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EnergyIn general, the concept of energy refers to "the potential for causing changes." The word is used in several different contexts. The scientific use has a precise, well-defined meaning, whilst the many non-scientific uses often do not.
In physics, energy is the ability to do work and has many different forms . No matter what its form, physical energy has the same units as work; a force applied through a distance. The SI unit of energy, the joule, equals one newton applied through one meter, for example.
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Encyclopedia
In general, the concept of energy refers to "the potential for causing changes." The word is used in several different contexts. The scientific use has a precise, well-defined meaning, whilst the many non-scientific uses often do not. In physics, energy is the ability to do work and has many different forms . No matter what its form, physical energy has the same units as work; a force applied through a distance. The SI unit of energy, the joule, equals one newton applied through one meter, for example. EtymologyThe etymology of the term is from Greek e????e?a, e?- means "in" and ????? means "work"; the -?a suffix forms an abstract noun. The compound e?-e??e?a in Epic Greek meant "divine action" or "magical operation"; it is later used by Aristotle in a meaning of "activity, operation" or "vigour", and by Diodorus Siculus for "force of an engine." Historical perspectiveThe concept of energy, in the distant past, was used to explain easily observable phenomena, such as the effects observed on the properties of objects or any other changes. It was generally construed that all changes can in fact be explained through some sort of energy. Soon the idea, that energy could be stored in objects took its roots in scientific thought and the concept of energy came to embrace the idea of the potential for change as well as change itself. Such effects come in many different forms. While in spiritualism they were reflected in changes in a person, in physical sciences it is reflected in different forms of energy itself. For example, electrical energy stored in a battery, the chemical energy stored in a piece of food, the thermal energy of a water heater, or the kinetic energy of a moving train. In 1807, Thomas Young was the first to use the term "energy" instead of vis viva to refer to the product of the mass of an object and its velocity squared. Gustave-Gaspard Coriolis described "kinetic energy" in 1829 in its modern sense, and in 1853, William Rankine coined the term "potential energy." The development of steam engines required engineers to develop concepts and formulas that would allow them to describe the mechanical and thermal efficiencies of their systems. Engineers such as Sadi Carnot and James Prescott Joule, mathematicians such as Émile Claperyon and Hermann von Helmholtz , and amateurs such as Julius Robert von Mayer all contributed to the notions that the ability to perform certain tasks, called work, was somehow related to the amount of energy in the system. The nature of energy was elusive, however, and it was argued for some years whether energy was a substance or merely a physical quantity, such as momentum. William Thomson amalgamated all of these laws into his laws of thermodynamics, which aided in the rapid development of energetic descriptions of chemical processes by Rudolf Clausius, Josiah Willard Gibbs, Walther Nernst. In addition, this allowed Ludwig Boltzmann to describe entropy in mathematical terms, and to discuss, along with Jožef Stefan, the laws of radiant energy. During a 1961 lecture for undergraduate students at the California Institute of Technology, Richard Feynman, a celebrated physics teacher and a Nobel Laureate, had said "There is a fact, or if you wish, a law, governing natural phenomena that are known to date. There is no known exception to this law -- it is exact so far we know. The law is called conservation of energy [it states that there is a certain quantity, which we call energy that does not change in manifold changes which nature undergoes]. That is a most abstract idea, because it is a mathematical principle; it says that there is a numerical quantity, which does not change when something happens. It is not a description of a mechanism, or anything concrete; it is just a strange fact that we can calculate some number, and when we finish watching nature go through her tricks and calculate the number again, it is the same..." This lecture was later published in the Volume 1 of the The Feynman Lectures on Physics. Energy in Natural SciencesThe concept of energy change from one form to another, as a "driver" for natural processes, is useful in explaining many phenomena. In particular, since energy cannot be created or destroyed, the driver of energetic processes is not creation of energy per se, but rather the transformation of energy from one type to another. The direction of this transformation favors an increase in entropy. In practice, this means that in natural processes energy is transformed from more concentrated forms, to less concentrated and more randomly distributed forms, for example heat. The exact context of such changes and transformations varies from one natural science to another. Some examples include: Physics: In physics the transformation that constitute the context is the change in position or movement of an object which is brought about through the action of a force. Thus in the context of physics energy is said to be the ability to do work, the strict mathematical definition of energy in physics is always done via the amount of work itself . Because forces are usually classified by kind , so are the specific forms of work they produce . For example, a gravitational potential energy is defined as the amount of work to elevate a mass against a gravitational force; electrostatic energy is defined as the work done to rearrange electric charges against electric force, kinetic energy is defined as the amount of work to accelerate a body to a given velocity, etc. Work is, simplistically, a force multiplied by a distance . Units of energy are thus exactly the same as units of work . Because work is frame dependent , energy also becomes frame dependent. For example, a speeding bullet has kinetic energy in the reference frame of a non-moving observer, but it has zero kinetic energy in its proper reference frame -- because it takes zero work to accelerate a bullet from zero speed to zero speed. Of course, the selection of a reference state is completely arbitrary - and usually is dictated to maximally simplify the problem to be dealt with. However, when the total energy of a system cannot be decreased by simple choice of reference frame, then the energy remaining in the system is associated with an invariant mass of the system. In this special frame, called the center-of-momentum frame or center-of-mass frame, total energy of the system E and its invariant mass m are related by Einstein's famous equation E=mc². Chemistry: Because atoms and molecules have electrically charged particles, in them, electric forces are at work during the rearrangement of atoms . The energy associated with this movement of charge is what we call "chemical energy". A chemical reaction invariably absorbs or releases energy, either heat or light. A chemical transformation is possible only if so-called free energy decreases. The concept of free energy is a synthesis of energy and entropy. Free energy is a useful concept in chemistry, because energy considerations alone are not sufficient to decide the possibility of a chemical reaction. According to the second law of thermodynamics, the entropy of the universe must increase in all spontaneous processes , and energy is transformed from one form to another so long as the second law is not violated. For example, a gas may expand and thus allow some of its heat to do work, but this is only possible because the net entropy of the universe increases due to the gas expansion, more than it decreases due to the disappearance of heat. Expansion of gas, radiation, and heat into empty space are still important processes which allow or cause the transformation of energy, in the modern universe. The speed of a permitted spontaneous chemical reaction is determined by yet another concept: activation energy. It refers to the minimum energy E which reactant molecules must have, in order to be able to produce product molecules. At a given temperature the fraction of molecules with this energy is usually proportional to the Boltzmann's population factor of exp. Biology: Energy transformation, is essential for the sustenance of life. Living organisms survive because of exchange of energy within and without; with the exchange always acting in a direction to increase the entropy of the universe, as a whole . Nearly all transformations of energy in biology are due to the chemical synthesis and decompositions ultimately originating in the energy absorbed from photons in sunlight . In a living organism chemical bonds are constantly broken and made to make the exchange and transformation of energy possible. These chemical bonds are most often bonds in carbohydrates, including sugars. Other chemical bonds include bonds in ATP and acetate, which in turn is derived from fats and oils. These molecules, along with oxygen, are common stores of concentrated energy for biological processes. Energy diffusion from more to less concentrated forms is the driving force of all biological processes as all biochemical processes are a subset of chemical processes. Molecular biology and biochemistry are essentially the making and breaking of certain chemical bonds in the molecules found in biological organisms. Biologists also study the way energy flows from one organism to another as food or nutrient as part of the science of ecology. The total energy captured by photosynthesis in green plants from the solar radiation is 91 x 10 26 joules of energy per year .
Meteorology The Earth's weather patterns, including energy-releasing processes like lightning, hurricanes, snow avalanches, and floods, are all powered ultimately by the energy of sunlight striking the Earth. Although this amount varies a little each year, as a result of solar flares, prominences and the sunspot cycle, it has been estimated that the average total solar incoming radiation is 342 watts per square meter incident to the summit of the atmosphere, at the equator at midday, a figure known as the Solar Constant. Some 34% of this is immediately reflected by the planetary albedo, as a result of clouds, snowfields, and even reflected light from water, rock or vegetation. As more energy is received in the tropics than is re-radiated, whilst more energy is radiated at the poles than is received, climatic homeostasis is only maintained by a transfer of energy from the tropics to the poles. This transfer of energy is what drives the winds and the ocean currents. Like biological processes, weather processes involve turning energy from a concentrated form such as sunlight , ultimately into a less concentrated form, such as far infrared radiation . However, energy may be temporarily locally stored during this process, and the sudden release of such stored sources is responsible for the most dramatic processes mentioned above.
