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. Electric power is usually measured in Watts, kilowatts (1,000 watts), and megawatts (1,000,000 watts). The amount of electrical energy used by an appliance is found by multiplying its consumed power by the length of time of operation. The units of electrical energy are usually watt-seconds (joules), watt-hours, or kilowatt-hours. For commercial purposes the kilowatt-hour is the unit of choice.

Sources of Electrical Energy

Electrical energy occurs naturally, but seldom in forms that can be used. For example, although the energy dissipated as lightning exceeds the world's demand for electricity by a large factor, lightning has not been put to practical use because of its unpredictability and other problems. Generally, practical electric-power-generating systems convert the mechanical energy of moving parts into electrical energy (see generator). While systems that operate without a mechanical step do exist, they are at present either excessively inefficient or expensive because of a dependence on elaborate technology. While some electric plants derive mechanical energy from moving water (hydroelectric power), the vast majority derive it from heat engines in which the working substance is steam. Roughly 89% of power in the United States is generated this way. The steam is generated with heat from combustion of fossil fuels or from nuclear fission (see nuclear energy; nuclear reactor).

Steam as an Energy Source

The conversion of mechanical energy to electrical energy can be accomplished with an efficiency of about 80%. In a hydroelectric plant, the losses occur in the turbines, bearings, penstocks, and generators. The basic limitations of thermodynamics fix the maximum efficiency obtainable in converting heat to electrical energy. The necessity of limiting the temperature to safe levels also helps to keep the efficiency down to about 41% for a fossil-fuel plant. Most nuclear plants use low-pressure, low-temperature steam operation, and have an even lower efficiency of about 30%. Nuclear plants have been able to achieve efficiency up to 40% with liquid-metal cooling. It is thought that by using magnetohydrodynamic "topping" generators in conjunction with normal steam turbines, the efficiency of conventional plants can be raised to close to 50%. These devices remove the restrictions imposed by the blade structure of turbines by using the steam or gasses produced by combustion as the working fluid.

Environmental Concerns

The heat generated by an electric-power plant that is not ultimately converted into electrical energy is called waste heat. The environmental impact of this waste is potentially catastrophic, especially when, as is often the case, the heat is absorbed by streams or other bodies of water. Cooling towers help to dispose waste heat into the atmosphere. Associated with nuclear plants, in addition to the problem of waste heat, are difficulties attending the disposal and confinement of reaction products that remain dangerously radioactive for many thousands of years and the adjustment of such plants to variable demands for power. Public concern about such issues—fueled in part by the accidents at the Three Mile Island nuclear plant in Harrisburg Pennsylvania in 1979, and the nuclear plant explosion in the Soviet Union at Chernobyl in 1986—forced the U.S. government to introduce extensive safety regulations for nuclear plants. Partly because of those regulations, nuclear plants are proving to be uneconomical. Several are being shut down and replaced by conventionally fueled plants.

Alternative Energy Sources

Fuel cells develop electricity by direct conversion of hydrogen, hydrocarbons, alcohol, or other fuels, with an efficiency of 50% to 60%. Although they have been used to produce electric power in space vehicles and some terrestrial locations, several problems have kept them from being widely used. Most important, the catalyst, which is an important component of a fuel cell, especially one that is operating at around room temperature, is very expensive. Controlled nuclear fusion could provide a virtually unlimited source of heat energy to produce steam in generating plants; however, many problems surround its development, and no appreciable contribution is expected from this source in the near future.

Solar energy has been recognized as a feasible alternative. It has been suggested that efficient collection of the solar energy incident on 14% of the western desert areas of the United States would provide enough electricity to satisfy current demands. Two main solar processes could be used. Photovoltaic cells (see solar cell) convert sunlight directly into electrical energy. Another method would use special coatings that absorb sunlight readily and emit infrared radiation slowly, making it possible to heat fluids to 1,000°F; (540°C;) by solar radiation. The heat in turn can be converted to electricity. Some of this heat would be stored to allow operation at night and during periods of heavy cloud cover. The projected efficiency of such a plant would be about 30%, but this fairly low efficiency must be balanced against the facts that energy from the sun costs nothing and that the waste heat from such a plant places virtually no additional burden on the environment. The principal problem with this and other exotic systems for generating electricity is that the time needed for their implementation may be considerable.

