Electricity is a fundamental entity of nature. It consists mainly of negatively and positively charged particles commonly found in the atom. Through man-made influence and natural phenomena, it is possible to observe how the electron (negatively charged) and the proton (positively charged) interact magnetically.
The attraction and repulsion principles of magnetism are used to make electricity perform work. Magnetic principles determine certain reactions; for example, the attraction or repulsion of two magnetically charged objects. These principles can be used in a motor to cause motion and to turn a water pump. Electricity, in other words, uses the magnetic properties of subatomic particles to develop magnetic fields at a given place and time to perform work.
Taking a magnetically neutral atom and artificially separating the electron from the rest of the atom leaves a positive ion. Exciting this atom through mechanical or chemical means prevents the electron and positive ion from returning to its natural state. Nature seeks equilibrium or a natural balance and order.
A battery or generator forces all the electrons to one terminal and positive ions to the other terminal. As long as the atoms are stimulated, this imbalance or difference between the terminals remains. If excitation of the atoms is stopped nature will cause the negative electrons to return to their positive ions through the principles of magnetism.
If excitation of the atoms continues and a complete path of conductive material connects the two terminals (where the negative electrons and positive ions have gathered), a complete circuit is created. Because positive and negative polarities attract, the electron follows this path from its terminal to the positive ion terminal seeking equilibrium. In doing so, a magnetic field from the electron is developing in the entire circuit.
An electron is surrounded by a magnetic field. Wherever an electron is present there is also the magnetic field. The more electrons, the greater the magnetic field in the circuit. The greater the magnetic field in the circuit, the greater the ability to attract or repel other magnets or ferrometallic objects.
Current is measured in amperes and is known mathematically as a quantity of electrons passing a specific point in a circuit in a given time period (coulomb per second).
Voltage is the force that allows the electron to be available to be attracted to the positive ion. Initially, when the electrically neutral atom was excited, a difference in potential was created. This produced negative electrons at one terminal and positive ions at the other terminal. The greater the difference in potential, the greater the number of electrons gathered at one terminal and positive ions at the other terminal. The greater this difference, the greater the potential to do work as the electrons move throughout the circuit carrying their magnetic field. As long as an imbalance at the terminals results from the exciting of atoms artificially, there will be a difference in potential, which is another term for voltage. The greater the difference in potential, the greater the voltage. The greater the negative and positive attraction, the greater the force to attract electrons back to the positive ions seeking equilibrium.
All the electrons (current) will move through the circuit at once, unless impeded or slowed down by some outside force. Wire size or an electrical light filament will restrict or resist the flow of the electrons returning to the positive ions. Everything that prevents or resists the maximum flow of electrons in their natural desire to seek out their positive ions is called resistance. If there is no resistance, a short circuit, which is a very dangerous condition, exists.
Matter is anything that occupies space. Examples of matter are air, water, automobiles, clothing, and even our own bodies. Matter can be found in any one of three states: solid, liquid, and gaseous.
Subatomic particles are the building blocks of all matter. Even though these particles cannot be measured by the usual mechanical tools, they are nonetheless matter. Over 99 percent of the matter in the universe is subatomic material called plasma. Plasma exists throughout the universe as interstellar gases and stars. Plasma is a kind of "subatomic particle soup." Plasma exists on earth only in small quantities. It is seen in the form of the Aurora Borealis, inside neon lamps, lightning bolts, and electricity. Plasma is a collection of positive and negative charges, about equal in number or density and forming a neutral charge (distribution) of matter. Plasma is considered the fourth state of matter.
Elements and Compounds
An element is a substance that cannot be reduced to a simpler substance by chemical means. It is composed of only one type of atom. Some examples are iron, gold, silver, copper, and oxygen. Now more than 100 elements are known. All substances are composed of one or more of these elements.
When two or more elements are chemically combined, the resulting substance is a compound. A compound is a chemical combination of elements that can be separated by chemical but not by physical means. Examples of common compounds are water (hydrogen and oxygen) and table salt (sodium and chlorine). A mixture is a combination of elements and/or 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 salts and water.
Atoms and Molecules
An atom is the smallest particle of an element that retains the characteristics of that element. The atoms of one element differ from the atoms of all other elements. Since more than 100 elements are known, there must be more than 100 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 the proper atoms.