Geology: volcanos, earthquakes, landslides, and tsunamis are all results of similar sudden releases of stored energy, in the crust of earth. The source of this energy is energy transformations in the Earth as a whole. Recent studies suggest that the Earth transforms about 6.18 x 10-12 watts per kilogram. Given the Earth's mass of about 5.97 x 1024 kilograms, this means that the rate of energy transformations inside the Earth is about 37 x 1012 watts per year. From the study of neutrinos radiated from the Earth , scientists have recently estimated that about 24 terawatts of this energy comes from radioactive decay , with the remaining 12.9 terawatts coming from energies produced by the continuing gravitational sorting of the core and mantle of the earth, energies left over from the formation of the Earth, about 4.57 billion years ago.
The magnitude of both these forms of energy decline over time, and based on half-life alone, it has been estimated that the current radioactive energy of the planet represents less than 1% of that which was available at the time the planet formed. As a result, geological forces of continental accretion, subduction and sea floor spreading, which release up to 90% of this available energy, were more active in the Archaean and Proterozoic periods than they are today. The remaining 10% of geological tectonic energy comes through hotspots produced by mantle plumes, resulting in shield volcanoes like Hawaii, geyser activity like Yellowstone or flood basalts like Iceland.
Tectonic process, driven by heat from the Earth's interior, metamorphose weathered rocks, and during orogeny periods, lift them up into mountain ranges. The potential energy represented by the mountain range's weight and height thus represents heat from the core of the Earth which has been partly transformed into gravitational potential energy. This potential energy may be suddenly released in landslides or tsunamis. Similarly, the energy release which drives an earthquake represents stresses in rocks that are mechanical potential energy which has been similarly stored from tectonic processes.
The remaining energy which is responsible for the geological processes of erosion and deposition is a result of the interaction of solar energy and gravity. An estimated 23% of the total insolation is used to drive the water cycle. When water vapour condenses to fall as rain, it dissolves small amounts of carbon dioxide, making a weak acid. This acid acting upon the metallic silicates that form most rocks produces chemical weathering, removing the metals, and leading to the production of rocks and sand, carried by wind and water downslope through gravity to be deposited at the edge of continents in the sea. Physical weathering of rocks is produced by the expansion of ice crystals, left by water in the joint planes of rocks. A geologic cycle is continued when these eroded rocks are later uplifted into mountains.
Cosmology all stellar phenomena are driven by various forms of energy release and diffusion. The source of this energy is ultimately derived either from gravitational collapse of matter which was distributed in the Big Bang, or else from fusion of lighter elements created in the Big Bang. These light elements were spread too fast and too thinly in the Big Bang process to be able to form the most stable and low-energy kinds of atoms, which have medium-sized atomic nuclei, like iron and nickel. The later formation of such atoms powers the energy-releasing reactions in stars. However, cosmologists are still unable to explain all the cosmological phenomena purely on the basis of known conventional forms of energy and often invoke another form called dark energy to account for certain cosmological observations.
Forms and relations between different forms
In the context of natural sciences, energy has different forms: thermal, chemical, electrical, radiant, nuclear etc. They can all be, in fact, reduced to kinetic energy or potential energy.
Thus energy can be divided into two broad categories.
Kinetic
- According to kinetic theory, the microscopic kinetic energies, of the particles in a gas comprise the internal energy of a system. By the equipartition theorem each degree of freedom of a particle has an associated energy, , such that the energy per particle is proportional to temperature. For a monatomic gas having N particles each with three degrees of freedom, the internal energy is:
- where k is the Boltzmann constant
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and T is absolute temperature. Whereas all internal energy is kinetic in an ideal gas, in solids half of it is stored in electromagnetic potential energy between particles. Thermal internal energy is present in all macroscopic objects in the universe. Although some heat transfer is mediated by the kinetic energy of a system's constituent particles, this kinetic energy exhibits Brownian motion, a highly disorganized state.
