Conductors and resistors

Type of physical science: Classical physics

Field of study: Electromagnetism

When electrons travel from one point to another through a substance, that substance is said to be acting as a conductor of electricity. Each substance found in nature conducts electricity differently, at a rate that is directly related to the amount of resistance those electrons encounter along the way. Those substances that are highly resistant to electron flow are poor conductors, while those that offer low resistance are good conductors of electricity.

Overview

A well-known story about Benjamin Franklin, the American patriot and inventor, describes his attempt to attract a flow of electricity by sending a kite aloft during a lightning storm. Franklin theorized that lightning was the visible process whereby electrical energy in the atmosphere discharges into the ground, often taking the path of least resistance. That path might be a tree, a building, or any object or person unfortunate enough to be touching both the atmosphere and the ground when the conditions are just right. In the case of Franklin's kite, the intended path was the kite string. Scientists in England and France, acting upon Franklin's proposal, had performed the experiment long before Franklin's attempt in 1752. These experiments helped demonstrate two principles: that electrical energy "flows" and that the flow, or electric current, consists of positively or negatively charged particles.

Today, the term "conductor" denotes the path taken by those particles as they travel from one point to another. This process is called conduction. Its nature varies from substance to substance, depending upon the amount of resistance to the flow of electricity present. Conduction is also affected by temperature and other factors.

All naturally occurring substances--including gases and liquids--are capable of conducting electricity. Nevertheless, some substances contain atoms whose proton and electron structures provide better conditions for the creation of electron flow than do other substances.

These substances are more efficient conductors of electricity and are used most often in the application of electrical science to everyday life. They include pure metals such as aluminum, copper, gold, iron, silver, and tin. It should be noted that a "normal" temperature range is assumed, which is usually defined as 20 degrees Celsius. In fact, some substances that are poor conductors of electricity at normal temperatures perform very well at extremely cold temperatures. During the late 1980's, researchers discovered new substances that led to renewed interest in the study of materials that become "super conductors"--materials that conduct electricity with virtually no resistance--at low temperatures. These included some synthetic ceramics. Superconductors would make it possible to transmit electricity and operate electronic equipment with tremendous efficiency at low cost.

The primary factor in determining whether a substance is a good conductor or a poor one is the amount of resistance encountered by electrons as they pass through the material. This resistance depends on the nature of the atomic structure of the material, its molecular structure, and the impurities present. In some materials where atoms contain large numbers of free electrons, those electrons move with relative ease, so that electrical energy is passed from one end of the conductor to the other with little loss of energy. In other substances with fewer free electrons, flow is impeded. When that occurs, energy escapes in the form of heat, light energy, or other forms of electromagnetic radiation. Perhaps the most familiar example of this effect is the glow that emanates from an incandescent light bulb, the result of intentional resistance built into the circuit using materials that offer properties of resistance ideal for that purpose.

To impede the escape of both current and electromagnetic radiation, conductors are insulated. Insulators are constructed of materials that have very poor conductive properties.

Insulators assure that as current moves along the primary conductor, it is not sidetracked when it comes in contact with other materials along its length. Early telegraph wires of the nineteenth century, for example, often traveled across ceramic or glass insulators where the wires came in contact with support poles. These insulating materials are poor conductors of electricity at normal temperatures. Soon, engineers discovered that naturally occurring electromagnetic phenomena interfered with transmission during storms. As the number of electric wires traversing the landscape increased, many of them strung along the same poles, electromagnetic interference also became a problem. A new form of insulation had to be devised. Eventually, wire was encapsulated in rubber casings lined internally with paper or some other form of nonconducting material to prevent disruptive interference. Later, metallic liners were incorporated that served to shield the internal conductor from incoming electromagnetic interference as well as to prevent outgoing radiation.

Materials with highly conductive properties are desirable because they require less energy than do poor conductors at the input stage relative to the amount of energy output that results from transmission. This principle is usually applied to all technologies in the form of input-to-output ratios called efficiency ratings. Based upon that premise, resistance as a factor in electrical transmission would appear to have no intrinsic value. In fact, where resistance is controlled, as in the case of the light bulb, it has great value. Resistance is a fundamental principle incorporated in the design of all electrical systems.