Any particle that is a chemical combination of two or more atoms is a molecule. In a compound, the molecule is the smallest particle that has all the characteristics of that compound. Water, for example, is a compound made up of two atoms of hydrogen and one atom of oxygen. It may be chemically or electrically divided into its separate atoms, but it cannot be divided by physical means.
The electrons, protons, and neutrons of one element are identical to those of any other element. However, the number and arrangement of electrons and protons within the atom are different for each element.
The electron is a small negative charge of electricity. The proton has a positive charge equal and opposite to the electron. Scientists have measured the mass and size of the electron and proton and found the mass of the proton is approximately 1,837 times that of the electron. In the nucleus is a neutral particle called the neutron. A neutron has a mass approximately equal to that of a proton, but with no electrical charge. According to a popular theory, the electrons, protons, and neutrons of the atoms are arranged like a miniature solar system. The protons and neutrons form the heavy nucleus with a positive charge around which the very light electrons revolve.
Figure 2-1 is a theoretical representation of 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 around it. The helium atom has a nucleus made up of two protons and two neutrons, with two electrons rotating outside the nucleus. Elements are classified numerically according to the complexity of their atoms. The number of protons in the atom's nucleus determines its atomic number.
Individually, an atom contains an equal number of protons and electrons. An atom of hydrogen, which contains one proton and one electron, has an atomic number of 1. Helium, with 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 that determines the radius of the electron orbit. To keep this orbit, an electron must neither gain nor lose energy.
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. If a photon of sufficient energy collides with an orbital electron, the electron absorbs the photon's energy (Figure 2-2). 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 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 represents one of a large number of energy levels that the electron may attain. However, 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.
In addition to being numbered, the shells are given letter designations (Figure 2-3). 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 full or complete when they contain the following quantities of electrons: 2 in the K shell, 8 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 four subshells, labeled s, p, d, and f. Like the major shells, the subshells are limited as to the number of electrons they can contain. Thus, the s subshell is complete when it contains 2 electrons, the p subshell when it contains 6, the d subshell when it contains 10, and the f subshell when it contains 14 electrons.
Since the K shell can contain no more than two electrons, it must have only one subshell, the s subshell. The M shell has three subshells: s, p, and d. Adding together the electrons in the s, p, and d subshells equals 18, the exact number required to fill the M shell. Figure 2-4 shows the electron configuration for copper. 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.
One of the easiest ways to create a static charge is by friction. When two pieces of matter are rubbed together, electrons can be wiped off one material onto the other. If both materials are good conductors, it is hard to obtain a detectable charge on either since equalizing currents can flow easily between the conducting materials. These currents equalize the charges almost as fast as they are created. A static charge is more easily created between nonconducting materials. When a hard rubber rod is rubbed with fur, the rod will accumulate electrons given up by the fur (Figure 2-5). Since both materials are poor conductors, very little equalizing current can flow and an electrostatic charge builds up. When the charge becomes great enough, current will flow regardless of the poor conductivity of the materials. These currents cause visible sparks and produce a crackling sound.
A simple experiment demonstrates the law of charged bodies. Suspend two pith (paper pulp) balls near one another by threads (Figure 2-6). Rub a hard rubber rod with fur to give it a negative charge. Then hold it against the right-handball (view A). The rod will give off a negative charge to the ball. The right-hand ball has a negative charge with respect to the left-hand ball. Release the two balls. They will be drawn together (view A). They will touch and remain in contact until the left-hand ball gains a portion of the negative charge of the right-handball. Then they will swing apart. If a positive or a negative charge is placed on both balls (views B and C), the balls will repel each other.
The field about a charged body is normally represented by lines called electrostatic lines of force. These imaginary lines represent the direction and strength of the field. To avoid confusion, the lines of force exerted by a positive charge are always shown leaving the charge. For a negative charge they are shown entering. Figure 2-7 shows these lines to represent the field about charged bodies. View A shows the repulsion of like-charged bodies and their associated fields. View B shows the attraction of unlike-charged bodies and their associated fields.
The magnetic force surrounding a magnet is not uniform. There is a great concentration of force at each end of the magnet and a very weak force at the center. To prove this fact, dip a magnet into iron filings (Figure 2-8). Many filings will cling to the ends of the magnet, while very few adhere to the center. The two ends, which are the regions of concentrated lines of force, are called the poles of the magnet. Magnets have two magnetic poles, and both poles have equal magnetic strength.
