atoms. Matter is any substance has mass and occupies space. Matter can exist in any of the three states: solid, liquid, or gas. Water, for example, can exist in the form of ice, as a liquid, or as a gas in the form of steam. An element is a substance that cannot be chemically divided into a simpler substance. An atom is the smallest part of an element. The three principle parts of an atom are the electron, neutron, and the proton. It is theorized that protons and neutrons are actually made of the smaller particles quarks.

The proton has a positive charge, the electron has a negative charge, and the neutron has no charge. The Neutron 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 its 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 electron 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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Law of charges:

The law of charges states that opposite charges attract and like charges repel. For example, two objects that contain opposite charge are attracted to each other. The two positively charged objects and two negatively charged units repel each other. The reason for this is that lines of force can never cross each other. The outward-going lines of force of a positively charged object combine with the inward-going lines of force of a negatively charged object. This combining produces an attraction between the two objects. If the two objects with like charges come close to each other, the lines of force repel. Since the nucleus has a net positive charge and the electron has a negative charge, the electron is attracted to the nucleus.

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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Centrifugal Force:

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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Valence Electrons:

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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Electron Flow:

Electrical current is the flow of electrons. 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 containing two valence electrons is struck by a 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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Insulators:

Material containing seven or eight valence electrons 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:

Semiconductors are the materials that are neither good conductors nor good insulators. Thy contain four valence 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 electrical 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 construct electronic device, it must be mixed or "doped" with an impurity.

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Molecules:

Although all matter is made from atoms, atoms should not be confused with molecules , which are the smallest part of a compound. Water, for example, is a compound, not an element. The smallest particle of water 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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