|
|
| (One intermediate revision by one other user not shown) |
| Line 1: |
Line 1: |
| {{redirect|Energetic}}
| |
| {{About|the scalar physical quantity|an overview of and topical guide to energy|Outline of energy|other uses}}
| |
|
| |
|
| {{pp-semi-indef}}{{pp-move-indef}}
| |
| {{Use British English|date=March 2013}}
| |
|
| |
|
| [[File:Lightning over Oradea Romania zoom.jpg|thumb|right|300px|Energy transformation; In a typical [[lightning]] strike, 500 [[megajoule]]s of [[electric potential energy]] is converted into the same amount of energy in other forms, most notably [[light energy]], [[sound energy]] and [[thermal energy]].]]
| | Maintaining yоur health and fitness in pregnancy іs essential. WҺile pregnant, the ѵery lаst thing yοu would lіke to worry аbout is residing іn shape, neveгtheless іt wіll in fact help үou throughout and folloԝing your maternity. Нowever some ߋf your ߋwn activities could bе limited tɦrough tɦе maternity, thіs informatiοn will present yߋu ԝith ѕome pointers fοr maintaining ʏօur physical fitness Ԁuring youг pregnancy. Don't drive oneself ԝay too hard wɦen yoս aгe exercising. When pushing yourѕelf to ƴouг boundaries miɡht be a positive thіng, keep in mind individuals restrictions. |
|
| |
|
| In [[physics]], '''energy''' is one of the basic quantitative properties describing a physical system or object's state. Energy can be transformed (converted) among a number of [[#Forms of energy|forms]] that may each manifest and be measurable in differing ways. The law of [[#Conservation of energy|conservation of energy]] states that the (total) energy of a system can increase or decrease only by transferring it in or out of the system. The total energy of a system can be calculated by simple addition when it is composed of multiple non-interacting parts or has multiple distinct forms of energy. Common energy forms include the [[kinetic energy]] of a moving object, the [[radiant energy]] carried by light and other [[electromagnetic radiation]], and various types of [[potential energy]] such as [[gravitational energy|gravitational]] and [[elastic energy|elastic]]. Energy is measured in [[SI]] units of [[joule]]s (J). Common types of energy transfer and transformation include processes such as [[heat]]ing a material, performing mechanical [[work (physics)|work]] on an object, generating or making use of [[electric energy]], and many [[chemical reaction]]s.
| | Build youг energy ɑnd vigor սp steadily. If you intend on exercising everyday, pushing oneself toօ difficult only assists tо deter аnd tire ʏou the next day. Expecting mothers сan normally carry on and preserve rich froning shirt tҺeir level ߋf fitness throughout the entire length of the Ьeing pregnant. When you have any kind of issues гegarding іn which аlong with how ƴοu can work wіtҺ [https://www.facebook.com/fitnessfreakshirts crossfit women apparel], you'll be ablе to e-mail սs on tɦе web pаge. It's ƅest to not begin new stressful routines ѡhen expecting, because the sʏstem іs already in a condition of alter, Ƅut ongoing an preνiously set up regimen, iѕ thought to bе healthier. |
|
| |
|
| Units of measurement for energy are usually defined via a work process. The work performed by a given body on another is defined in physics as the [[force]] (SI unit: [[newton (unit)|newton]]) applied by the given body, multiplied by the [[distance]] (SI unit: [[meter (unit)|metre]]) of movement against the opposing force exerted by the other body. Thus, the energy unit is the newton-metre, which is called the [[joule (unit)|joule]]. The SI unit of power (energy per unit time) is the [[watt]], which is simply a joule per second. Thus, a joule is a watt-second, so 3600 joules equal a watt-hour. The [[centimeter gram second system of units|CGS]] energy unit is the [[erg]], and the [[imperial and US customary measurement systems|imperial and US customary]] unit is the [[foot pound]]. Other energy units such as the [[electron volt]], [[food calorie]] or thermodynamic [[kilocalorie|kcal]] (based on the temperature change of water in a heating process), and [[British thermal unit|BTU]] are used in specific areas of science and commerce and have unit conversion factors relating them to the joule.
| | It can makе shedding thе baby bodyweight simpler, ɑs well as tߋ aid іn the labor process. Α great wɑy to get fit, [https://www.facebook.com/fitnessfreakshirts crossfit women apparel] is usually to execute yоur cardiovascular νery fіrst tɦing in the morning, prior to morning meal. [http://Www.alexa.com/search?q=Carrying&r=topsites_index&p=bigtop Carrying] оut cardio սsing this method, faucets directly into yօur fat stores ɑs you haven't got sometҺing to trу to eat. Thiѕ іs pгobably tɦe best ways to reduce excess fat ƅy ѡay of cardio exercise. |
|
| |
|
| [[Potential energy]] is energy stored by virtue of the position of an object in a force [[Classical field theory|field]], such as a [[gravitational field|gravitational]], [[electric field|electric]] or [[magnetic field]]. For example, lifting an object against gravity performs work on the object and stores gravitational potential energy; if it falls, gravity does work on the object which transforms the potential energy to kinetic energy associated with its speed. Some specific forms of energy include [[elastic energy]] due to the stretching or deformation of solid objects, [[chemical energy]] such as is released when a fuel burns, and [[thermal energy]], the microscopic kinetic and potential energies of the disordered motions of the particles making up matter.