Law of Magnetic Poles. To demonstrate the law of magnetic poles, suspend a bar magnet freely on a string (Figure 2-9). It will align itself in a north and south direction. Repeat this experiment. The same pole of the magnet will always swing toward the north geographical pole of the earth. Therefore, it is called the north-seeking pole or simply the north pole. The other pole of the magnet is the south-seeking pole or the south pole.
The Earth's Magnetic Poles. The fact that a compass needle always aligns itself in a particular direction, regardless of its location on earth, indicates that the earth is a huge natural magnet. The distribution of the magnetic force about the earth is the same as that which might be produced by a giant bar magnet running through the center of the earth (Figure 2-10). The magnetic axis of the earth is about 15 degrees from its geographical axis, thereby locating the magnetic poles some distance from the geographical poles. The ability of the north pole of the compass needle to point toward the north geographical pole is due to the presence of the magnetic pole nearby. This magnetic pole of the earth is popularly considered the magnetic north pole. However, it actually must have the polarity of magnet's south pole since it attracts the north pole of a compass needle. The reason for this conflict in terminology can be traced to the early users of the compass. Because they did not know that opposite magnetic poles attract, they called the end of the compass needle that pointed toward the north geographical pole the north pole of a compass needle. However, the north pole of a compass needle (a small bar magnet) can be attracted only by an unlike magnetic pole, a pole with the same magnetic polarity as the south pole of a magnet.
Weber's Theory. A popular theory of magnetism considers the molecular alignment of the material. This is known as Weber's Theory. This theory assumes that all magnetic substances are composed of tiny molecular magnets. Any magnetized material has the magnetic forces of its molecular magnets, thereby eliminating any magnetic effect. A magnetized material will have most of its molecular magnets lined up so that the north pole of each molecule points in one direction and the south pole faces the opposite direction. A material with its molecules thus aligned will then have one effective north pole and one effective south pole. Figure 2-11 illustrates Weber's Theory. When a steel bar is stroked several times in the same direction by a magnet, the magnetic force from the north pole of the magnet causes the molecules to align themselves.
An electron has a magnetic field about it along with an electric field. The number of electrons spinning in each direction determines the effectiveness of the magnetic field of an atom. If an atom has equal numbers of electrons spinning in opposite directions, the magnetic fields surrounding the electrons cancel one another and the atom is unmagnetized. However, if more electrons spin in one direction than another, the atom is magnetized. An atom with an atomic number of 26, such as iron, has 26 protons in the nucleus and 26 revolving electrons orbiting its nucleus. If 13 electrons are spinning in a clockwise direction and 13 electrons are spinning in a counterclockwise direction, the opposing magnetic fields will be neutralized. When more than 13 electrons spin in either direction, the atom is magnetized. Figure 2-12 shows an example of a magnetized atom of iron.
The space surrounding a magnet where magnetic forces act is the magnetic field. Magnetic forces have a pattern of directional force observed by performing an experiment with iron filings. Place a piece of glass over a bar magnet. Then sprinkle iron filings on the surface of the glass. The magnetizing force of the magnet will be felt through the glass, and each iron filing becomes a temporary magnet. Tap the glass gently. The iron particles will align themselves with the magnetic field surrounding the magnet just as the compass needle did previously. The filings form a definite pattern, which is a visible representation of the forces comprising the magnetic field. The arrangements of iron filings in Figure 2-13 indicate that the magnetic field is very strong at the poles and weakens as the distance from the poles increases. They also show that the magnetic field extends from one pole to the other in a loop around the magnet.
To further describe and work with magnetic phenomena, lines are used to represent the force existing in the area surrounding a magnet (Figure 2-14). These magnetic lines of force are imaginary lines used to illustrate and describe the pattern of the magnetic field. The magnetic lines of force are assumed to emanate from the north pole of a magnet, pass through the surrounding space, and enter the south pole. They then travel inside the magnet from the south pole to the north pole, thus completing a closed loop.
When two magnetic poles are brought close together, the mutual attraction or repulsion of the poles produces a more complicated pattern than that of a single magnet. These magnetic lines of force can be plotted by placing a compass at various points throughout the magnetic field, or they can be roughly illustrated using iron filings as before. Figure 2-15 shows a diagram of magnetic poles placed close together.
