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electricity, class of phenomena arising from the existence of charge. The basic unit of charge is that on the proton or electron—the proton's charge is designated as positive while the electron's is negative. There are three basic systems of units used to measure electrical quantities, the most common being the one in which the ampere is the unit of current, the coulomb is the unit of charge, the volt is the unit of electromotive force, and the ohm is the unit of resistance, reactance, or impedance (see electric and magnetic units).
Properties of Electric Charges
According to modern theory, most elementary particles of matter possess charge, either positive or negative. Two particles with like charges, both positive or both negative, repel each other, while two particles with unlike charges are attracted (see Coulomb's law). The electric force between two charged particles is much greater than the gravitational force between the particles. The negatively charged electrons in an atom are held near the nucleus because of their attraction for the positively charged protons in the nucleus.
If the numbers of electrons and protons are equal, the atom is electrically neutral; if there is an excess of electrons, it is a negative ion; and if there is a deficiency of electrons, it is a positive ion. Under various circumstances, the number of electrons associated with a given atom may change; chemical bonding results from such changes, with electrons being shared by more than one atom in covalent bonds or being transferred from one atom to another in ionic bonds (see chemical bond). Thus many of the bulk properties of matter ultimately are due to the electric forces among the particles of which the substance is composed. Materials differ in their ability to allow charge to flow through them (see conduction; insulation); materials that allow charge to pass easily are called conductors, while those that do not are called insulators, or dielectrics. A third class of materials, called semiconductors, conduct charge under some conditions but not under others.
Properties of Charges at Rest
Electrostatics is the study of charges, or charged bodies, at rest. When positive or negative charge builds up in fixed positions on objects, certain phenomena can be observed that are collectively referred to as static electricity. The charge can be built up by rubbing certain objects together, such as silk and glass or rubber and fur; the friction between the objects causes electrons to be transferred from one to the other—from a glass rod to a silk cloth or from fur to a rubber rod—with the result that the object that has lost the electrons has a positive charge and the object that has gained them has an equal negative charge. An electrically neutral object can be charged by bringing it in contact with a charged object: if the charged object is positive, the neutral object gains a positive charge when some of its electrons are attracted onto the positive object; if the charged object is negative, the neutral object gains a negative charge when some electrons are attracted onto it from the negative object.
A neutral conductor may be charged by induction using the following procedure. A charged object is placed near but not in contact with the conductor. If the object is positively charged, electrons in the conductor are drawn to the side of the conductor near the object. If the object is negatively charged, electrons are drawn to the side of the conductor away from the object. If the conductor is then connected to a reservoir of electrons, such as the ground, electrons will flow onto or off of the conductor with the result that it acquires a charge opposite to that of the charged object brought near it.
See also pole, in electricity and magnetism.
Properties of Charges in Motion
Electrodynamics is the study of charges in motion. A flow of electric charge constitutes an electric current. Historically, the direction of current was described in terms of the motion of imaginary positive charges; this convention is still used by many scientists, although it is directly opposite to the direction of electron flow, which is now known to be the basis of electric current in solids. Current considered to be composed of imaginary positive charges is often called conventional current. In order for a current to exist in a conductor, there must be an electromotive force (emf), or potential difference, between the conductor's ends. An electric cell, a battery of cells, and a generator are all sources of electromotive force; any such source with an external conductor connected from one of the source's two terminals to the other constitutes an electric circuit. If the source is a battery, the current is in one direction only and is called direct current (DC). If the source is a generator without a commutator, the current direction reverses twice during each rotation of the armature, passing first in one direction and then in the other; such current is called alternating current (AC). The number of times alternating current makes a double reversal of direction each second is called the frequency of the current; the frequency of ordinary household current in the U.S. is 60 cycles per sec (60 Hz), and electric devices must be designed to operate at this frequency.