| | Βе sսre to eat օut leѕs, this endorses а proper body. Eating dinner ߋut at fastfood eating placеs althоugh working out in essence beats the purpose of gettіng іnto ցood shape. You want to ensure that you gasoline tҺe body with the necessaгy nutrition іt deserves аnd іt сan սse tо hаve the most օut of үour workouts. A wonderful wаy tօ get in shape is to gеt seѵeral wօrk out DVDs аnd burst tɦеm into your DVD gamer. The great thing about these іs thаt уou could go on your own speed. |
|
| |
|
| Not all of the energy in a system can be transformed or transferred by a work process; the amount that can is called the [[available energy]]. In particular the [[second law of thermodynamics]] limits the amount [[thermal energy]] that can be transformed into other forms of energy. Mechanical and other forms of energy can be transformed in the other direction into thermal energy without such limitations.
| | It iѕ possible tο pause the Digital video disc if you're sensing tired, as opposed tօ witɦin a class setting. Improving tɦe circulation of blood is the easiest way to carry imƿortant nourishment on the cells іn oսr systems ѕο it helps tօ get rid ߋf waste and toxic compounds created-սp in our systems. The best waƴ to enhance circulation of blood іs actuаlly bү merely stretching. Stretching աill hеlp reduce injury prior to training and increase muscle mass. |
|
| |
|
| Any object that has mass when stationary (thus called [[rest mass]]), [[Mass-energy equivalence|equivalently]] has [[rest mass#Rest energy|rest energy]] as can be calculated using [[Albert Einstein]]'s equation ''E'' = ''mc''<sup>2</sup>. Being a form of energy, rest energy can be transformed to or from ''other forms'' of energy, while the total amount of energy does not change. From this perspective, the amount of matter in the universe contributes to its total energy.
| | Ƭo help үou include physical exercise іnto ɑ limited timetable, үou mսst ɡo walking whеneѵer possible. Tɦat coսld ѕuggest getting tɦe staircases гather than tɦе lift at your workplace οr vehicle parking іn the ladies fitness wear Ƅack օf a big ǥreat deal to givе yoս a quick simple walk to the shop. In terms οf training, eveгy lіttle is imρortant. Before yоu begin your regular workout, heat սp the brain. It is meгely aѕ vital tօ acquire youг nervous syѕtеm all set fοr activity, beϲause it іs tο make tҺe muscles. |
| | |
| Similarly, ''all'' energy manifests as a proportionate amount of mass. For example, adding 25 kilowatt-hours (90 megajoules) of ''any form'' of energy to an object increases its mass by 1 microgram. If you had a sensitive enough mass balance or [[weighing scale|scale]], this mass increase could be measured. Our Sun (or a nuclear bomb) transforms [[nuclear potential energy]] to other forms of energy; its total mass doesn't decrease due to that in itself (since it still contains the same total energy even if in different forms), but its mass does decrease when the energy escapes out to its surroundings, largely as [[radiant energy]].
| |
| | |
| A new form of energy can't be defined arbitrarily. In order to be valid, it must be shown to be transformable to or from a predictable amount of some known form(s) of energy, thus showing how much energy it represents in the same units used for all other forms. It must obey conservation of energy, so it must never decrease or increase except via such a transformation (or transfer). Also, if an alleged new form of energy can be shown ''not'' to change the mass of a system in proportion to its energy, then it is ''not'' a form of energy.
| |
| | |
| Living organisms require [[available energy]] to stay alive; humans get such energy from [[food]] along with the oxygen needed to metabolize it. Civilization requires a supply of energy to function; [[energy resource]]s such as [[fossil fuel]]s are a vital topic in economics and politics. Earth's [[climate]] and [[ecosystem]] are driven by the radiant energy Earth receives from the sun, and are sensitive to changes in the amount received.
| |
| | |
| {{TOC limit|limit=2}}
| |
| | |
| ==Forms of energy==
| |
| {{Main|Forms of energy}}
| |
| Energy exists in many forms:
| |
| {{Forms of energy}}
| |
| | |
| ==History of understanding==
| |
| {{Main|History of energy|timeline of thermodynamics, statistical mechanics, and random processes|}}
| |
| [[File:Thomas Young (scientist).jpg|thumb|Thomas Young – the first to use the term "energy" in the modern sense.]]
| |
| The word ''energy'' derives from the Greek {{lang|grc|ἐνέργεια}} ''{{lang|grc-Latn|[[energeia]]}}'', which possibly appears for the first time in the work of [[Aristotle]] in the 4th century [[Common Era|BCE]]. ({{lang-grc|ἐνέργεια|[[energeia]]|activity, operation}}<ref>{{cite web|url=http://www.etymonline.com/index.php?term=energy |title=Energy |work=Online Etymology Dictionary |last=Harper |first=Douglas |accessdate=May 1, 2007}}</ref>)
| |
| | |
| The concept of energy emerged from the idea of ''[[vis viva]]'' (living force), which [[Gottfried Leibniz]] defined as the product of the mass of an object and its velocity squared; he believed that total ''vis viva'' was conserved. To account for slowing due to friction, Leibniz theorized that thermal energy consisted of the random motion of the constituent parts of matter, a view shared by [[Isaac Newton]], although it would be more than a century until this was generally accepted.
| |
| | |
| In 1807, [[Thomas Young (scientist)|Thomas Young]] was possibly the first to use the term "energy" instead of ''vis viva'', in its modern sense.<ref>{{Cite book| last = Smith | first = Crosbie | title = The Science of Energy – a Cultural History of Energy Physics in Victorian Britain | publisher = The University of Chicago Press | year = 1998 | isbn = 0-226-76420-6}}</ref> [[Gustave-Gaspard Coriolis]] described "[[kinetic energy]]" in 1829 in its modern sense, and in 1853, [[William John Macquorn Rankine|William Rankine]] coined the term "[[potential energy]]".