Windmills, once widely used for pumping water, have become viable for electric-power generation because of advances in their design and the development of increasingly efficient generators. Windmill "farms," at which rows of windmills are joined together as the source of electrical energy, serve as a significant, though minor, source of electrical energy in coastal and plains areas. However, the vagaries of the wind make this a difficult solution to implement on a large scale.

See also energy, sources of.

Transmission of Electrical Energy

Electrical energy is of little use unless it can be made available at the place where it is to be used. To minimize energy losses from heating of conductors and to economize on the material needed for conductors, electricity is usually transmitted at the highest voltages possible. As modern transformers are virtually loss free, the necessary steps upward or downward in voltage are easily accomplished. Transmission lines for alternating current using voltages as high as 765,000 volts are not uncommon. For voltages higher than this it is advantageous to transmit direct current rather than alternating current. Recent advances in rectifiers, which turn alternating current into direct current, and inverters, which convert direct into alternating, have made possible transmission lines that operate at 800,000 volts and above. Such lines are still very expensive, however.

Electric utilities are tied together by transmission lines into large systems called power grids. They are thus able to exchange power so that a utility with a low demand can assist another with a high demand to help prevent a blackout, which involves the partial or total shutdown of a utility. Under such a system a utility experiencing too great a load, as when peak demand coincides with equipment failure, must remove itself from the grid or endanger other utilities. During periods in which demand exceeds supply a utility can reduce the power drawn from it by lowering its voltage. These voltage reductions, which are normally of 3%, 5%, or 8%, result in power reductions, or brownouts, of about 6%, 10%, or 15%, causing inefficient operation of some electrical devices. The power distribution system, because of its generation of low-frequency electromagnetic fields, has been suggested as a possible source of health problems.

Reactive Power

Reactive power is a concept used by engineers to describe the loss of power in a system arising from the production of electric and magnetic fields. Although reactive loads such as inductors and capacitors dissipate no power, they drop voltage and draw current, which creates the impression that they actually do. This "imaginary power" or "phantom power" is called reactive power. It is measured in a unit called Volt-Amps-Reactive (VAR). The actual amount of power being used, or dissipated, is called true power, and is measured in the unit of watts. The combination of reactive power and true power is called apparent power, and it is the product of a circuit's voltage and current. Apparent power is measured in the unit of Volt-Amps (VA). Devices which store energy by virtue of a magnetic field produced by a flow of current are said to absorb reactive power; those which store energy by virtue of electric fields are said to generate reactive power. Reactive power is significant because it must be provided and maintained to insure continuous, steady voltage on transmission networks. Reactive power thus is produced for maintenance of the system and not for end-use consumption. Power losses incurred in transmission from heat and electromagnetic emissions are included in the total reactive power requirement as are the needs of power hungry devices, such as electric motors, electromagnetic generators, and alternators. This power is supplied for many purposes by condensers, capacitors, and similar devices, which can react to changes in current flow by releasing energy to normalize the flow. If elements of the power grid cannot get the reactive power they need from nearby sources, they will pull it across transmission lines and destabilize the grid. In this way, poor management of reactive power can cause major blackouts.

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). The potential at the reference point is considered to be zero, and the reference point itself is usually chosen to be at infinity. It can be shown that the potential associated with a charged body at a given point in a static electric field is independent of the path along which the body has traveled in passing from infinity to the given point. Potential is measured in volts and is sometimes called voltage.

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 work. The traditional electric motor consists of a conducting loop that is mounted on a rotatable shaft. Current fed in by carbon blocks, called brushes, enters the loop through two slip rings. The magnetic field around the loop, supplied by an iron core field magnet, causes the loop to turn when current is flowing through it. In an alternating current (AC) motor, the current flowing in the loop is synchronized to reverse direction at the moment when the plane of the loop is perpendicular to the magnetic field and there is no magnetic force exerted on the loop. Because the momentum of the loop carries it around until the current is again supplied, continuous motion results. In alternating current induction motors the current passing through the loop does not come from an external source but is induced as the loop passes through the magnetic field. In a direct current (DC) motor, a device known as a split ring commutator switches the direction of the current each half rotation to maintain the same direction of motion of the shaft. In any motor the stationary parts constitute the stator, and the assembly carrying the loops is called the rotor, or armature. As it is easy to control the speed of direct-current motors by varying the field or armature voltage, these are used where speed control is necessary. The speed of AC induction motors is set roughly by the motor construction and the frequency of the current; a mechanical transmission must therefore be used to change speed. In addition, each different design fits only one application. However, AC induction motors are cheaper and simpler than DC motors. To obtain greater flexibility, the rotor circuit can be connected to various external control circuits. Most home appliances with small motors have a universal motor that runs on either DC or AC. Where the expense is warranted, the speed of AC motors is controlled by employing special equipment that varies the power-line frequency, which in the United States is 60 hertz (Hz), or 60 cycles per second. Brushless DC motors are constructed in a reverse fashion from the traditional form. The rotor contains a permanent magnet and the stator has the conducting coil of wire. By the elimination of brushes, these motors offer reduced maintainance, no spark hazard, and better speed control. They are widely used in computer disk drives, tape recorders, CD drives, and other electronic devices. Synchronous motors turn at a speed exactly proportional to the frequency. The very largest motors are synchronous motors with DC passing through the rotor.