In a solid the current consists not of a few electrons moving rapidly but of many electrons moving slowly; although this drift of electrons is slow, the impulse that causes it when the circuit is completed moves through the circuit at nearly the speed of light. The movement of electrons in a current is not steady; each electron moves in a series of stops and starts. In a direct current, the electrons are spread evenly through the conductor; in an alternating current, the electrons tend to congregate along the surface of the conductor. In liquids and gases, the current carriers are not only electrons but also positive and negative ions.
History of Electricity
The Leyden Jar and the Quantitative Era
Progress quickened after the Leyden jar was invented in 1745 by Pieter van Musschenbroek. The Leyden jar stored static electricity, which could be discharged all at once. In 1747 William Watson discharged a Leyden jar through a circuit, and comprehension of the current and circuit started a new field of experimentation. Henry Cavendish, by measuring the conductivity of materials (he compared the simultaneous shocks he received by discharging Leyden jars through the materials), and Charles A. Coulomb, by expressing mathematically the attraction of electrified bodies, began the quantitative study of electricity.
A new interest in current began with the invention of the battery. Luigi Galvani had noticed (1786) that a discharge of static electricity made a frog's leg jerk. Consequent experimentation produced what was a simple electron cell using the fluids of the leg as an electrolyte and the muscle as a circuit and indicator. Galvani thought the leg supplied electricity, but Alessandro Volta thought otherwise, and he built the voltaic pile, an early type of battery, as proof. Continuous current from batteries smoothed the way for the discovery of G. S. Ohm's law (pub. 1827), relating current, voltage (electromotive force), and resistance (see Ohm's law), and of J. P. Joule's law of electrical heating (pub. 1841). Ohm's law and the rules discovered later by G. R. Kirchhoff regarding the sum of the currents and the sum of the voltages in a circuit (see Kirchhoff's laws) are the basic means of making circuit calculations.
Era of Electromagnetism
In 1819 Hans Christian Oersted discovered that a magnetic field surrounds a current-carrying wire. Within two years André Marie Ampère had put several electromagnetic laws into mathematical form, D. F. Arago had invented the electromagnet, and Michael Faraday had devised a crude form of electric motor. Practical application of a motor had to wait 10 years, however, until Faraday (and earlier, independently, Joseph Henry) invented the electric generator with which to power the motor. A year after Faraday's laboratory approximation of the generator, Hippolyte Pixii constructed a hand-driven model. From then on engineers took over from the scientists, and a slow development followed; the first power stations were built 50 years later (see power, electric).
In 1873 James Clerk Maxwell had started a different path of development with equations that described the electromagnetic field, and he predicted the existence of electromagnetic waves traveling with the speed of light. Heinrich R. Hertz confirmed this prediction experimentally, and Marconi first made use of these waves in developing radio (1895). John Ambrose Fleming invented (1904) the diode rectifier vacuum tube as a detector for the Marconi radio. Three years later Lee De Forest made the diode into an amplifier by adding a third electrode, and electronics had begun. Theoretical understanding became more complete in 1897 with the discovery of the electron by J. J. Thomson. In 1910–11 Ernest R. Rutherford and his assistants learned the distribution of charge within the atom. Robert Millikan measured the charge on a single electron by 1913.
See D. L. Anderson, Discovery of the Electron: The Development of the Atomic Concept of Electricity (1964); W. T. Scott, The Physics of Electricity and Magnetism (2d ed. 1966); M. Kaufman and J. A. Wilson, Basic Electricity (1973); E. T. Whittaker, History of Theories of Aether and Electricity (1954, repr. 1987).