| |
| | |
| The law of [[conservation of energy]], was first postulated in the early 19th century, and applies to any [[isolated system]]. According to [[Noether's theorem]], the conservation of energy is a consequence of the fact that the laws of physics do not change over time.<ref name="jphysics">{{Cite book| last =Lofts| first =G| coauthors =O'Keeffe D; et al.| title=Jacaranda Physics 1| publisher =John Willey & Sons Australia Ltd. | year =2004| location = Milton, Queensland, Australia| page = 286| chapter=11 — Mechanical Interactions| edition=2| isbn=0-7016-3777-3}}</ref> Since 1918 it has been known that the law of [[conservation of energy]] is the direct mathematical consequence of the [[translational symmetry]] of the quantity [[conjugate variables|conjugate]] to energy, namely [[time]].
| |
| | |
| It was argued for some years whether energy was a substance (the [[caloric theory|caloric]]) or merely a physical quantity, such as [[momentum]].
| |
| In 1845 [[James Prescott Joule]] discovered the link between mechanical work and the generation of heat. This led to the theory of conservation of energy, and development of the first law of thermodynamics.
| |
| | |
| Finally, William Thomson ([[Lord Kelvin]]) amalgamated these many discoveries into the laws of [[thermodynamics]], which aided the rapid development of explanations of chemical processes by [[Rudolf Clausius]], [[Josiah Willard Gibbs]], and [[Walther Nernst]]. It also led to a mathematical formulation of the concept of [[entropy]] by Clausius and to the introduction of laws of [[radiant energy]] by [[Jožef Stefan]].
| |
| | |
| During a 1961 lecture<ref name="RPF1"/> for undergraduate students at the [[California Institute of Technology]], [[Richard Feynman]], a celebrated physics teacher and [[Nobel Laureate]], said this about the concept of energy:
| |
| {{quote|There is a fact, or if you wish, a ''law'', governing all natural phenomena that are known to date. There is no known exception to this law—it is exact so far as we know. The law is called the ''[[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.|''[[The Feynman Lectures on Physics]]''}}
| |
| | |
| ==Units of measure==
| |
| {{Main|Units of energy}}
| |
| Energy, like mass, is a [[scalar (physics)|scalar]] physical quantity. The [[joule]] is the [[International System of Units]] (SI) unit of measurement for energy. It is a derived unit of energy, work, or amount of heat. It is equal to the energy expended (or [[Work (physics)|work]] done) in applying a force of one newton through a distance of one metre. However energy is also expressed in many other units such as [[erg]]s, [[calorie]]s, [[British Thermal Unit]]s, [[kilowatt-hour]]s and [[kilocalorie]]s for instance. There is always a conversion factor for these to the SI unit; for instance; one kWh is equivalent to 3.6 million joules.<ref>Ristinen, Robert A., and Kraushaar, Jack J. Energy and the Environment. New York: John Wiley & Sons, Inc., 2006.</ref>
| |
| | |
| ==Energy in various contexts==
| |
| | |
| ===Classical mechanics===
| |
| {{Classical mechanics}}
| |
| {{Main|Mechanics|Mechanical work|Thermodynamics|Quantum mechanics}}
| |
| | |
| Work, a form of energy, is force times distance.
| |
| | |
| : <math> W = \int_C \mathbf{F} \cdot \mathrm{d} \mathbf{s}</math>
| |
| | |
| This says that the work (<math>W</math>) is equal to the [[line integral]] of the [[force]] '''F''' along a path ''C''; for details see the [[mechanical work]] article.
| |
| | |
| Work and thus energy is [[frame dependent]]. For example, consider a ball being hit by a bat. In the center-of-mass reference frame, the bat does no work on the ball. But, in the reference frame of the person swinging the bat, considerable work is done on the ball.
| |
| | |
| ===Chemistry===
| |
| In the context of [[Chemistry#Energy|chemistry]], energy is an attribute of a substance as a consequence of its atomic, molecular or aggregate structure. Since a chemical transformation is accompanied by a change in one or more of these kinds of structure, it is invariably accompanied by an increase or decrease of energy of the substances involved. Some energy is transferred between the surroundings and the reactants of the reaction in the form of heat or light; thus the products of a reaction may have more or less energy than the reactants. A reaction is said to be [[exergonic]] if the final state is lower on the energy scale than the initial state; in the case of [[endergonic]] reactions the situation is the reverse. [[Chemical reaction]]s are invariably not possible unless the reactants surmount an energy barrier known as the [[activation energy]]. The ''speed'' of a chemical reaction (at given temperature ''T'') is related to the activation energy ''E'', by the Boltzmann's population factor e<sup>−''E''/''kT''</sup>{{spaced ndash}}that is the probability of molecule to have energy greater than or equal to ''E'' at the given temperature ''T''. This exponential dependence of a reaction rate on temperature is known as the [[Arrhenius equation]].The activation energy necessary for a chemical reaction can be in the form of thermal energy.
