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energy, sources of,

origins of the power used for transportation, for heat and light in dwelling and working areas, and for the manufacture of goods of all kinds, among other applications. The development of science and civilization is closely linked to the availability of energyenergy,
in physics, the ability or capacity to do work or to produce change. Forms of energy include heat, light, sound, electricity, and chemical energy. Energy and work are measured in the same units—foot-pounds, joules, ergs, or some other, depending on the system of
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 in useful forms. Modern society consumes vast amounts of energy in all forms: light, heat, electrical, mechanical, chemical, and nuclear. The rate at which energy is produced or consumed is called powerpower,
in physics, time rate of doing work or of producing or expending energy. The unit of power based on the English units of measurement is the horsepower, devised for describing mechanical power by James Watt, who estimated that a horse can do 550 ft-lb of work per sec; a
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, although this term is sometimes used in common speech synonymously with energy.

Types of Energy

Chemical and Mechanical Energy

An early source of energy, or prime mover, used by humans was animal power, i.e., the energy obtained from domesticated animals. Later, as civilization developed, wind power was harnessed to drive ships and turn windmillswindmill,
apparatus that harnesses wind power for a variety of uses, e.g., pumping water, grinding corn, driving small sawmills, and driving electrical generators. Windmills were probably not known in Europe before the 12th cent.
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, and streams and rivers were diverted to turn water wheelswater wheel,
device for utilizing the power of flowing or falling water. The Norse wheel is the oldest type known. Despite its name it probably originated in the Middle East, where the swift stream required by this type of wheel is common.
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 (see water powerwater power,
mechanical energy derived from falling or flowing water, e.g., rivers, streams, and the overflow of dams. The wooden water wheel, long utilized for driving machinery in flour mills and factories, was largely supplanted by the steam engine in the early 19th cent.
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). The rotating shaft of a windmill or water wheel could then be used to crush grain, to raise water from a well, or to serve any number of other uses. The motion of the wind and water, as well as the motion of the wheel or shaft, represents a form of mechanical energy. The source of animal power is ultimately the chemical energy contained in foods and released when digested by humans and animals. The chemical energy contained in wood and other combustible fuelsfuel,
material that can be burned or otherwise consumed to produce heat. The common fuels used in industry, transportation, and the home are burned in air. The carbon and hydrogen in fuel rapidly combine with oxygen in the air in an exothermal reaction—one that liberates
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 has served since the beginning of history as a source of heat for cooking and warmth. At the start of the Industrial Revolution, water power was used to provide energy for factories through systems of belts and pulleys that transmitted the energy to many different machines.

Heat Energy

The invention of the steam enginesteam engine,
machine for converting heat energy into mechanical energy using steam as a medium, or working fluid. When water is converted into steam it expands, its volume increasing about 1,600 times. The force produced by the conversion is the basis of all steam engines.
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, which converts the chemical energy of fuels into heat energy and the heat into mechanical energy, provided another source of energy. The steam engine is called an external-combustion engine, since fuel is burned outside the engine to create the steam used inside it. During the 19th cent. the internal-combustion engineinternal-combustion engine,
one in which combustion of the fuel takes place in a confined space, producing expanding gases that are used directly to provide mechanical power.
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 was developed; a variety of fuels, depending on the type of internal-combustion engine, are burned directly in the engine's chambers to provide a source of mechanical energy. Both steam engines and internal-combustion engines found application as stationary sources of power for different purposes and as mobile sources for transportation, as in the steamship, the railroad locomotive (both steam and diesel), and the automobile. All these sources of energy ultimately depend on the combustion of fuels for their operation.

Electrical Energy

Early in the 19th cent. another source of energy was developed that did not necessarily need the combustion of fuels—the electric generatorgenerator,
in electricity, machine used to change mechanical energy into electrical energy. It operates on the principle of electromagnetic induction, discovered (1831) by Michael Faraday.
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, or dynamo. The generator converts the mechanical energy of a conductor moving in a magnetic field into electrical energy, using the principle of electromagnetic inductioninduction,
in electricity and magnetism, common name for three distinct phenomena. Electromagnetic induction is the production of an electromotive force (emf) in a conductor as a result of a changing magnetic field about the conductor and is the most important of the
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. The great advantage of electrical energy, or electric power, as it is commonly called, is that it can be transmitted easily over great distances (see power, electricpower, 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.
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). As a result, it is the most widely used form of energy in modern civilization; it is readily converted to light, to heat, or, through the electric motormotor, 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.
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, to mechanical energy again. The large-scale production of electrical energy was made possible by the invention of the turbineturbine,
rotary engine that uses a continuous stream of fluid (gas or liquid) to turn a shaft that can drive machinery.