There are two basic forms of resistance. Distributed resistance refers to that resistance found throughout a circuit, called inherent resistance or internal resistance; it generally describes the resistive character of the material used to conduct electricity. The other form of resistance is called lumped resistance, which refers to resistance that occurs in concentrations at one place or another along a circuit. These concentrated areas of resistance often are desirable in electronic circuitry and when introduced in the form of components are called resistors. Resistors offer a way to reduce intentionally the amount of electrical current allowed to flow through a circuit at particular points within that circuit. To know how much resistance is too much, too little, or just enough, it is necessary to measure the resistance.

In 1826, a German physicist, Georg Simon Ohm, discovered a relationship between the amount of resistance in a circuit, the amount of unvarying current applied to that circuit, and the amount of voltage applied to it. He discovered a direct relationship between the amount of current and the amount of voltage, and an inverse relationship between the amount of current and the amount of resistance. From his discovery sprang the definition of the basic unit of resistance, called, appropriately, the ohm. Often, a complete electrical circuit will contain several regions of electronic structure designed to perform functions somewhat independent of one another, with each requiring different levels of current; these values are denoted in ohms.

Resistors, which come in all shapes, sizes, and ohm values, perform this function. They are made from numerous alloys and nonmetallic materials called semiconductors, including germanium, silicon, some nonmetal forms of carbon, and many other materials. The kinds of materials used to manufacture resistors are often related in size requirements and other environmental conditions, such as operating temperatures, that must be factored into their design.

There is a class of resistors known as "nonlinear," whose behavior does not conform to ohmic principles. In some cases, their performance varies only slightly, while in others the departure is significant. In some, called voltage-dependent resistors, or varistors, electric current increases at a faster rate than the rate at which increased voltage is applied. In others, called thermistors, resistance either increases or decreases when the resistor is heated. In all cases, the ohm value of the resistor is either fixed or variable.

Applications

Conductors and resistors have been used since the early nineteenth century in numerous electrical applications. During the 1830's, Samuel Finley Breese Morse demonstrated that an electric current could be sent over long distances via insulated steel cables. The experiment took place before the era of the telegraph was introduced. In 1856, Cyrus West Field laid the first transatlantic telegraph cable, and by the end of the nineteenth century, the American Telephone and Telegraph Company (AT&T) was operating long-distance telephone circuits using copper wire with amplification circuits placed strategically along the way.

Over the years, researchers have improved continually upon the conductive efficiency of the materials employed in that technology. Steel gave way to copper, which gradually became purer, thereby increasing efficiency. Greater efficiency meant the same amount of current could be sent over smaller and lighter conductors for longer distances. Eventually, that range was extended even farther with the invention of amplifiers that could be placed strategically within the circuit.

With the arrival of the computer age, the desire for high capacity and lightning speed in transmission circuitry resulted in the development of exotic new conductors that transmit light energy. These include fibrous glass, plastics, ceramics, and other materials broadly categorized as fiber optics.

Conductors are used to transmit electricity in many forms: as direct and alternating current-to-drive electrical devices; as analog voice and data signals in telecommunications circuitry; and as light energy for high-speed, high-quality digital transmission. Conductors of all types are found in virtually all electrical devices and transmission facilities and consist of many different materials. The most popular conductors are those that exhibit the least amount of resistance. This attribute is particularly important in the transmission of high-voltage electric current that must travel long distances. High resistance in transmission lines leads to energy loss through heat and radiation. The resulting low transmission efficiency can be costly. High resistance is also an important concern in the modern telecommunications environment, where significant conductor resistance can result in external radiation of modulated electrical signals.

Once radiated, these signals are vulnerable to interception, making them a threat to the security of proprietary voice and data transmission. Increasingly, however, other factors are becoming important. For example, light fiber exhibits virtually no traceable radiation of energy and a much higher transmission capacity than metal conductors of comparable dimension. Other conductors in the development stage include lasers that can project extremely high levels of energy over long distances at the speed of light.

Resistors also are used widely in electrical devices. Linear type variable resistors like the volume control found on radios, televisions, and stereos vary the ohmic value of electric current allowed to move through the electric circuit. Nonlinear types called varistors are used principally as voltage regulators, lightning arresters, contact protectors, and transient suppressors.

Thermistors are used primarily as heat sensors and the basic components of altimeters, flow meters, gas analyzers, fire alarms, and infrared detectors.

Context

The modern age is an electronic age whose lifeblood runs through billions of kilometers of conductive materials. Virtually all technology relies heavily upon electrical circuitry to provide the conduit for the electricity required to drive the engines of productivity.