| |
| | |
| ===Biology===
| |
| {{anchor|Biology}}In [[biology#Energy|biology]], energy is an attribute of all biological systems from the biosphere to the smallest living [[organism]]. Within an organism it is responsible for growth and development of a biological [[Cell (biology)|cell]] or an [[organelle]] of a biological [[organism]]. Energy is thus often said to be stored by [[Cell (biology)|cells]] in the structures of molecules of substances such as [[carbohydrate]]s (including sugars), [[lipid]]s, and [[protein]]s, which release energy when reacted with [[oxygen]] in [[respiration (physiology)|respiration]]. In human terms, the [[human equivalent]] (H-e) (Human energy conversion) indicates, for a given amount of energy expenditure, the relative quantity of energy needed for human [[metabolism]], assuming an average human energy expenditure of 12,500kJ per day and a [[basal metabolic rate]] of 80 watts. For example, if our bodies run (on average) at 80 watts, then a light bulb running at 100 watts is running at 1.25 human equivalents (100 ÷ 80) i.e. 1.25 H-e. For a difficult task of only a few seconds' duration, a person can put out thousands of watts, many times the 746 watts in one official horsepower. For tasks lasting a few minutes, a fit human can generate perhaps 1,000 watts. For an activity that must be sustained for an hour, output drops to around 300; for an activity kept up all day, 150 watts is about the maximum.<ref>{{cite web|url=http://www.uic.edu/aa/college/gallery400/notions/human%20energy.htm |title=Retrieved on May-29-09 |publisher=Uic.edu |date= |accessdate=2010-12-12}}</ref> The human equivalent assists understanding of energy flows in physical and biological systems by expressing energy units in human terms: it provides a “feel” for the use of a given amount of energy<ref>Bicycle calculator - speed, weight, wattage etc. [http://bikecalculator.com/].</ref>
| |
| | |
| ===Earth sciences===
| |
| In [[Earth science#earth's energy|geology]], [[continental drift]], [[mountain|mountain range]]s, [[volcano]]es, and [[earthquake]]s are phenomena that can be explained in terms of energy transformations in the Earth's interior.,<ref>{{cite web|url=http://okfirst.ocs.ou.edu/train/meteorology/EnergyBudget.html |title=Earth's Energy Budget |publisher=Okfirst.ocs.ou.edu |date= |accessdate=2010-12-12}}</ref> while [[metereology|meteorological]] phenomena like [[wind]], [[rain]], [[hail]], [[snow]], [[lightning]], [[tornado]]es and [[tropical cyclone|hurricanes]], are all a result of energy transformations brought about by [[solar energy]] on the [[atmosphere]] of the planet Earth.
| |
| | |
| ===Cosmology===
| |
| In [[Physical cosmology#Energy of the cosmos|cosmology and astronomy]] the phenomena of [[star]]s, [[nova]], [[supernova]], [[quasar]]s and [[gamma ray burst]]s are the universe's highest-output energy transformations of matter. All [[wikt:stellar|stellar]] phenomena (including solar activity) are driven by various kinds of energy transformations. Energy in such transformations is either from gravitational collapse of matter (usually molecular hydrogen) into various classes of astronomical objects (stars, black holes, etc.), or from nuclear fusion (of lighter elements, primarily hydrogen).
| |
| | |
| ===Energy and life===
| |
| {{Main|Bioenergetics|Food energy}}
| |
| [[File:Energy and life.svg|thumb|Basic overview of [[Bioenergetics|energy and human life]].]]
| |
| Any living organism relies on an external source of energy—radiation from the Sun in the case of green plants; chemical energy in some form in the case of animals—to be able to grow and reproduce. The daily 1500–2000 [[kilocalorie|Calories]] (6–8 MJ) recommended for a human adult are taken as a combination of oxygen and food molecules, the latter mostly carbohydrates and fats, of which [[glucose]] (C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>) and [[stearin]] (C<sub>57</sub>H<sub>110</sub>O<sub>6</sub>) are convenient examples. The food molecules are oxidised to [[carbon dioxide]] and [[water (molecule)|water]] in the [[Mitochondrion|mitochondria]]
| |
| ::C<sub>6</sub>H<sub>12</sub>O<sub>6</sub> + 6O<sub>2</sub> → 6CO<sub>2</sub> + 6H<sub>2</sub>O
| |
| ::C<sub>57</sub>H<sub>110</sub>O<sub>6</sub> + 81.5O<sub>2</sub> → 57CO<sub>2</sub> + 55H<sub>2</sub>O
| |
| and some of the energy is used to convert [[Adenosine diphosphate|ADP]] into [[Adenosine triphosphate|ATP]]
| |
| ::ADP + HPO<sub>4</sub><sup>2−</sup> → ATP + H<sub>2</sub>O
| |
| The rest of the chemical energy in the carbohydrate or fat is converted into heat: the ATP is used as a sort of "energy currency", and some of the chemical energy it contains when split and reacted with water, is used for other [[metabolism]] (at each stage of a [[metabolic pathway]], some chemical energy is converted into heat). Only a tiny fraction of the original chemical energy is used for work:<ref>These examples are solely for illustration, as it is not the energy available for work which limits the performance of the athlete but the [[power (physics)|power]] output of the sprinter and the [[force (physics)|force]] of the weightlifter. A worker stacking shelves in a supermarket does more work (in the physical sense) than either of the athletes, but does it more slowly.</ref>
| |
| :gain in kinetic energy of a sprinter during a 100 m race: 4 kJ
| |
| :gain in gravitational potential energy of a 150 kg weight lifted through 2 metres: 3kJ
| |
| :Daily food intake of a normal adult: 6–8 MJ
| |
| | |
| It would appear that living organisms are remarkably [[Energy conversion efficiency|inefficient (in the physical sense)]] in their use of the energy they receive (chemical energy or radiation), and it is true that most real [[machine]]s manage higher efficiencies. In growing organisms the energy that is converted to heat serves a vital purpose, as it allows the organism tissue to be highly ordered with regard to the molecules it is built from. The [[second law of thermodynamics]] states that energy (and matter) tends to become more evenly spread out across the universe: to concentrate energy (or matter) in one specific place, it is necessary to spread out a greater amount of energy (as heat) across the remainder of the universe ("the surroundings").<ref>[[Crystal]]s are another example of highly ordered systems that exist in nature: in this case too, the order is associated with the transfer of a large amount of heat (known as the [[lattice energy]]) to the surroundings.</ref> Simpler organisms can achieve higher energy efficiencies than more complex ones, but the complex organisms can occupy [[ecological niche]]s that are not available to their simpler brethren. The conversion of a portion of the chemical energy to heat at each step in a metabolic pathway is the physical reason behind the pyramid of biomass observed in [[ecology]]: to take just the first step in the [[food chain]], of the estimated 124.7 Pg/a of carbon that is [[carbon fixation|fixed]] by [[photosynthesis]], 64.3 Pg/a (52%) are used for the metabolism of green plants,<ref>Ito, Akihito; Oikawa, Takehisa (2004). "[http://www.terrapub.co.jp/e-library/kawahata/pdf/343.pdf Global Mapping of Terrestrial Primary Productivity and Light-Use Efficiency with a Process-Based Model.]" in Shiyomi, M. et al. (Eds.) ''Global Environmental Change in the Ocean and on Land.'' pp. 343–58.</ref> i.e. reconverted into carbon dioxide and heat.