A water, or hydraulic, turbine is used to drive electric generators in hydroelectric power stations.
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, which efficiently converts the straight-line motion of falling water or expanding steam into the rotary motion needed to turn the rotor of a large generator.

Nuclear Energy

The development of nuclear energynuclear energy,
the energy stored in the nucleus of an atom and released through fission, fusion, or radioactivity. In these processes a small amount of mass is converted to energy according to the relationship E = mc2, where E is energy, m
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 made available another source of energy. The heat of a nuclear reactornuclear reactor,
device for producing controlled release of nuclear energy. Reactors can be used for research or for power production. A research reactor is designed to produce various beams of radiation for experimental application; the heat produced is a waste product and is
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 can be used to produce steam, which then can be directed through a turbine to drive an electric generator, the propellers of a large ship, or some other machine. In 1999, 23% of the electricity generated in the United States derived from nuclear reactors; however, since the 1980s, the construction and application of nuclear reactors in the United States has slowed because of concern about the dangers of the resulting radioactive waste and the possibility of a disastrous nuclear meltdown (see Three Mile IslandThree Mile Island,
site of a nuclear power plant 10 mi (16 km) south of Harrisburg, Pa. On Mar. 28, 1979, failure of the cooling system of the No. 2 nuclear reactor led to overheating and partial melting of its uranium core and production of hydrogen gas, which raised fears of
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; ChernobylChernobyl
, Ukr. Chornobyl, abandoned city, N Ukraine, near the Belarus border, on the Pripyat River. Ten miles (16 km) to the north, in the town of Pripyat, is the Chernobyl nuclear power station, site of the worst nuclear reactor disaster in history. On Apr.
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; FukushimaFukushima
, city (1990 pop. 277,528), capital of Fukushima prefecture, N Honshu, Japan, on the Kiso plain. A silk-textile center, it is a major commercial city of NE Japan, also producing cameras, automobiles, fruits, and bonsai trees.

Fukushima prefecture (1990 pop.
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).

Environmental Considerations

The demand for energy has increased steadily through much of the late 20th and early 21st cent., not only because of the growing population but also because of the greater number of technological goods available and the increased affluence that has brought these goods within the reach of a larger proportion of the population. For example, despite the introduction of more fuel-efficient motor vehicles (average miles per gallon increased by 34% between 1975 and 1990), the consumption of fuel by vehicles in America increased by 20% between 1975 and 1990. The rise in gasoline consumption is attributable to an increase in the number of miles the average vehicle traveled and to a 40% increase in the same period in the number of vehicles on the road. From 1990 to the mid-2000s, the average fuel efficiency decreased (a trend that had begun in the 1980s), due in part to the increasing use of light trucks as passenger vehicles, but it subsequently began to improve due to increasingly stringent federal standards. The number of miles traveled increased more slowly during the same period, and the total amount of fuel consumed declined for a time due to recession and then increased to around the levels of the mid-2000s.

As a result of the increase in the consumption of energy, concern has risen about the depletion of natural resources, both those used directly to produce energy and those damaged during the exploitation of the fuels or as a result of contamination by energy waste products (see under conservation of natural resourcesconservation of natural resources,
the wise use of the earth's resources by humanity. The term conservation came into use in the late 19th cent. and referred to the management, mainly for economic reasons, of such valuable natural resources as timber, fish, game, topsoil,
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). Most of the energy consumed is ultimately generated by the combustion of fossil fuels, such as coal, petroleum, and natural gas. Although the world has only a finite supply of these fuels, and concern was long focused on decreasing supply, the environmental damage caused by the use of such fuels, especially coal, is a greater concern. The production and combustion of these fuels releases various pollutants (see pollutionpollution,
contamination of the environment as a result of human activities. The term pollution refers primarily to the fouling of air, water, and land by wastes (see air pollution; water pollution; solid waste).
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), such as soot, carbon monoxide and sulfur dioxide, which pose health risks and contribute to acid rainacid rain
or acid deposition,
form of precipitation (rain, snow, sleet, or hail) containing high levels of sulfuric or nitric acids (pH below 5.5–5.6).
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, and carbon dioxide and methane, which contribute to global warmingglobal warming,
the gradual increase of the temperature of the earth's lower atmosphere as a result of the increase in greenhouse gases since the Industrial Revolution. Global warming and its effects, such as more intense summer and winter storms, are also referred to as climate
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. There are also destructive effects to sensitive wildlands (e.g., the tropical rain forests, the arctic tundra, and coastal marshes) during the exploitation of their resources.