Physical phenomena involving electric charges, their motions, and their effects. The motion of a charge is affected by its interaction with the electric field and, for a moving charge, the magnetic field. The electric field acting on a charge arises from the presence of other charges and from a time-varying magnetic field. The magnetic field acting on a moving charge arises from the motion of other charges and from a time-varying electric field. Thus electricity and magnetism are ultimately inextricably linked. In many cases, however, one aspect may dominate, and the separation is meaningful. See Electric charge, Electric field, Magnetism
The quantitative development of electricity began late in the eighteenth century. J. B. Priestley in 1767 and C. A. Coulomb in 1785 discovered independently the inverse-square law for stationary charges. This law serves as a foundation for electrostatics. See Coulomb's law, Electrostatics
In 1800 A. Volta constructed and experimented with the voltaic pile, the predecessor of modern batteries. It provided the first continuous source of electricity. In 1820 H. C. Oersted demonstrated magnetic effects arising from electric currents. The production of induced electric currents by changing magnetic fields was demonstrated by M. Faraday in 1831. In 1851 he also proposed giving physical reality to the concept of lines of force. This was the first step in the direction of shifting the emphasis away from the charges and onto the associated fields. See Electromagnetic induction, Electromagnetism, Lines of force
In 1865 J. C. Maxwell presented his mathematical theory of the electromagnetic field. This theory, which proposed a continuous electric fluid, not only synthesized a unified theory of electricity and magnetism, but also showed optics to be a branch of electromagnetism. See Electromagnetic radiation, Maxwell's equations
The developments of theories about electricity subsequent to Maxwell have all been concerned with the microscopic realm. Faraday's experiments on electrolysis in 1833 had indicated a natural unit of electric charge, thus pointing toward a discrete rather than continuous charge. The existence of electrons, negatively charged particles, was postulated by A. Lorenz in 1895 and demonstrated by J. J. Thomson in 1897. The existence of positively charged particles (protons) was shown shortly afterward (1898) by W. Wien. Since that time, many particles have been found having charges numerically equal to that of the electron. The question of the fundamental nature of these particles remains unsolved, but the concept of a single elementary charge unit is apparently still valid. See Baryon, Electron, Elementary particle, Hyperon, Meson, Proton, Quarks
The sources of electricity in modern technology depend strongly on the application for which they are intended.
The principal use of static electricity today is in the production of high electric fields. Such fields are used in industry for testing the ability of components such as insulators and condensers to withstand high voltages, and as accelerating fields for charged-particle accelerators. The principal source of such fields today is the Van de Graaff generator. See Particle accelerator
The major use of electricity arises in devices using direct current and low-frequency alternating current. The use of alternating current, introduced by S. Z. de Ferranti in 1885–1890, allows power transmission over long distances at very high voltages with a resulting low-percentage power loss followed by highly efficient conversion to lower voltages for the consumer through the use of transformers. See Electric current
Large amounts of direct current are used in the electrodeposition of metals, both in plating and in metal production, for example, in the reduction of aluminum ore.
The principal sources of low-frequency electricity are generators based on the motion of a conducting medium through a magnetic field. The moving charges interact with the magnetic field to give a charge motion that is normal to both the direction of motion and the magnetic field. In the most common form, conducting wire coils rotate in an applied magnetic field. The rotational power is derived from a water-driven turbine in the case of hydroelectric generation, or from a gas-driven turbine or reciprocating engine in other cases.
Many high-frequency devices, such as communications equipment, television, and radar, involve the consumption of only moderate amounts of power, generally derived from low-frequency sources. If the power requirements are moderate and portability is needed, the use of ordinary chemical batteries is possible. Ion-permeable membrane batteries are a later development in this line. Fuel cells, particularly hydrogen-oxygen systems, are being developed. They have already found extensive application in earth satellite and other space systems. The successful use of thermoelectric generators based on the Seebeck effect in semiconductors has been reported. See Thermoelectricity
The solar battery, also a semiconductor device, has been used to provide charging current for storage batteries in telephone service and in communications equipment in artificial satellites.
Direct conversion of mechanical energy into electrical energy is possible by utilizing the phenomena of piezoelectricity and magnetostriction. These have some application in acoustics and stress measurements. Pyroelectricity is a thermodynamic corollary of piezoelectricity. See Magnetostriction, Piezoelectricity, Pyroelectricity
the aggregate of phenomena that are caused by the existence, motion, and interaction of electrically charged bodies or particles. The interaction of electric charges occurs with the help of an electromagnetic field (in the case of electric charges at rest, with the help of an electrostatic field). Moving charges (electric current) create, along with an electric field, a magnetic field; that is, they generate an electromagnetic field, by way of which electromagnetic interactions are accomplished (the science of magnetism is thus an integral part of the science of electricity). Electromagnetic phenomena are described by classical electrodynamics, which is based on Maxwell’s equations. (See ELECTRIC CHARGE; ; ; , ; MAGNETISM; ; and MAXWELLS EQUATIONS.)
