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in physics, the ability or capacity to do work or to produce change. Forms of energy include heatheat,
nonmechanical energy in transit, associated with differences in temperature between a system and its surroundings or between parts of the same system. Measures of Heat
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, lightlight,
visible electromagnetic radiation. Of the entire electromagnetic spectrum, the human eye is sensitive to only a tiny part, the part that is called light. The wavelengths of visible light range from about 350 or 400 nm to about 750 or 800 nm.
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, soundsound,
any disturbance that travels through an elastic medium such as air, ground, or water to be heard by the human ear. When a body vibrates, or moves back and forth (see vibration), the oscillation causes a periodic disturbance of the surrounding air or other medium that
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, electricityelectricity,
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.
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, and chemical energy. Energy and work are measured in the same units—foot-pounds, joules, ergs, or some other, depending on the system of measurement being used. When a forceforce,
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.
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 acts on a body, the work performed (and the energy expended) is the product of the force and the distance over which it is exerted.

Potential and Kinetic Energy

Potential energy is the capacity for doing work that a body possesses because of its position or condition. For example, a stone resting on the edge of a cliff has potential energy due to its position in the earth's gravitational field. If it falls, the force of gravity (which is equal to the stone's weight; see gravitationgravitation,
the attractive force existing between any two particles of matter. The Law of Universal Gravitation

Since the gravitational force is experienced by all matter in the universe, from the largest galaxies down to the smallest particles, it is often called
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) will act on it until it strikes the ground; the stone's potential energy is equal to its weight times the distance it can fall. A charge in an electric field also has potential energy because of its position; a stretched spring has potential energy because of its condition. Chemical energy is a special kind of potential energy; it is the form of energy involved in chemical reactions. The chemical energy of a substance is due to the condition of the atoms of which it is made; it resides in the chemical bonds that join the atoms in compound substances (see chemical bondchemical 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 an
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Kinetic energy is energy a body possesses because it is in motion. The kinetic energy of a body with mass m moving at a velocity v is one half the product of the mass of the body and the square of its velocity, i.e., KE = 1-2mv2. Even when a body appears to be at rest, its atoms and molecules are in constant motion and thus have kinetic energy. The average kinetic energy of the atoms or molecules is measured by the temperaturetemperature,
measure of the relative warmth or coolness of an object. Temperature is measured by means of a thermometer or other instrument having a scale calibrated in units called degrees. The size of a degree depends on the particular temperature scale being used.
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 of the body.

The difference between kinetic energy and potential energy, and the conversion of one to the other, is demonstrated by the falling of a rock from a cliff, when its energy of position is changed to energy of motion. Another example is provided in the movements of a simple pendulum (see harmonic motionharmonic motion,
regular vibration in which the acceleration of the vibrating object is directly proportional to the displacement of the object from its equilibrium position but oppositely directed.
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). As the suspended body moves upward in its swing, its kinetic energy is continuously being changed into potential energy; the higher it goes the greater becomes the energy that it owes to its position. At the top of the swing the change from kinetic to potential energy is complete, and in the course of the downward motion that follows the potential energy is in turn converted to kinetic energy.

Conversion and Conservation of Energy

It is common for energy to be converted from one form to another; however, the law of conservation of energy, a fundamental law of physics, states that although energy can be changed in form it can be neither created nor destroyed (see conservation lawsconservation laws,
in physics, basic laws that together determine which processes can or cannot occur in nature; each law maintains that the total value of the quantity governed by that law, e.g., mass or energy, remains unchanged during physical processes.
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). The theory of relativity shows, however, that mass and energy are equivalent and thus that one can be converted into the other. As a result, the law of conservation of energy includes both mass and energy.

Many transformations of energy are of practical importance. Combustioncombustion,
rapid chemical reaction of two or more substances with a characteristic liberation of heat and light; it is commonly called burning. The burning of a fuel (e.g., wood, coal, oil, or natural gas) in air is a familiar example of combustion.
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 of fuels results in the conversion of chemical energy into heat and light. In the electric storage batterybattery, 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.
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 chemical energy is converted to electrical energy and conversely. In the photosynthesisphotosynthesis
, process in which green plants, algae, and cyanobacteria utilize the energy of sunlight to manufacture carbohydrates from carbon dioxide and water in the presence of chlorophyll. Some of the plants that lack chlorophyll, e.g.
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 of starch, green plants convert light energy from the sun into chemical energy. Hydroelectric facilities convert the kinetic energy of falling water into electrical energy, which can be conveniently carried by wires to its place of use (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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). The force of a nuclear explosion results from the partial conversion of matter to energy (see 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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The ability of one system to do work on another system. There are many kinds of energy: chemical energy from fossil fuels, electrical energy distributed by a utility company, radiant energy from the Sun, and nuclear energy from a reactor. The units of energy include ergs, joules, foot-pounds, and foot-poundals. Work and heat have the same units as energy, but are entirely different physical concepts. See Heat, Work

Any particle or system of particles subject to conservative forces has two kinds of energy, potential energy and kinetic energy. Potential energy is the energy due to position or configuration, and kinetic energy is the energy due to motion.