All substances that are attracted by a magnet can become magnetized. The fact that a material is attracted by a magnet indicates the material must itself be a magnet at the time of attraction. Knowing about magnetic fields and magnetic lines of force simplifies the understanding of how a material becomes magnetized when brought near a magnet. As an iron nail is brought close to a bar magnet (Figure 2-16), some flux lines emanating from the north pole of the magnet pass through the iron nail in completing their magnetic path. Since magnetic lines of force travel inside a magnet from the south pole to the north pole, the nail will be magnetized so its south pole will be adjacent to the north pole of the bar magnet.
Magnetic flux has no known insulator. If a nonmagnetic material is placed in a magnetic field, there is no appreciable change in flux. That is, the flux penetrates the nonmagnetic material. For example, a glass plate placed between the poles of a horseshoe magnet will have no appreciable effect on the field, although glass itself is a good insulator in an electric circuit. If a magnetic material such as soft iron is placed in a magnetic field, the flux may be redirected to take advantage of the greater permeability of the magnetic material (Figure 2-17). Permeability is the quality of a substance that determines the ease with which it can be magnetized.
Stray magnetic fields can influence the sensitive mechanisms of electric instruments and meters causing errors in their readings. Instrument mechanisms cannot be insulated against magnetic flux. Therefore, the flux must be directed around the instrument by placing a soft-iron case, called a magnetic screen or magnetic shield, about the instrument. Because the flux is established more readily through the iron (even though the path is larger) than through the air inside the case, the instrument is effectively shielded. Figure 2-18 shows a soft iron magnetic shield around a watch.
The bar magnet is most often used in schools and laboratories for studying the properties and effects of magnetism. The bar magnet helped demonstrate magnetic effects in Figure 2-14.
The ring magnet is used for computer memory cores. A common application for a temporary ring magnet is the shielding of electrical instruments.
The horseshoe magnet is most frequently used in electrical and electronic equipment. A horseshoe magnet is similar to a bar magnet but is bent in the shape of a horseshoe. The horseshoe magnet is magnetically stronger than a bar magnet of the same size and material because the magnetic poles are closer together. The magnetic strength from one pole to the other is greatly increased because the magnetic field is concentrated in a smaller area. Electrical measuring devices often use horseshoe magnets.
Care of Magnets
A piece of steel that has been magnetized can lose much of its magnetism by improper handling. If it is jarred or heated, its domains will be misaligned, and it loses some of its effective magnetism. If this piece of steel formed the horseshoe magnet of a meter, the meter would no longer operate or would give inaccurate readings. Therefore, be careful when handling instruments containing magnets. Severe jarring or subjecting the instrument to high temperatures will damage the device.
A magnet may also become weakened from loss of flux. When storing magnets, always try to avoid excess leakage of magnetic flux. Always store a horseshoe magnet with a keeper, a soft iron bar used to join the magnetic poles. By storing the magnet with a keeper, the magnetic flux continuously circulates through the magnet and does not leak off into space.
When storing bar magnets, follow the same principle. Always store bar magnets in pairs with a north pole and a south pole placed together. This provides a complete path for the magnetic flux without any flux leakage.
In the field of physical science, work is defined as the product of force and displacement. That is, the force applied to move an object and the distance the object is moved are the factors of work performed. No work is accomplished unless the force applied causes a change in position of a stationary object or a change in the velocity of a moving object. For example, a worker may tire by pushing against a heavy wooden crate, but unless the crate moves, no work will be accomplished.
In the study of energy and work, energy is defined as the ability to do work. To perform any kind of work, energy must be expended (converted from one form to another). Energy supplies the required force or power whenever any work is accomplished.
One form of energy is that contained by an object in motion. When a hammer is set in motion in the direction of a nail, it possesses energy of motion. As the hammer strikes the nail, the energy of motion is converted into work as the nail is driven into the wood. The distance the nail is driven into the wood depends on the velocity of the hammer at the time it strikes the nail. Energy contained in an object due to its motion is called kinetic energy.
If a hammer is suspended one meter above a nail by a string, gravity will pull the hammer downward. If the string is suddenly cut, the force of gravity will pull the hammer down against the nail, driving it into the wood. While the hammer is suspended above the nail, it has the ability to do work because of its elevated position in the earth's gravitational field. Since energy is the ability to do work, the hammer contains energy.
Energy contained in an object because of its position is called potential energy. The amount of potential energy available equals the product of the force required to elevate the hammer and the height to which it is elevated.
Another example of potential energy is that contained in a tightly coiled spring. The amount of energy released when the spring unwinds depends on the amount of force required to wind the spring initially.