- Radiation energy, also known as light energy, is the energy of electromagnetic radiation. It is carried in electric and magnetic fields. It is quantized, and the spacing between allowed levels is called a photon. A quantum of energy of the electromagnetic field is equal to: where f is the frequency of the photon and h is the Planck's constant. Photons move at the speed of light and carry energy and momentum. Because energy or momentum can code information, photons can be used to transfer information .
PotentialPotential energy is stored unreleased energy , or else required energy . This sort of energy may be positive or negative because it can represent work done on a system or work done by a system as a force result. For instance, using the power of a compressed spring to launch a dart uses the elastic potential energy stored within the spring. When the spring is released, this energy is converted into kinetic energy, and work is performed. There is a form of potential energy for each of the four basic forces in nature: gravity, electromagnetic, and strong and weak nuclear forces.
- Gravitational potential energy is the work of gravitational force during rearrangement of mutual positions of interacting masses - say, when masses are moved apart , or closer together . If the masses of the objects are considered point masses, this work is equal to: where m and M are the two masses in question, r is the distance between them, and G is the Gravitational constant. In case of small displacement h << r the above formula results in widely used E = mgh approximation.
- Electric potential energy is the work of electric forces during rearrangement of positions of charges, and also includes the common chemical potential energies . The energy released in lightning or from burning a litre of fuel oil, are some common kinds of electromagnetic potential energy. Electromagnetic potential energy is equal to: where q and Q are the electric charges on the objects in question, r is the distance between them, and e0 is the Electric constant of a vacuum.
- Energy can also be stored in a magnetic field, and is related to the relative motion of electric charges, for example, Superconducting magnetic energy storage can be called magnetic potential energy.This kind of potential energy is related to electric potential energy and is considered a form of it, since both types of potential are mediated by the electromagnetic field. Magnetic potential energy is most familar as the type of energy storage which allows transfer of power within an electrical transformer.
- Potential thermal energy is the part of thermal energy stored in "deformation" of atomic bonds during thermal motion of atoms . This energy is significant portion of thermal energy for strongly-bonded systems , but much less in gasses.
- Potential chemical energy is the energy which may potentially be liberated, when the bonds of chemical structures are rearranged . The mixture of a fuel and oxygen is an example, which stores chemical energy as compared to the products of combustion. Other common examples are: a rechargable battery or a food item.
- Potential elastic energy is the energy stored in the elastic nature of objects. In the ideal case, of Hooke's Law, the energy is equal to: where k is the spring constant, dependent on the individual spring, and x is the deformation of the object.
- Nuclear potential energy, along with Electric potential energy, provides the energy released from nuclear fission
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and nuclear fusion processes. In both cases strong nuclear forces bind nuclear particles more strongly and closely, after the reaction has completed. Weak nuclear forces provide the potential energy for certain kinds of radioactive decay, such as beta decay. Ultimately, the energy released in nuclear processes is so large that the change in mass is appreciable as being several parts per thousand in mass: where ?m is the amount of rest mass released into the surroundings as active energy , and c is the speed of light in a vacuum. Nuclear particles like protons and neutrons are not destroyed in fission and fusion processes, but collections of them have less mass than if they were individually free, and this mass difference is liberated as heat and radiation in nuclear reactions. The energy from the Sun also called the solar energy is an example of this form of energy.
Conservation of energy
Energy is subject to the law of conservation of energy . Thus, energy cannot be made or destroyed, it can only be converted from one form to another, that is, transformed.
In practice, during any energy transformation in system, some energy is converted into incoherent microscopic motion of parts of the system , and the entropy of the system increases. Due to mathematical impossibility to invert this process , the efficiency of energy conversion in a macroscopic system is always less than 100%.
