Foundations of Electronics
Matter is defined as anything that occupies space and has weight; that is, the weight and dimensions of matter can be measured. Examples of matter are air, water, automobiles, clothing, and even our own bodies. And even casual observation shows that matter can exist in any one of three states: as a solid, as a liquid, and as a gas.
Elements, Compounds and Mixtures
An element is a substance which cannot be reduced to a simpler substance by chemical means. Examples of elements with which you are in everyday contact are iron, gold, silver, copper, and oxygen. There are now over 115 known elements. All the different substances we know about are composed of one or more of these elements.
When two or more elements are chemically combined, the resulting substance is called a compound. A compound is a chemical combination of elements which can be separated by chemical but not by physical means. Examples of common compounds are water which consists of hydrogen and oxygen, and table salt, which consists of sodium and chlorine. A mixture, on the other hand, is a combination of elements and compounds, not chemically combined, that can be separated by physical means. Examples of mixtures are air, which is made up of nitrogen, oxygen, carbon dioxide, and small amounts of several rare gases, and sea water, which consists chiefly of salt and water.
A molecule is a chemical combination of two or more atoms, (atoms are described in the next paragraph). In a compound the molecule is the smallest particle that has all the characteristics of the compound. Consider water, for example. Water is matter, since it occupies space and has weight. Depending on the temperature, it may exist as a liquid (water), a solid (ice), or a gas (steam). Regardless of the temperature, however, it has the same composition. If we start with a quantity of water, pour out one half, and continue this process of pouring out half the remaining amount, we eventually end up with a quantity of water which cannot be further divided without ceasing to be water. This quantity is called a molecule of water. Dividing this molecule of water does not yield two smaller amounts of water, but rather one part of oxygen and two parts of hydrogen (H2O).
Molecules are made up of smaller particles called atoms. An atom is the smallest particle of an element that retains the characteristics of that element. The atoms of one element, however, differ from the atoms of all other elements. Since there are over 115 known elements, there must be over 115 different atoms, or a different atom for each element. Just as thousands of words can be made by combining the proper letters of the alphabet, so thousands of different materials can be made by chemically combining various combinations of atoms.
Any particle that is a chemical combination of two or more atoms is called a molecule. The oxygen molecule consists of two atoms of oxygen, and the hydrogen molecule consists of two atoms of hydrogen. Sugar, on the other hand, is a compound composed of atoms of carbon, hydrogen, and oxygen. These atoms are combined into sugar molecules. Since the sugar molecules can be broken down by chemical means into smaller and simpler units, we cannot have sugar atoms.
The atoms of each element are made up of electrons, protons, and, in most cases, neutrons, which are collectively called subatomic particles. Furthermore, the electrons, protons, and neutrons of one element are identical to those of any other element. The reason that there are different kinds of elements is that the number and the arrangement of electrons and protons within the atom are different for the different elements
The electron is considered to be a small negative charge of electricity. The proton has a positive charge of electricity equal and opposite to the charge of the electron. Scientists have measured the mass and size of the electron and proton, and they know how much charge each possesses. The electron and proton each have the same quantity of charge, although the mass of the proton is approximately 1837 times that of the electron. In some atoms there exists a neutral particle called a neutron. The neutron has mass approximately equal to that of a proton, but it has no electrical charge.
It is convenient and helpful to think of electrons, protons, and neutrons of the atoms as arranged in a manner similar to a miniature solar system. The protons and neutrons form a heavy nucleus with a positive charge, around which the very lightweight electrons revolve.
Figure 1-1 shows one hydrogen and one helium atom. Each has a relatively simple structure. The hydrogen atom has only one proton in the nucleus with one electron rotating about it. The helium atom is a little more complex. It has a nucleus made up of two protons and two neutrons, with two electrons rotating about the nucleus. Elements are classified numerically according to the complexity of their atoms. The atomic number of an atom is determined by the number of protons in its nucleus.
In a neutral state, an atom contains an equal number of protons and electrons. Therefore, an atom of hydrogen—which contains one proton and one electron—has an atomic number of 1; and helium, with 1-4 two protons and two electrons, has an atomic number of 2. The complexity of atomic structure increases with the number of protons and electrons
Since an electron in an atom has both mass and motion, it contains two types of energy. By virtue of its motion the electron contains kinetic energy. Due to its position it also contains potential energy. The total energy contained by an electron (kinetic plus potential) is the factor which determines the radius of the electron orbit. In order for an electron to remain in this orbit, it must neither gain or lose energy.