The study of electrostatics shows that a field of force exists in the space surrounding any electrical charge. The strength of the field depends directly on the force of the charge.
The charge of one electron might be used as a unit of electrical charge since displacing electrons creates charges. However, the charge of one electron is so small that it is impractical to use. The practical unit adopted for measuring charges is the coulomb, named after the scientist Charles Coulomb. A coulomb equals the charge 6,242,000,000,000,000,000 (six quintillion, two hundred forty-two quadrillion or 6.242 times 10 to the 18th power) electrons.
When a charge of 1 coulomb exists between two bodies, one unit of electrical potential energy exists. This difference in potential between the two bodies is called electromotive force (EMF) or voltage. The unit of measure is the volt.
Electrical charges are created by the displacement of electrons, so that there is an excess of electrons at one point and a deficiency at another point. Therefore, a charge must always have either a negative or positive polarity. A body with an excess of electrons is negative; a body with a deficiency of electrons is positive.
A difference in potential can exist between two points or bodies only if they have different charges. In other words, there is no difference in potential between two bodies if both have a deficiency of electrons to the same degree. If, however, one body is deficient by 6 coulombs (6 volts) and the other is deficient by 12 coulombs (12 volts), the difference in potential is 6 volts. The body with the greater deficiency is positive with respect to the other.
In most electrical circuits only the difference in potential between two points is important. The absolute potentials of the points are of little concern. Often it is convenient to use one standard reference for all of the various potentials throughout a piece of equipment. For this reason, the potentials at various points in a circuit are generally measured with respect to the metal chassis on which all parts of the circuit are mounted. The chassis is considered to be at zero potential and all other potentials are either positive or negative with respect to the chassis. When used as the reference point, the chassis is said to be at ground potential.
Sometimes rather large values of voltage may be encountered and the volt becomes too small a unit for convenience. In this situation, the kilovolt (kV), meaning 1,000 volts, is used. For example, 20,000 volts would be written as 20 kV. Sometimes the volt may be too large a unit when dealing with very small voltages. For this purpose, the millivolt (mV), meaning one-thousandth of a volt, and the microvolt (uV), meaning one-millionth of a volt, are used. For example, 0.001 volt would be written as 1 mV, and 0.000025 volt would be written as 25 uV.
When a difference in potential exists between two charged bodies connected by a conductor, electrons will flow along the conductor. This flow is from the negatively charged body to the positively charged body until the two charges are equalized and the potential difference no longer exists.
Figure 2-19 shows an analogy of this action in the two water tanks connected by a pipe and valve. At first, the valve is closed and all the water is in tank A. Thus, the water pressure across the valve is at maximum. When the valve is opened, the water flows through the pipe from A to B until the water level becomes the same in both tanks. The water then stops flowing in the pipe because there is no longer a difference in water pressure between the two tanks.
Electron movement through an electric circuit is directly proportional to the difference in potential or EMF across the circuit, just as the flow of water through the pipe in Figure 2-19 is directly proportional to the difference in water level in the two tanks.
A fundamental law of electricity is that the electron flow is directly proportional to the applied voltage. If the voltage is increased, the flow is increased. If the voltage is decreased, the flow is decreased.
It has been demonstrated that a charge can be produced by rubbing a rubber rod with fur. Because of the friction involved, the rod acquires electrons from the fur, making it negative. The fur becomes positive due to the loss of electrons. These quantities of charge constitute a difference in potential between the rod and the fur. The electrons that make up this difference in potential are capable of doing work if a discharge is allowed to occur.
To be a practical source of voltage, the potential difference must not be allowed to dissipate. It must be maintained continuously. As one electron leaves the concentration of negative charge, another must be immediately provided to take its place or the charge will eventually diminish to the point where no further work can be accomplished. A voltage source, therefore, is a device that can supply and maintain voltage while an electrical apparatus is connected to its terminals. The internal action of the source is such that electrons are continuously removed from one terminal to keep it positive and simultaneously supplied to the second terminal to keep it negative.
Presently, six methods for producing a voltage or electromotive force are known. Some are more widely used than others, and some are used mostly for specific applications. The six known methods of producing a voltage are --
Voltage Produced by Friction
The first method discovered for creating a voltage was generation by friction. The development of charges by rubbing a rod with fur is a prime example of the way friction generates voltage. Because of the nature of the materials producing this voltage, it cannot be conveniently used or maintained. There-fore, this method has very little practical use.