The first law of thermodynamics states that the total inflow of energy into a system must equal the total outflow of energy from the system, plus the change in the energy contained within the system. In other words, energy is neither created nor destroyed, only converted between forms. This law is used in all branches of physics, but frequently violated for short enough periods of time during which energy can not be mathematically defined yet . Noether's theorem relates the conservation of energy to the time invariance of physical laws.
The law of conservation of energy, a fundamental principle of physics, follows from the translational symmetry of time, a property of most phenomena below the cosmic scale that makes them independent of their locations on the time coordinate. Put differently, yesterday, today, and tomorrow are physically indistinguishable. The fact that energy can not be defined for arbitrary short periods of time in quantum mechanics follows from the definition of energy operator which results mathematically in the mutual uncertainty of time and energy known as the uncertainty principle:
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Despite being seemingly insignificant, this principle has profound impact on processes in our Universe. It results in the existence of virtual particles which carry momentum, exchange by which with real particles is responsible for creation of all known fundamental forces . Virtual photons are also responsible for spontaneous radiative decay of exited atomic and nuclear states, for the Casimir force, for Van der Vaals bond forces and some other observable phenomena.
Heat can be placed in a special class of energy, which has been "degraded" by giving it access to all parts of a system. While most heat consists of kinetic and potential energies associated with atomic motion, or with certain kinds of radiant energy , the energy associated with heat is in a "diffused" and non-direction form, in which the energy has spread out to occupy all of the possible states of a system which can store it. This happens at a certain equilibrium temperature, where "temperature" is a measure of energy concentration in a system. When all parts of a system reach the same temperature, the energy of heat cannot be directed into particular other kinds of energy , unless the system is "enlarged" in some fashion which allows the heat to diffuse into a particular direction, in which it is even less concentrated . Thus we see that heat is energy which has already reached a sort of minimal concentration or diffusion in the system it is in, and is useless for doing any kind of work unless the system is opened in such a way as to let the heat have access to a larger system.
Energy is always conserved in closed systems, if heat is taken into account. But the amount of useful energy is usually not conserved, since once energy is converted to heat, it loses some of its ability to do work, and therefore its ability to be convertible to other kinds of energy.
Conversion of energy into different forms
As a consequence of energy conservation law, one form of energy can often be readily transformed into another - for instance, a battery converts chemical energy into electrical energy. Similarly, gravitational potential energy is converted into the kinetic energy of moving water in a dam, which in turn is transformed into electric energy by a generator. Similarly in the case of a chemical explosion energy stored in chemical bonds that may be termed as chemical potential energy is converted to kinetic energy and heat in a very short time. Yet another example is that of a pendulum. At its highest points the kinetic energy is zero and the gravitational potential energy is at its maximum. At its lowest point the kinetic energy is at its maximum and is equal to the decrease of potential energy. If one unrealistically assumes that there is no friction, the energy will be conserved and the pendulum will continue swinging forever.
In practice, available energy is rarely perfectly conserved when a system changes state; in large systems , some energy will be converted into 'useless' energies, such as those associated with heat. This fraction, however, may be reduced arbitrarily toward zero. In large systems with little friction , motion may continue nearly indefinitely because useable energy is traded between usable kinetic and potential energies with so little conversion into heat. In small systems such the atom or in a vibrating molecule, where there may be no friction associated with the motion of electrons or the mututal vibration of nuclei, the possibility of indefinite motion, with perpetual conversion of kinetic and potential energy, is the case.
While energy in forms other than heat may be freely converted to other forms with efficiency approaching or even equaling 100%, once energy has been converted into heat, there are severe limitations in re-converting this energy into other useful forms, except chemical energy through biological organisms, and efficiency never reaches 100%. If this were not so, the creation of certain kinds of perpetual motion machines would be possible.
Work
Because energy is defined in terms of work, a definition of work is crucial to the understanding of energy.
Work is a defined as a line integral of force F over distance s:
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The equation above says that the work is equal to the integral of the dot product of the force on a body and the infinitesimal of the body's translation .
Depending on the kind of force F involved, work of this force results in a change of the corresponding kind of energy .