It is well known that light is a form of energy, but the physical form in which this energy exists is not known. One accepted theory proposes the existence of light as tiny packets of energy called photons. Photons can contain various quantities of energy. The amount depends upon the color of the light involved. Should a photon of sufficient energy collide with an orbital electron, the electron will absorb the photon's energy, as shown in figure 1-2. The electron, which now has a greater than normal amount of energy, will jump to a new orbit farther from the nucleus. The first new orbit to which the electron can jump has a radius four times as large as the radius of the original orbit. Had the electron received a greater amount of energy, the next possible orbit to which it could jump would have a radius nine times the original. Thus, each orbit may be considered to represent one of a large number of energy levels that the electron may attain. It must be emphasized that the electron cannot jump to just any orbit. The electron will remain in its lowest orbit until a sufficient amount of energy is available, at which time the electron will accept the energy and jump to one of a series of permissible orbits. An electron cannot exist in the space between energy levels. This indicates that the electron will not accept a photon of energy unless it contains enough energy to elevate itself to one of the higher energy levels. Heat energy and collisions with other particles can also cause the electron to jump orbits.
Once the electron has been elevated to an energy level higher than the lowest possible energy level, the atom is said to be in an excited state. The electron will not remain in this excited condition for more than a fraction of a second before it will radiate the excess energy and return to a lower energy orbit. To illustrate this principle, assume that a normal electron has just received a photon of energy sufficient to raise it from the first to the third energy level. In a short period of time the electron may jump back to the first level emitting a new photon identical to the one it received.
A second alternative would be for the electron to return to the lower level in two jumps; from the third to the second, and then from the second to the first. In this case the electron would emit two photons, one for each jump. Each of these photons would have less energy than the original photon which excited the electron.
This principle is used in the fluorescent light where ultraviolet light photons, which are not visible to the human eye, bombard a phosphor coating on the inside of a glass tube. The phosphor electrons, in returning to their normal orbits, emit photons of light that are visible. By using the proper chemicals for the phosphor coating, any color of light may be obtained, including white.
The basic principles just developed apply equally well to the atoms of more complex elements. In atoms containing two or more electrons, the electrons interact with each other and the exact path of any one electron is very difficult to predict. However, each electron lies in a specific energy band and the orbits will be considered as an average of the electron's position.
Shells and Subshells
The difference between the atoms, insofar as their chemical activity and stability are concerned, is dependent upon the number and position of the electrons included within the atom. How are these electrons positioned within the atom? In general, the electrons reside in groups of orbits called shells. These shells are elliptically shaped and are assumed to be located at fixed intervals. Thus, the shells are arranged in steps that correspond to fixed energy levels. The shells, and the number of electrons required to fill them, may be predicted by the employment of Pauli's exclusion principle. Simply stated, this principle specifies that each shell will contain a maximum of 2n2 electrons, where n corresponds to the shell number starting with the one closest to the nucleus. By this principle, the second shell, for example, would contain 2(2)2 or 8 electrons when full.
In addition to being numbered, the shells are also given letter designations, as pictured in figure 1-3. Starting with the shell closest to the nucleus and progressing outward, the shells are labeled K, L, M, N, O, P, and Q, respectively. The shells are considered to be full, or complete, when they contain the following quantities of electrons: two in the K shell, eight in the L shell, 18 in the M shell, and so on, in accordance with the exclusion principle. Each of these shells is a major shell and can be divided into subshells, of which there are four, labeled s, p, d, and f. Like the major shells, the subshells are also limited as to the number of electrons which they can contain. Thus, the "s" subshell is complete when it contains two electrons, the "p" subshell when it contains 10, and the "f" subshell when it contains 14 electrons.
Inasmuch as the K shell can contain no more than two electrons, it must have only one subshell, the S subshell. The M shell is composed of three subshells: s, p, and d. If the electrons in the s, p, and d subshells are added, their total is found to be 18, the exact number required to fill the M shell. Notice the electron configuration for copper illustrated in figure 1-4. The copper atom contains 29 electrons, which completely fill the first three shells and subshells, leaving one electron in the "s" subshell of the N shell.