While searching for ways to produce larger amounts of voltage with more practical nature, machines were developed that transferred charges from one terminal to another by rotating glass discs or moving belts. The most notable of these machines is the Van de Graaff generator. It is used today to produce potentials in the order of millions of volts for nuclear research. As these machines have little value outside the field of research, their theory of operation will not be described here.
Voltage Produced by Pressure
One specialized method of generating an EMF uses the characteristics of certain ionic crystals such as quartz, Rochelle salts, and tourmaline. These crystals can generate a voltage whenever stresses are applied to their surfaces. Thus, if a crystal of quartz is squeezed, charges of opposite polarity appear on two opposite surfaces of the crystal. If the force is reversed and the crystal is stretched, charges again appear but are of the opposite polarity from those produced by squeezing. If a crystal of this type is vibrated, it produces a voltage of reversing polarity between two of its sides. Quartz or similar crystals can thus be used to convert mechanical energy into electrical energy. Figure 2-20 shows this phenomenon, called the piezoelectric effect. Some of the common devices that use piezoelectric crystals are microphones, phonograph cartridges, and oscillators used in radio transmitters, radio receivers, and sonar equipment. This method of generating an EMF is not suitable for applications having large voltage or power requirements. But it is widely used in sound and communications systems where small signal voltages can be effectively used.
When a length of metal, such as copper, is heated at one end, valence electrons tend to move away from the hot end toward the cooler end. This is true of most metals. However, in some metals such as iron, the opposite takes place and electrons tend to move toward the hot end. Figure 2-21 illustrates these characteristics. The negative charges (electrons) are moving through the copper away from the heat and through the iron toward the heat. They cross from the iron to the copper through the current meter to the iron at the cold junction. This device is called a thermocouple.
The photosensitive materials most commonly used to produce a photoelectric voltage are various compounds of silver oxide or copper oxide. A complete device which operates with photoelectric voltage is a photoelectric cell. Many different sizes and types of photoelectric cells are in use, and each serves the special purpose for which it is designed. Nearly all, however, have some of the basic features of the photoelectric cells in Figure 2-22.
The cell in view A has a curved, light-sensitive surface focused on the central anode. When light from the direction shown strikes the sensitive surface, it emits electrons toward the anode. The more intense the light, the greater the number of electrons emitted. When a wire is connected between the filament and the back, or dark side of the cell, the accumulated electrons will flow to the dark side. These electrons will eventually pass through the metal of the reflector and replace the electrons leaving the light-sensitive surface. Thus, light energy is converted to a flow of electrons, and a usable current is developed.
The cell in view B is constructed in layers. A base plate of pure copper is coated with light-sensitive copper oxide. An extremely the semitransparent layer of metal is placed over the copper oxide. This additional layer serves two purposes:
An externally connected wire completes the electron path, the same as in the reflector-type cell. The photocell's voltage is used as needed by connecting the external wires to some other device, which amplifies (enlarges) it to a usable level.
The power capacity of a photocell is very small. However, it reacts to light-intensity variations in an extremely short time. This characteristic makes the photocell very useful in detecting or accurately controlling many operations. For instance, the photoelectric cell, or some form of the photoelectric principle, is used in television cameras, automatic manufacturing process controls, door openers, and burglar alarms.
Voltage Produced by Chemical Action
Voltage may be produced chemically when certain substances are exposed to chemical action. If two dissimilar substances, usually metals or metallic materials, are immersed in a solution that produces a greater chemical action on one substance than on the other, a difference in potential exists between the two. If a conductor is then connected between them, electrons flow through the conductor to equalize the charge. This arrangement is called a primary cell. The two metallic pieces are electrodes, and the solution is the electrolyte. The voltaic cell in Figure 2-23 is a simple example of a primary cell. The difference in potential results from the fact that material from one or both of the electrodes goes into the electrolyte. In the process, ions form near the electrodes. Due to the electric field associated with the charged ions, the electrodes acquire charges. The amount of difference in potential between the electrodes depends mainly on the metals used.
Batteries are formed when several cells are connected together to increase electrical output.
Three fundamental conditions must exist before a voltage can be produced by magnetism:
Figure 2-24 shows the three conditions needed to create an induced voltage. There is a magnetic field between the poles of the C-shaped magnet. The copper wire is the conductor. The wire is moved back and forth across the magnetic field for relative motion.