The laws of the classical theory of electricity encompass a large number of electromagnetic processes. Among the four types of interactions existing in nature—electromagnetic, gravitational, strong, and weak interactions—electromagnetic interactions exhibit the widest range and greatest variety of manifestations. This is associated with the fact that all matter is made up of electrically charged particles of opposite signs, interactions between which on the one hand are many orders of magnitude stronger than gravitational or weak interactions, but on the other hand are long-range in comparison with strong interactions. The structure of atomic shells, how atoms are held together in molecules (chemical forces), and the formation of a condensed substance are determined by electromagnetic interactions.
History. The simplest electric and magnetic phenomena have been known since antiquity. Some minerals were found to attract small pieces of iron, and it was observed that amber (elektron in Greek; hence the term “electricity”) after being rubbed with wool attracted light objects (electrification by friction). However, it was not until 1600 that W. Gilbert for the first time ascertained the difference between electric and magnetic phenomena. Gilbert discovered the existence of magnetic poles and their inseparability from one another and established that the earth is a gigantic magnet.
In the 17th and first half of the 18th centuries, numerous experiments were performed with electrified bodies, the first electrostatic machines were constructed based on electrification by friction, the existence of two types of electric charges was established (C. F. Dufay), and the electrical conductivity of metals was discovered (the English scientist S. Gray). With the invention of the first capacitor, the Leyden jar (1745), it became possible to store large amounts of electric charge. In the period 1747–53, B. Franklin advanced the first consistent theory of electric phenomena, established the electric nature of lightning, and invented the lightning rod.
The quantitative investigation of electric and magnetic phenomena began in the second half of the 18th century, when the first measuring instruments—electroscopes of various designs and electrometers—appeared. H. Cavendish (1773) and C. A. de Coulomb (1785) established experimentally the law for the interaction of electric point charges at rest (Cavendish’s works were not published until 1879). This fundamental law of electrostatics (seeCOULOMBS LAW) made it possible to create for the first time a method of measuring electric charges by means of the forces of interaction between them. Coulomb also established the law for the interaction between the poles of long magnets and introduced the concept of magnetic charges concentrated at the ends of the magnets.
The next stage in the development of the science of electricity is linked with L. Galvani’s discovery of “animal electricity” in the late 1700’s and the research of A. Volta, who correctly interpreted Galvani’s experiments by having present in a closed circuit two different metals in the liquid and who invented the first source of electric current—the galvanic cell, called a voltaic pile (1800), which produced a prolonged continous direct current. In 1802, V. V. Petrov constructed a galvanic cell of considerably greater capacity, discovered the electric arc and studied its properties, and indicated the possibility of using the electric arc for illumination and for melting and welding metals. H. Davy obtained by the electrolysis of aqueous solutions of alkalies (1807) the previously unknown metals sodium and potassium. J. P. Joule determined (1841) that the amount of heat generated in a conductor by the passage of an electric current is proportional to the square of the current; this law was substantiated by the accurate experiments of H. F. E. Lenz (1842; Joule’s law). G. S. Ohm established (1826) a quantitative relationship between the electric current and voltage in a circuit. K. F. Gauss formulated (1830) the basic theorem of electrostatics (seeGAUSS’ THEOREM).
The most fundamental discovery was made in 1820 by H. C. Oersted, who observed the action of an electric current on a magnetic needle, a phenomenon that indicated a relationship between electricity and magnetism. Later that year, A. M. Ampere established the law of the interaction of electric currents (seeAMPERES LAW); he also demonstrated that the properties of permanent magnets could be explained proceeding from the assumption that electric currents are continuously circulating in the molecules of magnetized bodies (molecular currents). Thus, according to Ampere, all magnetic phenomena are reduced to the interactions of currents, and magnetic charges do not exist. From the time of Oersted’s and Ampere’s discoveries, the science of magnetism became an integral part of the science of electricity.