The number of electrons in the outermost shell determines the valence of an atom. For this reason, the outer shell of an atom is called the valence shell, and the electrons contained in this shell are called valence electrons. The valence of an atom determines its ability to gain or lose an electron, which in turn determines the chemical and electrical properties of the atom. An atom that is
When the atom loses electrons or gains electrons in this process of electron exchange, it is said to be ionized. For ionization to take place, there must be a transfer of energy which results in a change in the internal energy of the atom. An atom having more than its normal amount of electrons acquires a negative charge, and is called a negative ion. The atom that gives up some of its normal electrons is left with less negative charges than positive charges and is called a positive ion. Thus, ionization is the process by which an atom loses or gains electrons.
CONDUCTORS, SEMICONDUCTORS, AND INSULATORS
In this study of electricity and electronics, the association of matter and electricity is important. Since
every electronic device is constructed of parts made from ordinary matter, the effects of electricity on
matter must be well understood. As a means of accomplishing this, all elements of which matter is made
may be placed into one of three categories: CONDUCTORS, SEMICONDUCTORS, and
INSULATORS, depending on their ability to conduct an electric current. CONDUCTORS are elements
which conduct electricity very readily, INSULATORS have an extremely high resistance to the flow of
electricity. All matter between these two extremes may be called SEMICONDUCTORS.
The electron theory states that all matter is composed of atoms and the atoms are composed of
smaller particles called protons, electrons, and neutrons. The electrons orbit the nucleus which contains
the protons and neutrons. It is the valence electrons that we are most concerned with in electricity. These
are the electrons which are easiest to break loose from their parent atom. Normally, conductors have three
or less valence electrons; insulators have five or more valence electrons; and semiconductors usually
have four valence electrons.
The electrical conductivity of matter is dependent upon the atomic structure of the material from
which the conductor is made. In any solid material, such as copper, the atoms which make up the
molecular structure are bound firmly together. At room temperature, copper will contain a considerable
amount of heat energy. Since heat energy is one method of removing electrons from their orbits, copper
will contain many free electrons that can move from atom to atom. When not under the influence of an
external force, these electrons move in a haphazard manner within the conductor. This movement is equal
in all directions so that electrons are not lost or gained by any part of the conductor. When controlled by
an external force, the electrons move generally in the same direction. The effect of this movement is felt
almost instantly from one end of the conductor to the other. This electron movement is called an
Some metals are better conductors of electricity than others. Silver, copper, gold, and aluminum are
materials with many free electrons and make good conductors. Silver is the best conductor, followed by
copper, gold, and aluminum. Copper is used more often than silver because of cost. Aluminum is used
where weight is a major consideration, such as in high-tension power lines, with long spans between
supports. Gold is used where oxidation or corrosion is a consideration and a good conductivity is
The protons within an atom are much heavier than the electrons. Therefore, in an atom of gas, the electrons knocked loose when ionization occurs will move much more easily if some electric force is applied than will the much heavier protons. Ionization of gases is important to you because it happens in electronic equipment, such as radio and television receivers. During the study of electron tubes, many of which are similar to those in home radio receivers, you will see that ionization is sometimes desirable and at other times undesirable.
Although this course is mainly about charges in motion, a good understanding of static fields will be helpful to you.
That certain objects attract paper and other light materials when rubbed with various kinds of cloth has been known a long time. The early Greeks were familiar with this method of producing what is now called static electricity. They knew that amber, which they called electron, attracted light objects when rubbed with cloth. The English words electron and electricity are derived from this Greek word for amber. A great deal of our early knowledge about electricity was obtained by experiments on charged bodies, or electricity, at rest.
When static electricity converts to energy, the effects can sometimes be quite startling. Lightning discharges and the crackling sound in a radio receiver are manifestations of static electricity, releasing its stored up energy.
Charged Bodies and the Force Between Them. Bodies can be charged with static electricity various ways. To understand how, you need to know the following. A charged body merely means that the object has more or less than its normal number of electrons. In the uncharged state, each atom has an equal number of electrons and protons; therefore, in order to charge a body positively, it is necessary to remove some of the electrons. When that happens, there will be an excess of protons or positive charges. The electrons, which were removed are now on some other object, causing it to be negatively charged. (Recall both negatively and positively charged atoms are ions.)