In view A, the conductor moves toward the front of the page and the electrons move from left to right. The movement of the electrons occurs because of the magnetically induced EMF acting on the electrons in the copper. The right-hand end becomes negative and the left-hand end positive. The conductor is stopped in view B, and motion is eliminated (one of the three required conditions). Since there is no longer an induced EMF, there is no longer any difference in potential between the two ends of the wire. In view C, the conductor is moving away from the front of the page. An induced EMF is again created. However, the reversal of motion has caused a reversal of direction in the induced EMF.
If a path for electron flow is provided between the ends of the conductor, electrons will leave the negative end and flow to the positive end. View D shows this condition. Electron flow will continue as long as the EMF exists. Note that the induced EMF in Figure 2-24 could also have been created by holding the conductor stationary and moving the magnetic field back and forth.
Electrons move through a conductor in response to a magnetic field. Electron current is the directed flow of electrons. The direction of electron movement is from a region of negative potential to a region of positive potential. Therefore, electron current flow in a material is determined by the polarity of the applied voltage.
Random Drift
All materials are composed of atoms, each capable of being ionized. If some form of energy, such as heat, is applied to a material, some electrons acquire enough energy to move to a higher energy level. As a result, some electrons are freed from their parent atoms, which then become ions. Other forms of energy, particularly light or an electric field, will also cause ionization.
The number of free electrons resulting from ionization depends on the quantity of energy applied to a material and the atomic structure of the material. At room temperature, some materials, classified as conductors, have an abundance of free electrons. Under a similar condition, materials classified as insulators exchange relatively few free electrons.
In a study of electric current, conductors are of major concern. Conductors consist of atoms with loosely bound electrons in their outer orbits. Due to the effects of increased energy, these outermost electrons frequently break away from their atoms and freely drift throughout the material. The free electrons take an unpredictable path and drift hap-hazardly about the material. This movement is called random drift. Random drift of electrons occurs in all materials. The degree of random drift is greater in a conductor than in an insulator.
Directed Drift
Associated with every charged body is an electrostatic field. Bodies with like charges repel one another, and bodies with unlike charges attract each other. An electron is affectedly an electrostatic field in the same manner as any negatively charged body. It is repelled by a negative charge and attracted by a positive charge. If a conductor has a difference in potential impressed across it, a direction is imparted to the random drift (Figure 2-25). This causes the free electrons to be repelled away from the negative terminal and attracted toward the positive terminal. This constitutes a general migration of electrons from one end of the conductor to the other. The directed migration of free electrons due to the potential difference is called directed drift.
The directed movement of the electrons occurs at a relatively low velocity (rate of motion in a particular direction). The effect of this directed movement, however, is almost instantaneous (Figure 2-26). As a difference in potential is impressed across the conductor, the positive terminal of the battery attracts electrons from point A. Point A now has a deficiency of electrons. As a result, electrons are attracted from point B to point A. Point B now has an electron deficiency therefore, it will attract electrons. This same effect occurs throughout the conductor and repeats itself from points D to C. At the same instant the positive battery terminal attracts electrons from point A, the negative terminal repels electrons toward point D. These electrons are attracted to point D as it gives up electrons to point C. This process continues for as long as a difference in potential exists across the conductor. Though an individual electron moves quite slowly through the conductor, the effect of a directed drift occurs almost instantly. As an electron moves into the conductor at point D, an electron is leaving at point A. This action takes place at approximately the speed of light.
Consider a simple metallic substance. Most metals are crystalline in structure and consist of atoms that are tightly bound in the lattice network. The atoms of such elements are so close together that the electrons in the outer shell of the atom are associated with one atom as much as with its neighbor (Figure 2-27 view A). As a result, the force of attachment of an outer electron with an individual atom is practically zero. Depending on the metal, at least one electron, sometimes two, and, in a few cases, three electrons per atom exist in this state. In such a case, a relatively small amount of additional electron energy would free the outer electrons from the attraction of the nucleus. At normal room temperature, materials of this type have many free electrons and are good conductors. Good conductors have a low resistance.
If the atoms of a material are farther apart, the electrons in the outer shells will not be equally attached to several atoms as they orbit the nucleus (view B). They are attracted to the nucleus of the parent atom only. Therefore, a greater amount of energy is required to free any of these electrons. Materials of this type are poor conductors and have a high resistance.