Beginning in the second quarter of the 19th century, electricity penetrated rapidly into technology. In the 1820’s the first electromagnets appeared. One of the first applications of electricity was the telegraph. Electric motors and generators were constructed in the 1830’s and 1840’s and electric illumination and other devices, in the 1840’s. Subsequently, the practical uses of electricity progressively increased, which in turn had an important influence on the science of electricity.
Major contributions to the development of the science of electricity were made in the 1830’s and 1840’s by M. Faraday, founder of the general science of electromagnetic phenomena, in which all electric and magnetic phenomena are treated from a single viewpoint. Faraday demonstrated experimentally that the effects of electric charges and currents do not depend on the method of obtaining them; before Faraday, a distinction was made between “normal” (obtained by triboelectrification), atmospheric, “galvanic,” magnetic, thermoelectric, “animal,” and other forms of electricity. In 1831, Faraday discovered electromagnetic induction, that is, the generation of an electric current in a circuit placed in a changing magnetic field; this phenomenon, which had also been observed in 1832 by J. Henry, constitutes the foundation of electrical engineering. In 1833–34, Faraday determined the laws of electrolysis, which marked the birth of electrochemistry (see). Later he sought to find the relationship between electric, magnetic, and optical phenomena. He also discovered the polarization of dielectrics (1837), the phenomena of paramagnetism and diamagnetism (1845), and the magnetic rotation of the plane of polarized light (1845).
Faraday was the first to introduce the concept of electric and magnetic fields. He refuted the concept of action at a distance, whose adherents believed that bodies acted on one another directly (through a vacuum) from a distance. According to Faraday’s ideas, the interaction between charges and currents is accomplished by means of intermediate agents: the charges and currents create in the surrounding space, respectively, electric or magnetic fields by way of which the interaction is transmitted from point to point (the concept of short-range action). Underlying his ideas about electric and magnetic fields was the concept of lines of force, which Faraday regarded as mechanical formations in a hypothetical medium, the ether, similar to extended elastic threads or cords (seeLINES OF FORCE).
Faraday’s ideas about the reality of an electromagnetic field were not immediately accepted. The first mathematical formulation of the laws of electromagnetic induction was given by F. Neumann in 1845 in the language of action at a distance. Neumann introduced the important concepts of the coefficients of self-induction and mutual induction for currents. The significance of these concepts was revealed completely later, when W. Thomson (Lord Kelvin) evolved (1853) the theory of electric oscillations in a circuit composed of a capacitor (capacity) and a coil (inductance).
Of great importance to the development of the science of electricity was the creation of new instruments and methods in electrical measurements, along with the standard system of electrical and magnetic measuring units developed by Gauss and W. E. Weber (seeGAUSSIAN SYSTEM OF UNITS). In 1846, Weber indicated the relationship between current strength and the density of the electric charges in a conductor and the relationship between current strength and the velocity of the ordered motion of the charges. He also established the law for the interaction of moving point charges, which introduced a new universal electrodynamic constant, which is the ratio of the electrostatic and electromagnetic units of charge and has the dimension of velocity. In an experimental determination of this constant (Weber and R. H. A. Kohlrausch, 1856), a value was obtained close to the velocity of light, a definite indication of a relationship between electromagnetic and optical phenomena.
In the period 1861–73, the work of J. C. Maxwell developed and completed the science of electricity. On the basis of the empirical laws of electromagnetic phenomena, Maxwell formulated the fundamental equations of classical electrodynamics, now named after him, by introducing a hypothesis about the production of a magnetic field by a changing electric field. Like Faraday, he regarded electromagnetic phenomena as some form of mechanical process in an ether. A major new consequence was deduced from these equations—the existence of electromagnetic waves propagating with the velocity of light. Maxwell’s equations laid the foundations for the electromagnetic theory of light. Maxwell’s theory was substantiated in the period 1886–89, when H. R. Hertz demonstrated experimentally the existence of electromagnetic waves. Following Hertz’ discovery, attempts were made to communicate by means of electromagnetic waves, which culminated in the advent of radio and intensive research in the field of radio engineering.