It has been proved experimentally that charged bodies act upon each other with a force of attraction when their charges are unlike and a force of repulsion when their charges are like. Thus, the conclusion is that electrons and protons attract each other, that electrons repel other electrons, and that protons repel other protons. This attraction and repulsion may be stated as the following laws:
Electric charges of like kind repel each other and charges of unlike kind attract each other.
The forces of attraction and repulsion are directly proportional to the product of the charges and inversely proportional to the square of the distance between them.
While a unit of electrical charge could be taken as the charge associated with an electron or proton, it would not be practical because it is so small. A more practical unit of charge, called a coulomb, is used. It is about equal to a charge of 6.28 x 1018 electrons. The coulomb derives its name from Charles A. Coulomb, a Frenchman who reduced the two laws above to the following formula or law:
The law (Coulomb's Law) can be expressed algebraically as:
where Q1 and Q2 represent the charges in electrostatic units (2.1 x 109 electrons), d the distance in centimeters separating them, and K a constant which depends upon the material separating them. F(dyne) means force in the form of dynes. A dyne is that force which will give an acceleration of 1 centimeter per second, during each second, to a free mass of 1 gram.
Electrostatic or Dielectric Field of Force. The region surrounding and between charged bodies is called the electrostatic field of force. Since this force will act through free space or even through a vacuum, it is different from ordinary forces such as those caused by striking a sharp blow or by exerting steady pressure, like the pressure of water on a dam or the pressure of the air inside an automobile tire. These methods of applying force involve some mechanical connecting link. A field of force differs from these in that it requires no physical or mechanical connecting link, but can be applied through space or through a vacuum.
In order to visualize the various properties of fields of force and their relation to electrical phenomena, you can represent them by imaginary lines that show the direction and intensity of the field. Since it is impossible to imagine enough lines to represent all the paths through space along which the force acts, only a few are drawn, and those only in one plane. The force direction is indicated by an arrowhead and the field strength (or intensity) is indicated by the density or number of lines per unit area. The direction of force is the direction a small positive test object moves or tends to move when acted on by the force.
To test the direction of an electric field, the test object would have to be either a small positive charge or a small negative charge, because the force of a dielectric field will act on either. Scientists use a small positive charge for determining the direction of a dielectric field, and so, this course does, too. (A dielectric field is a field of force that exists between two charged bodies.) In other words, the field about an isolated positive charge (figure 1-3), is away from the charge because a positive test charge would be repelled. The field about an isolated negative charge (figure 1-3), is toward the charge, because a positive test charge would be attracted. The field between a positive and negative charge is from positive to negative for the same reason.
Figure 1-3. The Fields of Force About Single Charges
Note in figures 1-4 and 1-5 how lines of force apparently repel each other. In figure 1-4, although the two charges are attracted, the lines of force between the two are not parallel but bulge out at the center as if they were repelling each other. Also note that where they bulge at the center, they are in the same direction; that is, from right to left on the paper.
Figure 1-4. Dielectric Field About Two Unlike Charges.
Figure 1-5. Dielectric Field About Two Like Charges.
In figure 1-5, the lines of force which are in the region between the charges apparently are repelling each other, as you can judge by the direction of their bends. Although you can say, "like charges repel," the law is stated: Dielectric lines of force in the same direction repel each other. In dealing with certain electric phenomena, this rule is very convenient and useful.
If you briskly rub a rubber rod or comb over a piece of fur or woolen cloth a number of electrons from the fur or cloth adhere to the rubber. If you separate the two immediately, the rubber has an excess of electrons (is negatively charged). If you charge two pith balls oppositely by touching one of them with the rubber and the other with the cloth or fur, they will have an attraction for each other, showing that a force is present. See figure 1-6. You have established a dielectric field. If you allowed the bodies to come together after having been pulled apart, the energy expended in separating them would be regained in the form of force of attraction. This means that energy can be stored in a dielectric field.
Figure 1-6. Pith Ball Experimentation With Dielectric Fields.