fuse, electric, safety device used to protect an electric circuit against an excessive current. A fuse consists essentially of a strip of low-melting alloy enclosed in a suitable housing. It is connected in series with the circuit it is to protect. Because of its electrical resistance, the alloy strip in the fuse is heated by an electric current; if the current exceeds the safe value for which the fuse was designed, the strip melts, opening the circuit and stopping the current. The fuse housing is designed to resist the pressure generated if the overcurrent vaporizes the alloy strip, provided the voltage across the fuse does not exceed its rating. Some fuses, called slow-blow fuses, are designed to carry a small overload for a short time without opening the circuit, while others are designed to open very rapidly if the rated current is exceeded. The choice of one type or the other depends on the ruggedness of the equipment to be protected and whether large pulses of current often occur in the circuit; a slow-blow fuse is usually used to protect motors, and a fast-blow fuse to protect electronic equipment. A circuit can also be protected by a circuit breaker.

electric-light bug: see water bug.

electric shock, effect of the passage of a current of electricity through the body. Fatality may result from shocks of from 1 to 2 amperes and 500 to 1,000 volts. However, the effect of electric shock on the body depends not only on the strength of the current, but on such factors as wetness of the skin, area of contact, duration of contact, constitution of the victim, and whether or not the victim is well grounded. The general range of disturbances include a mild tingling (usually produced by common static electricity), spasm of the muscles, loss of consciousness, and sometimes death. In addition, burns occur where the current enters and leaves the body. A lethal dose of electricity may paralyze the respiratory organs and damage the central nervous system; the immediate cause of death, however, is usually an interruption of heart action. Electroconvulsive therapy is the use of electric shock to treat certain mental illnesses.

electric furnace: see furnace.

electric fish, name for various fish that produce electricity by means of organs usually developed from modified muscle tissue. The electric eel (Electrophorus electricus), a South American freshwater fish related to the carp, has organs along the ventral surface capable of producing from 450 to 600 volts of electricity—enough to light a neon bulb. Other electric fish include the electric ray, or torpedo; a freshwater electric catfish with a jellylike subcutaneous electric organ (probably of epidermal origin) that extends over the whole body; and various species of stargazer. All these fish produce electricity at will to paralyze or kill their prey, to repel their enemies, and to aid in navigation. Recent experiments have shown that when an electric eel is in motion it generates pulses of low-energy electricity which serve to detect the presence of nearby objects. Scientists believe that electric organs in fishes may function also in communication between individuals. Electric eels are classified in the phylum Chordata, subphylum Vertebrata, class Osteichthyes, order Cypriniformes, family Electrophoridae.

electric eye: see photoelectric cell.

electric eel: see electric fish.

electric current: see electricity.