For example, for the gravitational force F=mg acting on mass m, when the mass is elevated from some height h1 to the height h2, the work done against gravitational potential energy is therefore:
- W = mg · -= mgh2 - mgh1
and this work changes "gravitational potential" by U = mg?h.
Similarly, work by the force F = ma to accelerate a bullet from zero velocity to the velocity v is
- = mv2/2
and this work changes "kinetic energy" by K = mv2/2.
Other forms of energy are similarly defined via work.
Energy in Society
In the context of society the word energy is synonymous to energy resources, it most often refers to substances like fuels, petroleum products and electric power installations. This difference vis a vis energy in natural sciences can lead to some confusion, because energy resources are not conserved in nature in the same way as the energy is conserved in the context of say physics. People often talk about energy crisis and the need to conserve energy, something contrary to the principle of energy conservation in natural sciences.
Economics
Production and consumption of energy resources is very important to the global economy. All economic activity requires energy resources, whether to manufacture goods, provide transportation, run computers and other machines, or to grow food to feed workers, or even to harvest new fuels. Thus the way in which a human society uses its existing energy resources, develops means of their production or acquisition is a defining characteristic of its economy. The progression from animal power to steam power, then the internal combustion engine and electricity, are key elements in the development of modern civilization. The cost of energy resources depends on its demand and production at any particular time. Scarcity of cheap fuels is a key concern in future energy development.
Some attempts have been made to define "embodied energy" - the sum total of energy expended to deliver a good or service as it travels through the economy.
Environment
Consumption of energy resources, is apparently harmless. However, producing that energy requires resources and contributes to air and water pollution. Many electric power plants burn coal oil or natural gas in order to generate electricity for energy needs. While burning these fossil fuels produces a readily available and instantaneous supply of electricity, it also generates air pollutants including carbon dioxide , sulfur dioxide and trioxide and nitrogen oxides . Carbon dioxide is an important greenhouse gas which is thought to be responsible for some fraction of the rapid increase in global warming seen especially temperature records in the 19th century, as compared with tens of thousands of years worth of temperature records which can be read from ice cores taken in artic regions.
Burning fossil fuels for electricity generation also releases trace metals such as beryllium, cadmium, chromium, copper, manganese, mercury, nickel, and silver into the environment, which also act as pollutants. Certain renewable energy technologies do not pollute the environment in the same ways, and therefore can help contribute to a cleaner energy future for the world. Renewable energy technologies available for electricity production include biofuels, solar power, tidal power, wind turbines, hydroelectric power etc. Pollution associated with these technologies lies mainly in the unavoidable pollution associated with the manufacture and retirement of the materials associated with the machinery which produces the energy.
Management
Since the cost of energy has become a significant factor in the performance of economy of societies, management of energy resources has become very crucial.Energy management involves utilizing the available energy resources more effectively that is with minimum incremental costs. Many times it is possible to save expenditure on energy without incorporating fresh technology by simple management techniques. Most often energy management is the practice of using energy more efficiently by eliminating energy wastage or to balance justifiable energy demand with appropriate energy supply. The process couples energy awareness with energy conservation.
Politics
Since energy plays an essential role in industrial societies, the ownership and control of energy resources plays an increasing role in politics at the national level. Governments may seek to influence who owns resources within their borders and may also seek to influence the use of energy by individuals and business in an attempt to tackle environmental issues.
The strategic control of international energy resources has been cited by some as a cause of the Iraq War.
Production
Producing energy to sustain the social needs of all human beings is an essential social activity. Therefore it is not very surprising that a great deal of effort goes into this activity. While most of the effort in this direction is limited towards increasing the production of electricity and oil, newer ways of producing usable energy resources from the available energy resources are being explored. One such effort is to explore means of producing hydrogen from the abundant water. Water can be electrolyzed to produce hydrogen and oxygen, a mixture that is very environment friendly, but existing technologies are not very efficient. Research is underway to explore enzymatic decomposition of biomass and or water.
Similar is the case with many other forms of conventional energy resources. Coal gasification and liquification are recent technologies that are becoming attractive after the realization that oil reserves, the way they are being consumed, may be rather short lived.
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