electricity, class of phenomena arising from the existence of charge charge, property of matter that gives rise to all electrical phenomena (see electricity ). The basic unit of charge, usually denoted by e, is that on the proton or the electron ; that on the proton is designated as positive (+e
..... Click the link for more information. . The basic unit of charge is that on the proton proton, elementary particle having a single positive electrical charge and constituting the nucleus of the ordinary hydrogen atom. The positive charge of the nucleus of any atom is due to its protons.
..... Click the link for more information. or electron electron, elementary particle carrying a unit charge of negative electricity. Ordinary electric current is the flow of electrons through a wire conductor (see electricity ). The electron is one of the basic constituents of matter.
..... Click the link for more information. —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 ampere (ăm`pēr), abbr. amp or A, basic unit of electric current.
..... Click the link for more information. is the unit of current, the coulomb coulomb (k

`lŏm) [for C. A. de Coulomb ], abbr.
..... Click the link for more information. is the unit of charge, the volt volt [for Alessandro Volta ], abbr. V, unit of electric potential and electromotive force . It is defined as the difference of electric potential existing across the ends of a conductor carrying a constant current of 1 ampere when the power dissipated is 1 watt.
..... Click the link for more information. is the unit of electromotive force, and the ohm ohm (ōm) [for G. S. Ohm ], unit of electrical resistance , defined as the resistance in a circuit in which a potential difference of one
..... Click the link for more information. is the unit of resistance, reactance, or impedance (see electric and magnetic units electric and magnetic units, units used to express the magnitudes of various quantities in electricity and magnetism. Three systems of such units, all based on the metric system , are commonly used.
..... Click the link for more information. ).

Properties of Electric Charges

According to modern theory, most elementary particles elementary particles, the most basic physical constituents of the universe.

Basic Constituents of Matter

Molecules are built up from the atom , which is the basic unit of any chemical element .
..... Click the link for more information. 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 Coulomb's law (k
..... Click the link for more information. ). The electric force force, commonly, a "push" or "pull," more properly defined in physics as a quantity that changes the motion, size, or shape of a body. Force is a vector quantity, having both magnitude and direction.
..... Click the link for more information. between two charged particles is much greater than the gravitational force between the particles. The negatively charged electrons in an atom atom [Gr.,=uncuttable (indivisible)], basic unit of matter ; more properly, the smallest unit of a chemical element having the properties of that element.

Structure of the Atom

..... Click the link for more information. 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 ion, atom or group of atoms having a net electric charge .

Positive and Negative Electric Charges

A neutral atom or group of atoms becomes an ion by gaining or losing one or more electrons or protons.
..... Click the link for more information. ; 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 chemical bond, mechanism whereby atoms combine to form molecules . There is a chemical bond between two atoms or groups of atoms when the forces acting between them are strong enough to lead to the formation of an aggregate with sufficient stability to be regarded as
..... Click the link for more information. ). 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 conduction, transfer of heat or electricity through a substance, resulting from a difference in temperature between different parts of the substance, in the case of heat, or from a difference in electric potential , in the case of electricity.
..... Click the link for more information. ; insulation insulation (ĭn'səlā`shən, ĭn'sy
..... Click the link for more information. ); materials that allow charge to pass easily are called conductors, while those that do not are called insulators, or dielectrics dielectric (dī'ĭlĕk`trĭk), material that does not conduct electricity readily, i.e., an insulator (see insulation ).
..... Click the link for more information. . A third class of materials, called semiconductors semiconductor, solid material whose electrical conductivity at room temperature is between that of a conductor and that of an insulator (see conduction ; insulation ).
..... Click the link for more information. , conduct charge under some conditions but not under others.

Properties of Charges at Rest

Electrostatics electrostatics, study of phenomena associated with charged bodies at rest (see charge ; electricity ). A charged body has an excess of positive or negative charges, a condition usually brought about by the transfer of electrons to or from the body.
..... Click the link for more information. 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 pole, in electricity and magnetism, point where electric or magnetic force appears to be concentrated. A single electric charge located at a point is sometimes referred to as an electric monopole.
..... Click the link for more information. , in electricity and magnetism.