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 small lamp (the load) to which the free ends of the wires leading from the cell are attached. When the connections are made properly, current flows, the circuit is said to be "closed," and the lamp will light. The current flows from the cell along one wire to the lamp, through the lamp, and along the other wire back to the cell. When the wires are disconnected, the circuit is said to be "open" or "broken." In practice, circuits are opened by such devices as switches, fuses, and circuit breakers (see fuse, electric; circuit breaker; short circuit). Two general circuit classifications are series and parallel. The elements of a series circuit are connected end to end; the same current flows through its parts one after another. The elements of a parallel circuit are connected so that each component has the same voltage across its terminals; the current flow is divided among its parts. When two circuit elements are connected in series, their effective resistance (impedance if the circuit is being fed alternating current) is equal to the sum of the separate resistances; the current is the same in each component throughout the circuit. When circuit elements are connected in parallel, the total resistance is less than that of the element having the least resistance, and the total current is equal to the sum of the currents in the individual branches. A battery-powered circuit is an example of a direct-current circuit; the voltages and currents are constant in magnitude and do not vary with time. In alternating-current circuits, the voltage and current periodically reverse direction with time. A standard electrical outlet supplies alternating current. Lighting circuits and electrical machinery use alternating current circuits. Many other devices, including computers, stereo systems, and television sets, must first convert the alternating current to direct current. That is done by a special internal circuit usually called a power supply. A digital circuit is a special kind of electronic circuit used in computers and many other devices. Magnetic circuits are analogous to electric circuits, where magnetic materials are regarded as conductors of magnetic flux. Magnetic circuits can be part of an electric circuit; a transformer is an example. Equivalent circuits are used in circuit analysis as a modeling tool; a simple circuit made up of a resistor, and an inductor might be used to electrically represent a loudspeaker. Electrical circuits can also be used in other fields of studies. In the study of heat flow, for example, a resistor is used to represent thermal insulation. Operating electric circuits can be used for general problem solving (as in an analog computer).

electric charge: see charge.

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. One of these, the mksa-practical system, is defined in terms of the units of the mks system and has the ampere of electric current as its basic unit. The units of this system—the volt, ohm, watt, and farad—are those commonly used by scientists and engineers to make practical measurements. The two other systems are both based on the cgs system. Electrostatic units (cgs-esu) are defined in a way that simplifies the description of interactions between static electric charges; there are no corresponding magnetic units in this system. Electromagnetic units (cgs-emu), on the other hand, are defined especially for the description of phenomena associated with moving electric charges, i.e., electric currents and magnetic poles. The two cgs systems have been widely used in the past and are still found in many texts and papers. The official body for maintaining such units in the United States is the National Institute of Standards and Technology.

eel, electric: see electric fish.

current, electric, net movement or flow of electric charge from one point to another or across some boundary. See alternating current; direct current; electricity.

circuit, electric: see electric circuit.

battery, electric, device that converts chemical energy into electrical energy, consisting of a group of electric cells that are connected to act as a source of direct current. The term is also now commonly used for a single cell, such as the alkaline dry cell used in flashlights and portable tape players, but strictly speaking batteries are made up of connected cells encased in a container and fitted with terminals to provide a source of direct electric current at a given voltage. A cell consists of two dissimilar substances, a positive electrode and a negative electrode, that conduct electricity, and a third substance, an electrolyte, that acts chemically on the electrodes. The two electrodes are connected by an external circuit (e.g., a piece of copper wire); the electrolyte functions as an ionic conductor for the transfer of the electrons between the electrodes. The voltage, or electromotive force, depends on the chemical properties of the substances used, but is not affected by the size of the electrodes or the amount of electrolyte.

Batteries are classed as either dry cell or wet cell. In a dry cell the electrolyte is absorbed in a porous medium, or is otherwise restrained from flowing. In a wet cell the electrolyte is in liquid form and free to flow and move. Batteries also can be generally divided into two main types—rechargeable and nonrechargeable, or disposable. Disposable batteries, also called primary cells, can be used until the chemical changes that induce the electrical current supply are complete, at which point the battery is discarded. Disposible batteries are most commonly used in smaller, portable devices that are only used intermittently or at a large distance from an alternative power source or have a low current drain. Rechargeable batteries, also called secondary cells, can be reused after being drained. This is done by applying an external electrical current, which causes the chemical changes that occur in use to be reversed. The external devices that supply the appropriate current are called chargers or rechargers.

A battery called the 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. Each cell contains a number of alternately positive and negative plates separated by the liquid electrolyte. The positive plates of the cell are connected to form the positive electrode; similarly, the negative plates form the negative electrode. In the process of charging, the cell is made to operate in reverse of its discharging operation; i.e., current is forced through the cell in the opposite direction, causing the reverse of the chemical reaction that ordinarily takes place during discharge, so that electrical energy is converted into stored chemical energy. The storage battery's greatest use has been in the automobile where it was used to start the internal-combustion engine. Improvements in battery technology have resulted in vehicles—some in commercial use—in which the battery system supplies power to electric drive motors instead.