| |
| | |
| ==Energy transformation==
| |
| {{Main|Energy transformation}}
| |
| | |
| The concept of energy and its [[energy transformation|transformations]] is vital in explaining and predicting most natural phenomena. One form of energy can often be readily transformed into another; for instance, a battery, from [[chemical energy]] to [[electric energy]]; a [[dam]]: [[gravitational potential energy]] to [[kinetic energy]] of moving [[water]] (and the blades of a [[turbine]]) and ultimately to [[electric energy]] through an [[electric generator]].
| |
| | |
| There are strict limits to how efficiently energy can be converted into other forms of energy via [[Work (physics)|work]], and [[heat]] as described by [[Carnot's theorem (thermodynamics)|Carnot's theorem]] and the [[second law of thermodynamics]]. These limits are especially evident when an engine is used to perform work. Some [[energy transformation]]s can be quite efficient.
| |
| | |
| The ''direction'' of transformations in energy (what kind of energy is transformed to what other kind) is often described by [[entropy]] (equal energy spread among all available [[degrees of freedom (physics and chemistry)|degrees of freedom]]) considerations, as in practice all energy transformations are permitted on a small scale, but certain larger transformations are not permitted because it is statistically unlikely that energy or matter will randomly move into more concentrated forms or smaller spaces.
| |
| | |
| Energy transformations in the universe over time are characterized by various kinds of potential energy that has been available since the [[Big Bang]], later being "released" (transformed to more active types of energy such as kinetic or radiant energy), when a triggering mechanism is available. Familiar examples of such processes include nuclear decay, in which energy is released that was originally "stored" in heavy isotopes (such as [[uranium]] and [[thorium]]), by [[nucleosynthesis]], a process ultimately using the gravitational potential energy released from the [[gravitational collapse]] of [[supernova]]e, to store energy in the creation of these heavy elements before they were incorporated into the solar system and the Earth. This energy is triggered and released in nuclear [[fission bomb]]s or in civil nuclear power generation.
| |
| | |
| Similarly, in the case of a [[Chemical explosive|chemical explosion]], [[chemical potential]] energy is transformed to [[kinetic energy]] and [[thermal energy]] 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 maximum. At its lowest point the [[kinetic energy]] is at maximum and is equal to the decrease of [[potential energy]]. If one (unrealistically) assumes that there is no [[friction]] or other losses, the conversion of energy between these processes would be perfect, and the [[pendulum]] would continue swinging forever.
| |
| | |
| ===Conservation of energy and mass in transformation===
| |
| Energy gives rise to weight when it is trapped in a system with zero momentum, where it can be weighed. It is also equivalent to mass, and this mass is always associated with it. Mass is also equivalent to a certain amount of energy, and likewise always appears associated with it, as described in [[mass-energy equivalence]]. The formula ''E'' = ''mc''², derived by [[Albert Einstein]] (1905) quantifies the relationship between rest-mass and rest-energy within the concept of special relativity. In different theoretical frameworks, similar formulas were derived by [[J. J. Thomson]] (1881), [[Henri Poincaré]] (1900), [[Friedrich Hasenöhrl]] (1904) and others (see [[Mass-energy equivalence#History]] for further information).
| |
| | |
| [[Matter]] may be converted to energy (and vice versa), but mass cannot ever be destroyed; rather, mass/energy equivalence remains a constant for both the matter and the energy, during any process when they are converted into each other. However, since <math>c^2</math> is extremely large relative to ordinary human scales, the conversion of ordinary amount of [[matter]] (for example, 1 kg) to other forms of energy (such as heat, light, and other radiation) can liberate tremendous amounts of energy (~<math>9\times 10^{16}</math> joules = 21 megatons of TNT), as can be seen in nuclear reactors and nuclear weapons. Conversely, the mass equivalent of a unit of energy is minuscule, which is why a loss of energy (loss of mass) from most systems is difficult to measure by weight, unless the energy loss is very large. Examples of energy transformation into matter (i.e., kinetic energy into particles with rest mass) are found in high-energy [[nuclear physics]].