Energy is conserved for all isolated mechanical systems. This is because if a system A is isolated, there is no other system B that it can give any energy to, and its total energy must remain constant. This system A can convert kinetic energy to potential energy, and it can convert one form of potential energy to another, but the total energy must remain the same. The meaning of conserved total energy is that the system has the same value of total energy at all times. See Conservation of energy

In 1905 A. Einstein showed that at high velocities near the speed of light important modifications must be made in physical concepts. One particularly radical idea which he advanced was that space and time are not independent, but rather are two aspects of the same object, a space-time manifold. This necessitated a reexamination of the concept of energy and led to the conclusion that the inertia, or mass m, depends upon its energy through the mass-energy relation shown below, where

c is the speed of light in vacuum. Furthermore, energy and momentum conservation become joined in a single four-momentum conservation law in special relativity. See Internal energy, Relativity


Symbol: E . A measure of the capacity of a body or system for doing work, i.e. for changing the state of another body or system. The SI unit of energy is the joule; the erg (10–7 joule) is also used. There are various forms of energy including mechanical, electrical, nuclear, and radiant energy, all of which are interconvertible in the presence of matter. Mass is also regarded as a form of energy. In any closed system, the total energy, including mass, is always constant. See also conservation of mass-energy; kinetic energy; potential energy.


A resource such as oil, gas or coal from which usable energy can be produced; there are many alternative forms of energy that can be used to produce power.

blue energy

Called osmotic energy, it is generated from a chemical reaction between freshwater and seawater. It can either be installed near a saltwater resource or operated independently using stored water.

fuel cell energy

A kind of battery that produces electricity from the reaction between an externally supplied fuel and an oxidant, in the presence of an electrolyte. Fuel cells generate but do not store energy, and they can provide nonstop, continuous operation as long as the required energy flow is maintained. For home use they provide clean, renewable energy and do not need distribution lines. Small-size home fuel cell units produce heat as a by-product, which can then be used as a cogeneration system for domestic hot water and space heating. Fuel cells can be used in conjunction with power grid systems or used independently as an off-the-grid system in remote areas.

geothermal energy

Produced by using the heat below the earth’s surface. Geothermal heat pumps are necessary for residential use; for commercial use, power plants are built on the surface to convert geothermal energy into electricity.

hybrid systems

A method that uses two or more distinct power sources to run a device, such as an on-board rechargeable energy storage system with a fueled power source internal combustion engine or fuel cell, air and internal combustion engines, and photovoltaic modules and wind turbines with electric power.


Generated from the force of moving water, but a renewable energy resource that usually requires large installations, such as dammed water, to drive a water turbine and generator. There are small, mobile hydrogenerators available for individual home use.

solar photovoltaic

A nonmechanical device that converts sunlight into electricity. Consists of photovoltaic cells, mounting hardware, electrical connections, power-conditioning equipment, and an energy storage device. Individual cells can be grouped into modules to form larger collectors, which can in turn be further grouped into photovoltaic arrays, necessary for industrial-level electricity production. The number and size of the modules can vary depending on the availability and intensity of the sunlight, the geographical location of the modules, and the user’s needs.

solar thermal

Uses sunlight to create heat, but needs water to operate. Once solar thermal devices receive sunlight, they concentrate the light and generate heat. The generated heat warms the water, which can either be circulated and used directly, or can be used to drive a turbine that generates electricity. There are three types of solar thermal systems: parabolic troughs, parabolic dishes, and power towers.

wind energy

Wind energy turns a windmill’s blades on a rotor that is connected to a main shaft. The main shaft spins a generator, producing energy. The amount of energy generated depends on the speed and direction of the wind. One drawback is proximity of the wind generation facilities to the distribution centers and/or to the homes served, which affects the quality and cost of the energy.



a general quantitative measure of motion and interaction of all forms of matter. Energy in nature is neither created nor destroyed; it is only converted from one form to another. The concept of energy unifies all natural processes.

Different forms of energy are differentiated in correspondence with the various forms of the motion of matter, for example, mechanical, electromagnetic, and nuclear energy. This subdivision is somewhat arbitrary. Thus, chemical energy comprises the kinetic energy of the motion of electrons and the electric energy of the interaction of electrons with one another and with atomic nuclei. Internal energy is equal to the sum of the kinetic energy of the random motion of molecules relative to the center of mass of bodies and the potential energies of the interaction of molecules with one another. The energy of a system is uniquely determined by the parameters that characterize the state of the system. In the case of a continuous medium or field, additional concepts are introduced: energy density (energy per unit volume) and energy flux density (the product of the energy density and the rate of its displacement).

According to the theory of relativity, the energy E of a body is related to its mass m by the expression E = mc2, where c is the velocity of light in a vacuum. Any body has energy. If m0 is the mass of a body at rest, then its rest energy is E0 = m0c2; this energy may be converted to other forms of energy in particle transformations (for example, decays and nuclear reactions).

According to classical physics, the energy of any system changes continuously and can take on any values. According to quantum theory, the energy of microparticles moving in a bounded region of space—for example, electrons in atoms— takes on a discrete series of values. Atoms emit electromagnetic energy in the form of discrete batches, called light quanta, or photons (seePHOTON and QUANTUM MECHANICS).

Energy is measured in the same units as work: in ergs in the cgs system and in joules in the International System of Units (SI). In atomic and nuclear physics and the physics of elementary particles, a subsidiary unit, the electron volt, is ordinarily used.



The capacity for doing work.


The capacity to do work; the amount of work that a system is capable of doing.


a. the capacity of a body or system to do work
b. a measure of this capacity, expressed as the work that it does in changing to some specified reference state. It is measured in joules (SI units).
References in periodicals archive ?
Caption: Figure 1: The energy dependence of baryon chemical potential (a) and chemical freeze-out temperature (b) in our model.
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The final group of papers focuses on external relations of the EU, discussing issues of economic integration, Euro-Mediterranean relations, energy dependence and co-operation with the former countries of the Soviet Union, and transatlantic relations.
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In a speech on fighting the war on terror at UCLA in February, Kerry even got to the point about our "corporate and energy dependence" on the Saudis: "If I am President, we will embark on a historic effort to create alternative fuels and the vehicles of the future--to make this country energy independent of Mideast oil within ten years.
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