The late 19th and early 20th centuries heralded a new era in the development of the theory of electricity. Studies of electric discharges were crowned with success when J. J. Thomson discovered the discreteness of electric charges. In 1897, Thomson measured the ratio of the charge of an electron to its mass, and in 1898 he determined the absolute value of the charge. Based on Thomson’s discovery and the conclusions of the molecular-kinetic theory, H. Lorentz laid the foundations for the electron theory of the construction of matter (seeLORENTZ-MAXWELL EQUATIONS). In the classical electron theory, matter is viewed as an aggregate of electrically charged particles, whose motions are in accordance with the laws of classical mechanics. Maxwell’s equations are derived from the equations for the electron theory by means of statistical averaging.
Attempts to apply the laws of classical electrodynamics to electromagnetic processes in moving mediums met with intrinsic difficulties. While seeking to resolve these difficulties, A. Einstein (1905) formulated the theory of relativity, which refuted once and for all the existence of an ether endowed with mechanical properties (seeRELATIVITY, THEORY OF). After the creation of the theory of relativity, it became evident that the laws of electrodynamics could not be reduced to the laws of classical mechanics.
In small intervals of space and time, the quantum properties of the electromagnetic field, which are not taken into account by the classical theory of electricity, become essential. The quantum theory of electromagnetic processes (seeQUANTUM ELECTRODYNAMICS) was evolved in the second quarter of the 20th century. The quantum theory of matter and fields now extends beyond the realm of the science of electricity and studies the more fundamental problems of the laws of motion and the structure of elementary particles.
With the discovery of new facts and the creation of new theories, the value of the classical science of electricity has not diminished, but rather the limits of applicability of classical electrodynamics have been defined. Within these limits, Maxwell’s equations and the classical electron theory remain valid, forming the foundation of the modern theory of electricity. Classical electrodynamics underlies most branches of electrical engineering, radio engineering, electronics, and optics (except quantum electronics). With the help of its equations, an enormous number of theoretical and applied problems have been solved. Specifically, the numerous problems concerning the behavior of a plasma under laboratory conditions and in space are being resolved by means of Maxwell’s equations (seePLASMA; CONTROLLED FUSION; and STAR).
REFERENCESKudriavtsev, P. S. Istoriia fiziki. Moscow, 1956.
Liozzi, M. Istoriia fiziki. Moscow, 1970. (Translated from Italian.)
Maxwell, J. C. Izbr. soch. po teorii elektromagnitnogo polia. Moscow, 1952. (Translated from English.)
Lorentz, H. A. Teoriia elektronov i ee primenenie k iavleniiam sveta i teplovogo izlucheniia, 2nd ed. Moscow, 1953. (Translated from English.)
Tamm, 1. E. Osnovy teorii elektrichestva, 9th ed. Moscow, 1976.
G. IA. MIAKISHEV
electricityAn electric charge. Electricity travels at the speed of light (approximately 186,000 miles per second or 300 million meters per second). In a single second, electricity would circle the earth at the equator more than seven times. As it travels in a wire, the speed is slowed a small amount due to the resistance in the metal.
Electricity Is a Weird Phenomenon
Because an electrical current flows in a circuit, it is natural to think of individual electrons flowing. However, they technically do not flow. Each electron pushes the one next to it. Therefore, in an alternating current, they are constantly pushing back and forth against each other.
Volts, Amps and Watts
Electrical pressure (force) is measured in "volts," and its flow (current) is measured in "amperes" or simply "amps." The amount of work electricity produces is measured in "watts" (amps X volts). See wireless electricity, volt, ampere, watt, nanosecond, electron, electrical and electronic.