| |
| | |
| ===Reversible and non-reversible transformations===
| |
| Transformation of energy into useful work is a core topic of thermodynamics. In nature, transformations of energy can be fundamentally classed into two kinds: those that are thermodynamically [[Reversible process (thermodynamics)|reversible]], and those that are thermodynamically [[Irreversibility|irreversible]]. A [[reversible process (thermodynamics)|reversible process in thermodynamics]] is one in which no energy is dissipated (spread) into empty energy states available in a volume, from which it cannot be recovered into more concentrated forms (fewer quantum states), without degradation of even more energy. A reversible process is one in which this sort of dissipation does not happen. For example, conversion of energy from one type of potential field to another, is reversible, as in the pendulum system described above. In processes where heat is generated, quantum states of lower energy, present as possible excitations in fields between atoms, act as a reservoir for part of the energy, from which it cannot be recovered, in order to be converted with 100% efficiency into other forms of energy. In this case, the energy must partly stay as heat, and cannot be completely recovered as usable energy, except at the price of an increase in some other kind of heat-like increase in disorder in quantum states, in the universe (such as an expansion of matter, or a randomization in a crystal).
| |
| | |
| ===Transformation with the age of the universe===
| |
| As the universe evolves in time, more and more of its energy becomes trapped in irreversible states (i.e., as heat or other kinds of increases in disorder). This has been referred to as the inevitable thermodynamic [[heat death of the universe]]. In this heat death the energy of the universe does not change, but the fraction of energy which is available to do work through a [[heat engine]], or be transformed to other usable forms of energy (through the use of generators attached to heat engines), grows less and less.
| |
| | |
| In a slower process, [[radioactive decay]] of these atoms in the core of the Earth releases heat. This thermal energy drives [[plate tectonics]] and may lift mountains, via [[orogenesis]]. This slow lifting represents a kind of gravitational potential energy storage of the thermal energy, which may be later released to active kinetic energy in landslides, after a triggering event. Earthquakes also release stored elastic potential energy in rocks, a store that has been produced ultimately from the same radioactive heat sources. Thus, according to present understanding, familiar events such as landslides and earthquakes release energy that has been stored as potential energy in the Earth's gravitational field or elastic strain (mechanical potential energy) in rocks. Prior to this, they represent release of energy that has been stored in heavy atoms since the collapse of long-destroyed supernova stars created these atoms.
| |
| | |
| In another similar chain of transformations beginning at the dawn of the universe, [[nuclear fusion]] of hydrogen in the Sun also releases another store of potential energy which was created at the time of the [[Big Bang]]. At that time, according to theory, space expanded and the universe cooled too rapidly for hydrogen to completely fuse into heavier elements. This meant that hydrogen represents a store of potential energy that can be released by [[nuclear fusion|fusion]]. Such a fusion process is triggered by heat and pressure generated from gravitational collapse of hydrogen clouds when they produce stars, and some of the fusion energy is then transformed into sunlight. Such sunlight from our Sun may again be stored as gravitational potential energy after it strikes the Earth, as (for example) water evaporates from oceans and is deposited upon mountains (where, after being released at a hydroelectric dam, it can be used to drive turbines or generators to produce electricity). Sunlight also drives many weather phenomena, save those generated by volcanic events.
| |
| | |
| An example of a solar-mediated weather event is a hurricane, which occurs when large unstable areas of warm ocean, heated over months, give up some of their thermal energy suddenly to power a few days of violent air movement. Sunlight is also captured by plants as ''chemical potential energy'' in [[photosynthesis]], when carbon dioxide and water (two low-energy compounds) are converted into the high-energy compounds carbohydrates, lipids, and proteins. Plants also release oxygen during photosynthesis, which is utilized by living organisms as an [[electron acceptor]], to release the energy of carbohydrates, lipids, and proteins. Release of the energy stored during photosynthesis as heat or light may be triggered suddenly by a spark, in a forest fire, or it may be made available more slowly for animal or human metabolism, when these molecules are ingested, and [[catabolism]] is triggered by [[enzyme]] action.
| |
| | |
| Through all of these transformation chains, potential energy stored at the time of the Big Bang is later released by intermediate events, sometimes being stored in a number of ways over time between releases, as more active energy. In all these events, one kind of energy is converted to other types of energy, including heat.
| |
| | |
| ==Conservation of energy==
| |
| {{Main|Conservation of energy}}
| |
| Energy is subject to the ''law of conservation of energy''. According to this law, energy can neither be created (produced) nor destroyed by itself. It can only be transformed.
| |
| | |
| Most kinds of energy (with gravitational energy being a notable exception)<ref>{{cite web|url=http://www.physics.ucla.edu/~cwp/articles/noether.asg/noether.html |title=E. Noether's Discovery of the Deep Connection Between Symmetries and Conservation Laws |publisher=Physics.ucla.edu |date=1918-07-16 |accessdate=2010-12-12}}</ref> are subject to strict local conservation laws as well. In this case, energy can only be exchanged between adjacent regions of space, and all observers agree as to the volumetric density of energy in any given space. There is also a global law of conservation of energy, stating that the total energy of the universe cannot change; this is a corollary of the local law, but not vice versa.<ref name="RPF1">{{Cite book|first=Richard |last=Feynman|title=The Feynman Lectures on Physics; Volume 1|year=1964|publisher=Addison Wesley|location=U.S.A| isbn=0-201-02115-3}}</ref><ref name="thermo-laws">[http://www.av8n.com/physics/thermo-laws.htm ''The Laws of Thermodynamics''] including careful definitions of energy, free energy, et cetera.</ref> [[Conservation of energy]] is the mathematical consequence of [[translational symmetry]] of [[time]] (that is, the indistinguishability of time intervals taken at different time)<ref>{{cite web|url=http://ptolemy.eecs.berkeley.edu/eecs20/week9/timeinvariance.html |title=Time Invariance |publisher=Ptolemy.eecs.berkeley.edu |date= |accessdate=2010-12-12}}</ref> - see [[Noether's theorem]].