Batteries are made of a wide variety of electrodes and electrolytes to serve a wide variety of uses. Batteries consisting of carbon-zinc dry cells connected in various ways (as well as batteries consisting of other types of dry cells) are used to power such devices as flashlights, lanterns, and pocket-sized radios and CD players. Alkaline dry cells are an efficient battery type that is both economical and reliable. In alkaline batteries, the hydrous alkaline solution is used as an electrolyte; the dry cell lasts much longer as the zinc anode corrodes less rapidly under basic conditions than under acidic conditions. In the United States the lead storage battery is commonly used. A more expensive type of lead-acid battery called a gel battery (or gel cell) contains a semisolid electrolyte to prevent spillage. More portable rechargeable batteries include several dry-cell types, which are sealed units and are therefore useful in appliances like mobile phones and laptops. Cells of this type (in order of increasing power density and cost) include nickel-cadmium (nicad or NiCd), nickel metal hydride (NiMH), and lithium-ion (Li-Ion) cells.

There is evidence that primitive batteries were used in Iraq and Egypt as early as 200 B.C. for electroplating and precious metal gilding. In 1748, Benjamin Franklin coined the term battery to describe an array of charged glass plates. However, most historians date the invention of batteries to about 1800 when experiments by Alessandro Volta resulted in the generation of electrical current from chemical reactions between dissimilar metals. Experiments with different combinations of metals and electrolytes continued over the next 60 years. In the 1860s, Georges Leclanche of France developed a carbon-zinc wet cell; nonrechargeable, it was rugged, manufactured easily, and had a reasonable shelf life. Also in the 1860s, Raymond Gaston Plant invented the lead-acid battery. It had a short shelf life, and about 1881 Émile Alphonse Faure developed batteries using a mixture of lead oxides for the positive plate electrolyte with faster reactions and higher efficiency. In 1900, Thomas Alva Edison developed the nickel storage battery, and in 1905 the nickel-iron battery. During World War II the mercury cell was produced. The small alkaline battery was introduced in 1949. In the 1950s the improved alkaline-manganese battery was developed. In 1954 the first solar battery or solar cell was introduced, and in 1956 the hydrogen-oxygen fuel cell was introduced. The 1960s saw the invention of the gel-type electrolyte lead-acid battery. Lithium-ion batteries, wafer thin and powering portable computers, cell phones, and space probes were introduced in the 1990s. Computer chips and sensors now help prolong battery life and speed the charging cycle. Sensors monitor the temperature inside a battery as chemical reactions during the recharging cause it to heat up; microchips control the power flow during recharging so that current flows in rapidly when the batteries are drained and then increasingly slowly as the batteries become fully charged. Another source of technical progress is nanotechnology; research indicates that batteries employing carbon nanotubes will have twice the life of traditional batteries.

See also electric circuit; fuel cell; solar cell.

Amount of work needed to move a unit electric charge from a reference point to a specific point against an electric field. The potential energy of a positive charge increases when it moves against an electric field, and decreases when it moves with the field. Electric potential can be thought of as potential energy per unit charge. The work done in moving a unit charge from one point to another, as in an electric circuit, is equal to the difference in potential energies at each point. Electric potential is expressed in units of joules per coulomb, or volts.

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Physical effect of an electric current that enters the body, ranging from a minor static-electricity discharge to a power-line accident or lightning strike but most often resulting from house current. The effects depend on the current (not the voltage), and the worst damage occurs along its path from the entry to the exit point. Causes of immediate death are ventricular fibrillation and paralysis of the brain's breathing centre or of the heart. Cardiopulmonary resuscitation is the best first aid. Though most survivors recover completely, aftereffects may include cataract, angina pectoris, or nervous-system disorders.

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Any of the aquatic rays (families Torpedinidae, Narkidae, and Temeridae) that produce an electrical shock. They are found worldwide in warm and temperate seas, mostly in shallow water but some (genus Benthobatis) at depths greater than 3,000 ft (900 m). Slow-moving bottom-dwellers, they feed on fishes and invertebrates. They range in length from less than 1 ft (30 cm) to about 6 ft (1.8 m) and have a short, stout tail. They are soft and smooth-skinned, with a circular or nearly circular body disk formed by the head and pectoral fins. They are harmless unless touched or stepped on. The electric organs, composed of modified muscle tissue, are in the disk near the head. The shock from these organs, which may reach 220 volts and is strong enough to fell a human adult, is used for defense, sensory location, and capturing prey.