As the bond of that reliance has grown ever stronger, scientists have worked to identify and refine conductive materials that promise greater efficiencies in the tasks electronic conductors are designed to perform. As these conductors become more efficient, the cost of electrical energy relative to the value of the productivity created in the process goes down. The ultimate conductor, then, is the one that will allow electricity to flow over long distances without resistance so that the amount of electricity entering the conductor at one end equals the amount emerging at the other. Once that ideal state of conduction is achieved, electrical applications previously considered uneconomical or otherwise impractical will become commonplace.

Examples include electric commuter trains that can run economically over long distances at high speeds, and electronic equipment such as computers that will no longer require cooling systems to expel heat created by radiated electrical energy flowing through the circuitry. By eliminating radiated energy that is present in the form of heat, all electronic circuitry can be miniaturized. As a result, manufacturers will enjoy greater control over the design and function of virtually all electronic devices and will be able to enhance the productivity of those devices.

Another reason that scientists are seeking the ultimate conductor is the relatively new and very strong demand for high-capacity facilities designed to transmit digital information among computers and within computer networks. Like the conductors of electricity used by public utilities to provide energy to customers, these facilities must provide high-speed, high-quality transmission, but with the added burden of avoiding significant levels of electromagnetic interference. In other words, in order for data transmission to be reliable, the packets of digital information that move along the conductor must do so with no loss of data.

Otherwise, communication among computers and related equipment cannot occur, and the process becomes impractical. The existing public telecommunications network was designed to carry voice, not digital traffic; as a result, it is often unsuitable for all but the most basic data transmission. The need for a better and more versatile conductor led scientists to a revolutionary concept: the use of light in place of electricity to transmit impulses. These materials are called fiber optics.

During the 1980's, the cost of producing fiber-optic materials for use as conductors fell into a range competive with traditional metallic conductors. Fiber optics also provided a level of efficiency--virtually no radiation leakage--that was unmatched by the metals. Light was used instead of current to propel data instantaneously and could transmit much more data on strands many times smaller than standard-gauge copper wire. By 1990, U.S. telecommunications companies had created a fiber network that spanned the continent and were well on their way to producing fiber links that could provide truly universal voice and data transmission to each home.

Principal terms

AMPERES: the amount of electromotive force that would result from 1 volt of electricity conducted through a resistance of 1 ohm

CONDUCTOR: a substance through which an electric current passes

OHM: a unit measure of resistance; equal to the resistance of a circuit, where 1 volt of electromotive force maintains 1 ampere of electric current

RESISTOR: a device used in a circuit to create resistance

SEMICONDUCTOR: a material whose resistivity is between that of insulators and conductors

VOLTAGE: electromotive force expressed in volts

Bibliography

Albert, Arthur Lemuel. ELECTRONICS AND ELECTRON DEVICES. New York: Macmillan, 1956. Contains a lucid discussion of basic electronic theory that includes a particularly useful examination of electron emission. Other areas covered are amplifiers, rectifiers, oscillators, semiconductors, and photoelectric devices. Contains illustrations, diagrams, and an index.

Bureau of Naval Personnel. BASIC ELECTRONICS. Washington, D.C.: U.S. Bureau of Naval Personnel, Navy Training Course, 1968. A good introduction to electronics. Contains a discussion of antennae, receivers, and an introduction to computers and their electronics. Excellent glossary of electrical terminology.

Lurch, E. Norman. FUNDAMENTALS OF ELECTRONICS. New York: John Wiley & Sons, 1981. In addition to a good discussion of fundamental electronic principles, this volume contains an excellent in-depth discussion of the concept of amplification in electronic circuitry. Contains diagrams, illustrations, and an index.

Mullin, William F. ABC'S OF CAPACITORS. New York: Howard V. Sams, 1976. While this article does not address capacitors specifically, electronic circuitry cannot function without them. Presents the fundamental principles of capacitance, which are necessary to understand how electrical systems involving conductors and resistors are able to function. Contains diagrams and an index.

Turner, Rufus B. ABC'S OF RESISTANCE AND RESISTORS. New York: Howard V. Sams, 1974. This small volume offers a good introduction to the topic, stressing fundamental concepts of electricity and differentiating among the several types of resistance and resistors. Contains diagrams and an index.

Charges and Currents

Electrons and Atoms

Forces on Charges and Currents

Insulators and Dielectrics