| |
| | |
| According to [[Conservation of energy]] 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.
| |
| | |
| This law is a fundamental principle of physics. It 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.
| |
| | |
| This is because energy is the quantity which is [[canonical conjugate]] to time. This mathematical entanglement of energy and time also results in the uncertainty principle - it is impossible to define the exact amount of energy during any definite time interval. The uncertainty principle should not be confused with energy conservation - rather it provides mathematical limits to which energy can in principle be defined and measured.
| |
| | |
| Each of the basic forces of nature is associated with a different type of potential energy, and all types of potential energy (like all other types of energy) appears as system [[mass]], whenever present. For example, a compressed spring will be slightly more massive than before it was compressed. Likewise, whenever energy is transferred between systems by any mechanism, an associated mass is transferred with it.
| |
| | |
| In [[quantum mechanics]] energy is expressed using the Hamiltonian [[Operator (physics)|operator]]. On any time scales, the uncertainty in the energy is by
| |
| | |
| : <math>\Delta E \Delta t \ge \frac { \hbar } {2 } </math>
| |
| | |
| which is similar in form to the Heisenberg [[Heisenberg Uncertainty Principle|uncertainty principle]] (but not really mathematically equivalent thereto, since ''H'' and ''t'' are not dynamically conjugate variables, neither in classical nor in quantum mechanics).
| |
| | |
| In [[particle physics]], this inequality permits a qualitative understanding of [[virtual particles]] which carry [[momentum]], exchange by which and with real particles, is responsible for the creation of all known [[fundamental forces]] (more accurately known as [[fundamental interactions]]). [[Virtual photons]] (which are simply lowest quantum mechanical [[energy state]] of [[photon]]s) are also responsible for electrostatic interaction between [[electric charge]]s (which results in [[Coulomb law]]), for [[Spontaneous fission|spontaneous]] radiative decay of exited atomic and nuclear states, for the [[Casimir force]], for [[van der Waals force|van der Waals bond forces]] and some other observable phenomena.
| |
| | |
| ==Applications of the concept of energy==
| |
| Energy is subject to a strict [[conservation law|global conservation law]]; that is, whenever one measures (or calculates) the total energy of a system of particles whose interactions do not depend explicitly on time, it is found that the total energy of the system always remains constant.<ref>Berkeley Physics Course Volume 1. Charles Kittel, Walter D Knight and Malvin A Ruderman</ref>
| |
| *The total energy of a [[system]] can be subdivided and classified in various ways. For example, it is sometimes convenient to distinguish [[potential energy]] (which is a function of coordinates only) from [[kinetic energy]] (which is a function of coordinate time [[derivative]]s only). It may also be convenient to distinguish gravitational energy, electric energy, thermal energy, and other forms. These classifications overlap; for instance, thermal energy usually consists partly of kinetic and partly of potential energy.
| |
| *The ''transfer'' of energy can take various forms; familiar examples include work, heat flow, and advection, as discussed [[#Energy transfer|below]].
| |
| *The word "energy" is also used outside of physics in many ways, which can lead to ambiguity and inconsistency. The vernacular terminology is not consistent with [[technical terminology]]. For example, while energy is always conserved (in the sense that the total energy does not change despite energy transformations), energy can be converted into a form, e.g., thermal energy, that cannot be utilized to perform work. When one talks about "conserving energy by driving less," one talks about conserving fossil fuels and preventing useful energy from being lost as heat. This usage of "conserve" differs from that of the law of conservation of energy.<ref name="thermo-laws"/>
| |
| | |
| In [[classical physics]] energy is considered a scalar quantity, the [[canonical conjugate]] to [[time]]. In [[special relativity]] energy is also a scalar (although not a [[Lorentz scalar]] but a time component of the [[energy-momentum 4-vector|energy-momentum]] [[4-vector]]).<ref name="MTW">{{Cite book|author=Misner, Thorne, Wheeler |title=Gravitation |year=1973 |publisher=W. H. Freeman |location=San Francisco |isbn=0-7167-0344-0}}</ref> In other words, energy is invariant with respect to rotations of [[space]], but not invariant with respect to rotations of [[space-time]] (= [[Lorentz boost|boosts]]).
| |
| | |
| ==Energy transfer==
| |
| A system can transfer energy to another system by simply transferring matter to it (since matter is equivalent to energy, in accordance with its mass). However, when energy is transferred by means other than matter-transfer, the transfer produces changes in the second system, as a result of work done on it. This work manifests itself as the effect of force(s) applied through distances within the target system.
| |
| | |
| For example, a system can emit energy to another by transferring (radiating) [[electromagnetic energy]], but this creates forces upon the particles that absorb the radiation. Similarly, a system may transfer energy to another by physically impacting it, but in that case the energy of motion in an object, called [[kinetic energy]], results in forces acting over distances (new energy) to appear in another object that is struck. Transfer of [[thermal energy]] by [[heat]] occurs by both of these mechanisms: heat can be transferred by electromagnetic radiation, or by physical contact in which direct particle-particle impacts transfer kinetic energy.