Silver, gold, and aluminum are good conductors. Therefore, materials composed of their atoms would have a low resistance. The element copper is the conductor most widely used throughout electrical applications. Silver has a lower resistance than copper, but its cost limits usage to circuits where a high conductivity is demanded. Aluminum, which is much lighter than copper, is used as a conductor when weight is a major factor.
Effect of Physical Dimensions
Cross-sectional Area. Cross-sectional area greatly affects the magnitude of resistance. If the cross-sectional area of a conductor is increased, a greater quantity of electrons are available to move through the conductor. Therefore, a larger current will flow for a given amount of applied voltage. An increase in current indicates that when the cross-sectional area of a conductor is increased, the resistance must have decreased. If the cross-sectional area of a conductor is decreased, the number of available electrons decreases and, for a given applied voltage, the current through the conductor decreases. A decrease in current flow indicates that when the cross-sectional area of a conductor is decreased, the resistance must have increased. Thus, the resistance of a conductor is inversely proportional to its cross-sectional area.
Conductor Diameter. The diameter of conductors used in electronics is often only a fraction of an inch. Therefore, the diameter is expressed in mils (thousandths of an inch). It is also standard practice to assign the unit circular mil to the cross-sectional area of the conductor. The circular mil is found by squaring the diameter, when the diameter is expressed in mils. Thus, if the diameter is 35 mils (0.035 inch, the circular mil area equals 352 or 1,225 circular mils. Figure 2-28 shows a comparison between a square mil and circular mil.
Resistance is a property of every electrical component. At times, its effects will be undesirable. However, resistance is used in many varied ways. Resistors are components manufactured in many types and sizes to possess specific values of resistance. In a schematic representation, a resistor is drawn as a series of jagged lines (Figure 2-29).
The disadvantage of carbon resistors can be overcome by using wirewound resistors (Figure 2-29 views B and C). These resistors have very accurate values and can handle higher current than carbon resistors. The material often used to manufacture wirewound resistors is German silver, composed of copper, nickel, and zinc. The qualities and quantities of these elements in the wire determine the resistivity of the wire, which is the measure or ability of the wire to resist current. Usually the percent of nickel in the wire determines the resistivity. One disadvantage of the wirewound resistor is that it takes a large amount of wire to manufacture a resistor of high ohmic value, thereby increasing the cost. A variation of the wirewound resistor provides an exposed surface to the resistance wire on one side. An adjustable tap is attached to this side. Such resistors, sometimes with two or more adjustable taps, are used as voltage dividers in power supplies and in other applications where a specific voltage needs to be tapped off.
Types of Resistors
The two kinds of resistors are freed and variable. The freed resistor will have one value and will never change, other than through temperature, age, and so forth. The resistors in views A and B are fixed resistors. The tapped resistor in view B has several fixed taps which make more than one resistance value available. The sliding cent act resist or in view C has an adjustable collar that can be moved to tap off any resistance within the ohmic value range of the resistor.
There are two types of variable resistors: the potentiometer and the rheostat (views D and E). An example of the potentiometer is the volume control on your radio. An example of the rheostat is the dimmer control for the dash lights in an automobile. There is a slight difference between them. Rheostats usually have two connections: one fixed and the other movable. Any variable resistor can properly be called a rheostat. The potentiometer always has three connections: two fixed and one movable. Generally, the rheostat has a limited range of values and high current-handling capability. The potentiometer has a wide range of values, but it usually has a limited current-handling capability. Potentiometers are always connected as voltage dividers.
When a current is passed through a resistor, heat develops within the resistor. The resistor must be able to dissipate this heat into the surrounding air. Otherwise, the temperature of the resistor rises causing a change in resistance or possibly causing the resistor to burn out.
The resistor's ability to dissipate heat depends on the design of the resistor. It depends on the amount of surface area exposed t o the air. A resistor designed to dissipate a large amount of heat therefore must be large. The heat dissipating capability of a resistor is measured in watts. Some of the more common wattage ratings of carbon resistors are 1/8 watt, 1/4 watt, 1/2 watt, 1 watt, and 2 watts. In some of the newer state-of-the-art circuits, much smaller wattage resistors are used. Generally, the type that can be physically worked with are of the values above. The higher the wattage rating of the resistor, the larger its physical size. Resistors that dissipate very large amounts of power (watts) are usually wirewound resistors. Wirewound resistors with wattage ratings up to 50 watts are not uncommon. Figure 2-30 shows some resistors with different wattage ratings.