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Machine that converts mechanical energy to electricity for transmission and distribution over power lines to domestic, commercial, and industrial customers. Generators also produce the electric power required for automobiles, aircraft, ships, and trains. The mechanical power for an electric generator is usually obtained from a rotating shaft and is equal to the shaft torque multiplied by the rotational, or angular, velocity (speed). The mechanical power may come from various sources: turbines powered by water, wind, steam, or gas; gasoline engines; or diesel engines.

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Chamber heated with electricity to very high temperatures, for melting and alloying metals and refractories. Modern electric furnaces generally are either arc furnaces or induction furnaces. Arc furnaces produce roughly two-fifths of the steel made in the U.S. In the induction furnace, a coil carrying alternating electric current surrounds the container or chamber of metal; circulating eddy currents induced in the metal produce extremely high temperatures.

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or electric force

Force between two electric charges. The magnitude of the force math.F is proportional to the product of the two charges, math.q₁ and math.q₂, divided by the square of the distance math.r between them, or math.F = math.kmath.q₁math.q₂/math.r², where math.k is a constant that depends on the measurement system being used. The Coulomb force can be one of repulsion, such as the force between two objects having like charges, or it can be attractive, such as the force between two objects having opposite charges.

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Region around an electric charge in which an electric force is exerted on another charge. The strength of an electric field math.E at any point is defined as the electric force math.F exerted per unit positive electric charge math.q at that point, or math.E = math.F/math.q. An electric field has both magnitude and direction and can be represented by lines of force, or field lines, that start on positive charges and terminate on negative charges. The electric field is stronger where the field lines are close together than where they are farther apart. The value of the electric field has dimensions of force per unit charge and is measured in units of newtons per coulomb.

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or photoelectric cell or electric eye

Solid-state device with a photosensitive cathode that emits electrons when illuminated and an anode for collecting the emitted electrons. Illumination excites electrons, which are attracted to the anode, producing current proportional to the intensity of the illumination. In a photovoltaic cell, light is used to produce voltage. In a photoconductive cell, light is used to regulate the flow of current. Photocells are used in control systems, where interrupting a beam of light opens a circuit, actuating a relay that supplies power to a mechanism to bring about a desired operation, such as opening a door or setting off a burglar alarm. Photocells are also used in photometry and spectroscopy.

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Eel-shaped South American fish (Electrophorus electricus) capable of producing an electric shock strong enough to stun a human. The electric eel (not a true eel) is a sluggish inhabitant of slow freshwater, surfacing periodically to gulp air. Long, cylindrical, scaleless, and gray-brown, it sometimes reaches a length of 9 ft (2.75 m) and a weight of 49 lbs (22 kg). The tail region, bordered below by a long anal fin that the fish undulates to move about, contains the electric organs. The shock (up to 650 volts discharged at will) is used mainly to immobilize fish and other prey.

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or vapor lamp

Lighting device consisting of a transparent container within which a gas is energized by an applied voltage and made to glow. After practical generators were devised in the 19th century, many experimenters applied electric power to tubes of gas. From circa 1900, electric discharge lamps were in use in Europe and the U.S. Fluorescent, neon, mercury, sodium, and metal-halide lamps are of the electric discharge variety.

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Pair of equal and opposite electric charges, the centres of which do not coincide. An atom in which the centre of the negative cloud of electrons has been shifted slightly away from the nucleus by an external electric field is an induced electric dipole. When the external field is removed, the atom loses its dipolarity. A water molecule, in which two hydrogen atoms are situated to one side of an oxygen atom, is a permanent electric dipole. The oxygen side is always slightly negative, the hydrogen side slightly positive.

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Movement of electric charge carriers. In a wire, electric current is a flow of electrons that have been dislodged from atoms and is a measure of the quantity of electrical charge passing any point of the wire per unit time. Current in gases and liquids generally consists of a flow of positive ions in one direction together with a flow of negative ions in the opposite direction. Conventionally, the direction of electric current is that of the flow of the positive ions. In alternating current (AC) the motion of the charges is periodically reversed; in direct current (DC) it is not. A common unit of current is the ampere, a flow of one coulomb of charge per second, or 6.24 × 10¹⁸ electrons per second.