Even if you move the negatively charged rubber rod some distance away from the cloth or fur, a dielectric field still exists in the space around it. You can see it demonstrated by picking up small bits of paper with the rod or by charging both of the pith balls from it. The pith balls would then show a force of repulsion between them, indicating the pressure of a dielectric field.
If you use an external force to bring the two charged pith balls closer together, work is done, and the force of repulsion is increased due to the decrease of the natural distance between the two charged bodies. The energy used in decreasing this distance (recovered when you remove the external force) will be used in returning the pith balls to their original position. Here again, it is shown that energy is necessary to establish a force and that the recovered energy has been stored in the field.
If you isolate one negatively charged pith ball and bring the negatively charged rubber rod toward it from any direction, a force or repulsion will be present. If the pith ball is positively charged, it will have an attraction for a negative charge in any direction. The conclusion is that a dielectric field entirely surrounds a charged body.
The Electroscope. It has been shown experimentally that an electric charge can be detected because it attracts light objects such as pith balls, bits of paper, etc.
Any device used for detecting electric charges is called an electroscope. In its simplest form, an electroscope consists of a pith ball hanging on the end of a silk thread. By touching it with a body of a known charge, you have an instrument that can detect charged bodies and that can indicate the type of charge (polarity). To illustrate, if you touch the pith ball with a glass rod, which has been rubbed with silk, you charge the pith ball positively. Any other charged body that is brought near the pith ball will repel it if the body is positive or attract it if the body is negative. The force of repulsion or attraction indicates the strength of the field surrounding the charged bodies.
A better and more sensitive device is the leaf electroscope shown in figure 1-7. It is two thin sheets of metal foil (usually gold or aluminum) called leaves, supported by a wire or stem whose ends pass through a block of sealing wax or insulating material to a metal ball or cap. The leaves are usually sealed in a glass container to prevent air currents and moisture from affecting the instrument. The sensitivity of the instrument depends on several factors, the main two being the thickness and the type of material the leaves are made of.
Figure 1-7. Electroscope.
If the ball receives either a positive or a negative charge, it causes the leaves to spread apart. The leaves spread because like charges repel. When a charge of positive electricity is placed on the leaves, the spread of the leaves will increase when the ball is approached by a positively charged body. On the other hand, a negatively charged body brought near the ball or cap will decrease the spread.
You can place a charge on the leaves by bringing a charged body near, but without making physical contact with, the ball. This is charging by induction. As soon as you remove the charged body, the electroscope is no longer charged unless you provided some means for it to gain or to lose some electrons while the charge was being induced. You can do this by connecting a wire from the electroscope to some neutral conducting object, such as ground. Then, if a charged body is brought near the electroscope, electrons can leave if the charge is negative or enter if the charge is positive. If the wire is disconnected before the charged body is removed, the electroscope will remain charged oppositely to the charge that induced it. This is charging by conduction because the electroscope comes into direct contact with the charged body.
Conductors and Insulators
With certain materials, electrons can be quite readily separated from the atom. In fact, there is much evidence to show that in some metals there are free electrons. Experiments show either that free electrons unattached to atoms do exist or that there is a free interchange of orbital electrons between adjacent atoms. The effect is the same: at any instant, the metal seems full of free electrons. Such material, called a conductor, offers little opposition to the movement of electrons between atoms. While, in general, all metals are good conductors, silver, gold, copper, and aluminum are particularly good.
Materials (such as rubber, glass, silk, fur, mica, and air) which have few free electrons are classed as insulators. Such materials offer great opposition to the movement of electrons between atoms. Materials mentioned previously, on which a charge can be placed by rubbing with a dissimilar material, are insulators. If the center of a long rubber rod is rubbed with a piece of fur, an excess of electrons will locate in the rod's center instead of spreading immediately over the whole surface of the rod.
If, on the other hand, an excess of electrons could be placed at a point on a conductor of uniform cross section, they would immediately spread evenly over the entire surface of the conductor because of the free movement of the electrons.
Since all materials, to some extent, both permit and oppose the movement of electrons, there is no such thing as a perfect conductor or a perfect insulator. Even though there is no sharp dividing line between conductors and insulators, only good conductors are used as conductors and only good insulators are used as insulators.
|David L. Heiserman, Editor||
Copyright © SweetHaven
Revised: June 06, 2015