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energy, 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 measurement being used. When a force 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 gravitation) 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 bond).
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 temperature 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 motion). 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 laws). 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. Combustion of fuels results in the conversion of chemical energy into heat and light. In the electric storage battery chemical energy is converted to electrical energy and conversely. In the photosynthesis 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, electric). The force of a nuclear explosion results from the partial conversion of matter to energy (see nuclear energy).
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
energySymbol: 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.
fuel cell 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.
G. IA. MIAKISHEV