Properties of Charges in Motion

Electrodynamics electrodynamics, study of phenomena associated with charged bodies in motion and varying electric and magnetic fields (see charge ; electricity ); since a moving charge produces a magnetic field , electrodynamics is concerned with effects such as magnetism ,
..... Click the link for more information. 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 electromotive force, abbr. emf, difference in electric potential, or voltage, between the terminals of a source of electricity, e.g., a battery from which no current is being drawn. When current is drawn, the potential difference drops below the emf value.
..... Click the link for more information. (emf), or potential potential, electric, work per unit of electric charge expended in moving a charged body from a reference point to any given point in an electric field (see electrostatics ).
..... Click the link for more information. difference, between the conductor's ends. An electric cell, a battery storage battery is generally of the wet-cell type; i.e., it uses a liquid electrolyte and can be recharged many times. The storage battery consists of several cells connected in series.
..... Click the link for more information. of cells, and a generator generator, in electricity, machine used to change mechanical energy into electrical energy. It operates on the principle of electromagnetic induction , discovered (1831) by Michael Faraday.
..... Click the link for more information. 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 electric circuit, unbroken path along which an electric current exists or is intended or able to flow. A simple circuit might consist of an electric cell (the power source), two conducting wires (one end of each being attached to each terminal of the cell), and a
..... Click the link for more information. . 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

From the writings of Thales of Miletus it appears that Westerners knew as long ago as 600 B.C. that amber becomes charged by rubbing. There was little real progress until the English scientist William Gilbert in 1600 described the electrification of many substances and coined the term electricity from the Greek word for amber. As a result, Gilbert is called the father of modern electricity. In 1660 Otto von Guericke invented a crude machine for producing static electricity. It was a ball of sulfur, rotated by a crank with one hand and rubbed with the other. Successors, such as Francis Hauksbee, made improvements that provided experimenters with a ready source of static electricity. Today's highly developed descendant of these early machines is the Van de Graaf generator, which is sometimes used as a particle accelerator particle accelerator, apparatus used in nuclear physics to produce beams of energetic charged particles and to direct them against various targets. Such machines, popularly called atom smashers, are needed to observe objects as small as the atomic nucleus in studies
..... Click the link for more information. . Robert Boyle realized that attraction and repulsion were mutual and that electric force was transmitted through a vacuum (c.1675). Stephen Gray distinguished between conductors and nonconductors (1729). C. F. Du Fay recognized two kinds of electricity, which Benjamin Franklin and Ebenezer Kinnersley of Philadelphia later named positive and negative.

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 Ohm's law (ōm) [for G. S. Ohm ], law stating that the electric current i flowing through a given resistance r
..... Click the link for more information. ), 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 Kirchhoff's laws [for Gustav R. Kirchhoff ], pair of laws stating general restrictions on the current and voltage in an electric circuit . The first of these states that at any given instant the sum of the voltages around any closed path, or loop, in the network is
..... Click the link for more information. ) 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 motor, electric, machine that converts electrical energy into mechanical energy. When an electric current is passed through a wire loop that is in a magnetic field, the loop will rotate and the rotating motion is transmitted to a shaft, providing useful mechanical
..... Click the link for more information. . 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 power, electric, energy dissipated in an electrical or electronic circuit or device per unit of time. The electrical energy supplied by a current to an appliance enables it to do work or provide some other form of energy such as light or heat.
..... Click the link for more information. ).

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 diode (dī`ōd), two-terminal electronic device that permits current flow predominantly in only one direction.
..... Click the link for more information. 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 electronics, science and technology based on and concerned with the controlled flow of electrons or other carriers of electric charge, especially in semiconductor devices. It is one of the principal branches of electrical engineering .
..... Click the link for more information. 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.

Bibliography

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).

electricity

Phenomenon associated with stationary or moving electric charges. The word comes from the Greek elektron (“amber”); the Greeks discovered that amber rubbed with fur attracted light objects such as feathers. Such effects due to stationary charges, or static electricity, were the first electrical phenomena to be studied. Not until the early 19th century were static electricity and electric current shown to be aspects of the same phenomenon. The discovery of the electron, which carries a charge designated as negative, showed that the various manifestations of electricity are the result of the accumulation or motion of numbers of electrons. The invention of the incandescent lightbulb (1879) and the construction of the first central power station (1881) by Thomas Alva Edison led to the rapid introduction of electric power into factories and homes. See also James Clerk Maxwell.

The flow of electrons in a circuit. The speed of electricity is the speed of light (approximately 186,000 miles per second or 300,000,000 meters per second). In a wire, it is slowed due to the resistance in the material.

Its pressure, or force, is measured in "volts," and its flow, or current, is measured in "amperes" or simply "amps." The amount of work it produces is measured in "watts" (amps X volts).