The Search for New Sources of Energy

The environmental consequences of energy production have led many nations in the world to impose stricter guidelines on the production and consumption of energy. Further, the search for new sources of energy and more efficient means of employing energy has accelerated. The development of a viable nuclear fusion reactor is often cited as a possible solution to our energy problems. Presently, nuclear-energy plants use nuclear fission, which requires scarce and expensive fuels and produces potentially dangerous wastes. The fuel problem has been partly helped by the development of breeder reactors, which produce more nuclear fuel than they consume, but the long-term hopes for nuclear energy rest on the development of controlled sources using nuclear fusion rather than fission. The basic fuels for fusion are extremely plentiful (e.g., hydrogen, from water) and the end products are relatively safe. The basic problem, which is expected to take decades to solve, is in containing the fuels at the extremely high temperatures necessary to initiate and sustain nuclear fusion.

Another source of energy is solar energysolar energy,
any form of energy radiated by the sun, including light, radio waves, and X rays, although the term usually refers to the visible light of the sun. Solar energy is needed by green plants for the process of photosynthesis, which is the ultimate source of all food.
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. The earth receives huge amounts of energy every day from the sun, but the problem has been harnessing this energy so that it is available at the appropriate time and in the appropriate form. For example, solar energy is received only during the daylight hours, but more heat and electricity for lighting are needed at night. Technological advances in photovoltaic cells and in the cost of the their production have made solar energy a more financially competitive source of energy.

Wind energy, which has long been used as a source of mechanical energy for milling and pumping, can also be used to produced electricity. Modern propellerlike wind turbines, often joined together in wind farms, can produce 1.5 MW or more of electricity and can serve as a significant source of electric energy in plains and coastal areas (including offshore locations). Wind turbines have been most extensively used in Europe, but also have been used in many areas in the United States.

Some scientists have suggested using the earth's internal heat as a source of energy. Geothermal energy is released naturally in geysers and volcanoes. In California, some of the state's electricity is generated by the geothermal plant complex known as the Geysers, which has been in production since 1960, and in Iceland, which is geologically very active, roughly 90% of the homes are heated by geothermal energy. Still another possible energy source is tidal energy. A few systems have been set up to harness the energy released in the twice-daily ebb and flow of the ocean's tides, but they have not been widely used, because they cannot operate turbines continuously and because they must be built specifically for each site.

Another direction of research and development is in the search for alternatives to gasoline. Possibilities include methanol, which can be produced from wood, coal, or natural gas; ethanol, an alcohol produced from grain, sugarcane, and other agriculture plants and currently used in some types of U.S. motor fuel (e.g., gasoholgasohol,
a gasoline extender made from a mixture of gasoline (90%) and ethanol (10%; often obtained by fermenting agricultural crops or crop wastes) or gasoline (97%) and methanol, or wood alcohol (3%).
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 and E85, a mixture of 85% ethanol and 15% gasoline); and compressed natural gas, which is much less polluting than gasoline, but all of these contribute to a greater or lesser degree to air pollution and global warming. Hybrid vehicles, which use electric power from batteries in addition to an internal combustion engine, are less polluting, but rely in part on the engine to recharge the batteries. Fully electric vehicles, which are increasingly common, are significantly less polluting if the energy used to charge them is primarily derived from hydroelectric, solar, or wind sources.

Bibliography

See G. R. Harrison, The Conquest of Energy (1968); F. Barnaby, Man and the Atom: The Uses of Nuclear Energy (1971); W. G. Steltz and A. M. Donaldson, Aero-Thermodynamics of Steam Turbines (1981); T. N. Veziroglu, ed., Alternative Sources of Energy (1983 and 1985) and Renewable Energy Sources (Vol. 4, 1984); G. L. Johnson, Wind Energy Systems (1985).

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