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Electrically conducting pathway containing both inductance and capacitance elements. When these elements are connected in series, the circuit presents low electrical impedance to alternating current of the same frequency as the resonance frequency of the circuit and high impedance to current of other frequencies. The circuit's resonance frequency is determined by the values of inductance and capacitance. When the circuit elements are connected in parallel, the impedance is high at the resonance frequency and low at other frequencies. With their ability to pass only certain frequencies, tuned circuits are important in, for example, radio and television receivers.

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Electrical device in which the wiring and certain components consist of a thin coat of electrically conductive material applied in a pattern on an insulating substrate. Printed circuits replaced conventional wiring after World War II in much electronic equipment, greatly reducing size and weight while improving reliability and uniformity over the hand-soldered circuits formerly used. They are commonly used to mount integrated circuits on boards for use as plug-in units in computers, televisions, and other electronic devices. Mass-produced printed circuit boards allow automated assembly of electronic components, considerably reducing their cost.

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or microcircuit or chip or microchip

Assembly of microscopic electronic components (transistors, diodes, capacitors, and resistors) and their interconnections fabricated as a single unit on a wafer of semiconducting material, especially silicon. Early ICs of the late 1950s consisted of about 10 components on a chip 0.12 in. (3 mm) square. Very large-scale integration (VLSI) vastly increased circuit density, giving rise to the microprocessor. The first commercially successful IC chip (Intel, 1974) had 4,800 transistors; Intel's Pentium (1993) had 3.2 million, and more than a billion are now achievable.

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or electric circuit

Path that transmits electric current. A circuit includes a battery or a generator that gives energy to the charged particles; devices that use current, such as lamps, motors, or electronic computers; and connecting wires or transmission lines. Circuits can be classified according to the type of current they carry (see alternating current, direct current) or according to whether the current remains whole (series) or divides to flow through several branches simultaneously (parallel). Two basic laws that describe the performance of electric circuits are Ohm's law and Kirchhoff's circuit rules. Seealso tuned circuit.

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Quantity of electricity that flows in electric currents or that accumulates on the surfaces of dissimilar nonmetallic substances that are rubbed together briskly. It occurs in discrete natural units, equal to the charge of an electron or proton. It cannot be created or destroyed. Charge can be positive or negative; one positive charge can combine with one negative charge, and the result is a net charge of zero. Two objects that have an excess of the same type of charge repel each other, while two objects with an excess of opposite charge attract each other. The unit of charge is the coulomb, which consists of 6.24 × 10¹⁸ natural units of electric charge.

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Method of execution in which the condemned person is subjected to a heavy charge of electric current. The prisoner is shackled into a wired chair, and electrodes are fastened to the head and one leg so that the current will flow through the body. One electrical shock may not be enough to kill the person; if a doctor does not confirm the death, several shocks may be applied. The electric chair was first used in 1890. Electrocution also refers to death by other causes of electrical shock (e.g., accidental contact with high-voltage wiring).

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Battery-powered motor vehicle. Originating in the 1880s, electric cars were used for private passenger, truck, and bus transportation in cities, where their low speeds and limited battery range were not drawbacks, and the cars became popular for their quietness and low maintenance costs. Until 1920 they were competitive with gasoline-fueled cars; they became less so after the electric self-starter made gasoline-powered cars more attractive and mass production made them cheaper to produce. In Europe electric vehicles have been used as short-range delivery vans. Renewed interest in electric cars beginning in the 1970s, spurred especially by new consciousness of foreign oil dependency and environmental concern, led to improvements in speed and range. Recent laws, particularly in California, have mandated commercial production. “Hybrid” cars employing both electric and internal combustion engines and providing the best features of both technologies, have recently become commercially available. Experimental vehicles have used solar fuel cells.

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Type of electric furnace in which heat is generated by an arc between carbon electrodes above the surface of the material (commonly a metal) being heated. William Siemens first demonstrated the arc furnace in 1879 at the Paris Exposition by melting iron in crucibles; horizontally placed carbon electrodes produced an electric arc above the container of metal. The first commercial arc furnace in the U.S. (1906) had a capacity of four tons (3.6 metric tons) and was equipped with two electrodes. Modern furnaces range in heat size from a few tons up to 400 tons (360 metric tons), and the arcs strike directly into the metal bath from vertically positioned, graphite electrodes to remelt scrap steel or refine briquettes of direct-reduced iron ore.

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