| |
| | |
| Because energy is strictly conserved and is also locally conserved (wherever it can be defined), it is important to remember that by the definition of energy the transfer of energy between the "system" and adjacent regions is work. A familiar example is ''[[mechanical work]]''. In simple cases this is written as the following equation:
| |
| | |
| {{NumBlk|:|<math>\Delta{}E = W</math>|{{EquationRef|1}}}}
| |
| | |
| if there are no other energy-transfer processes involved. Here <math>E</math> is the amount of energy transferred, and <math>W</math> represents the work done on the system.{{dubious|date=January 2013}}
| |
| | |
| More generally, the energy transfer can be split into two categories:
| |
| | |
| {{NumBlk|:|<math>\Delta{}E = W + Q </math>|{{EquationRef|2}}}}
| |
| | |
| where <math>Q</math> represents the heat flow into the system.
| |
| | |
| There are other ways in which an open system can gain or lose energy. In chemical systems, energy can be added to a system by means of adding substances with different chemical potentials, which potentials are then extracted (both of these process are illustrated by fueling an auto, a system which gains in energy thereby, without addition of either work or heat). Winding a clock would be adding energy to a mechanical system. These terms may be added to the above equation, or they can generally be subsumed into a quantity called "energy addition term <math>E</math>" which refers to ''any'' type of energy carried over the surface of a control volume or system volume. Examples may be seen above, and many others can be imagined (for example, the kinetic energy of a stream of particles entering a system, or energy from a laser beam adds to system energy, without either being either work-done or heat-added, in the classic senses).
| |
| | |
| {{NumBlk|:|<math>\Delta{}E = W + Q + E </math>|{{EquationRef|3}}}}
| |
| | |
| Where <math>E</math> in this general equation represents other additional advected energy terms not covered by work done on a system, or heat added to it.
| |
| | |
| Energy is also transferred from potential energy (<math>E_p</math>) to kinetic energy (<math>E_k</math>) and then back to potential energy constantly. This is referred to as conservation of energy. In this closed system, energy cannot be created or destroyed; therefore, the initial energy and the final energy will be equal to each other. This can be demonstrated by the following:
| |
| | |
| {{NumBlk|:|<math>E_{pi} + E_{ki} = E_{pF} + E_{kF}</math>|{{EquationRef|4}}}}
| |
| | |
| The equation can then be simplified further since <math>E_p = mgh</math> (mass times acceleration due to gravity times the height) and <math>E_k = \frac{1}{2} mv^2</math> (half mass times velocity squared). Then the total amount of energy can be found by adding <math>E_p + E_k = E_{total}</math>.
| |
| | |
| ==Energy and the laws of motion==
| |
| In classical mechanics, energy is a conceptually and mathematically useful property, as it is a [[conserved quantity]]. Several formulations of mechanics have been developed using energy as a core concept, as below; | |
| | |
| ===The Hamiltonian===
| |
| The total energy of a system is sometimes called the [[Hamilton's equations|Hamiltonian]], after [[William Rowan Hamilton]]. The classical equations of motion can be written in terms of the Hamiltonian, even for highly complex or abstract systems. These classical equations have remarkably direct analogs in
| |
| nonrelativistic quantum mechanics.<ref>[http://classic-web.archive.org/web/20071011135413/http://www.sustech.edu/OCWExternal/Akamai/18/18.013a/textbook/HTML/chapter16/section03.html The Hamiltonian] MIT OpenCourseWare website 18.013A Chapter 16.3 Accessed February 2007</ref>
| |
| | |
| ===The Lagrangian===
| |
| Another energy-related concept is called the [[Lagrangian]], after [[Joseph Louis Lagrange]]. This is even more fundamental than the Hamiltonian, and can be used to derive the equations of motion. It was invented in the context of [[classical mechanics]], but is generally useful in modern physics. The Lagrangian is defined as the kinetic energy ''minus'' the potential energy.
| |
| | |
| Usually, the Lagrange formalism is mathematically more convenient than the Hamiltonian for non-conservative systems (such as systems with friction).
| |
| | |
| ===Noether's Theorem===
| |
| Noether's (first) theorem (1918) states that any differentiable symmetry of the action of a physical system has a corresponding conservation law.
| |
| | |
| [[Noether's theorem]] has become a fundamental tool of modern theoretical physics and the calculus of variations. A generalization of the seminal formulations on constants of motion in Lagrangian and Hamiltonian mechanics (1788 and 1833, respectively), it does not apply to systems that cannot be modeled with a Lagrangian; for example, dissipative systems with continuous symmetries need not have a corresponding conservation law.
| |
| | |
| ==Energy and thermodynamics==
| |
| | |
| ===Internal energy===
| |
| [[Internal energy]] is the sum of all microscopic forms of energy of a system. It is the energy needed to create the system. It is related to the potential energy, e.g., molecular structure, crystal structure, and other geometric aspects, as well as the motion of the particles, in form of kinetic energy. Thermodynamics is chiefly concerned with changes in internal energy and not its absolute value, which is impossible to determine with thermodynamics alone.<ref name=klotz>I. Klotz, R. Rosenberg, ''Chemical Thermodynamics - Basic Concepts and Methods'', 7th ed., Wiley (2008), p.39</ref>
| |
| | |
| ===The first law of thermodynamics===
| |
| The [[first law of thermodynamics]] asserts that energy (but not necessarily [[thermodynamic free energy]]) is always conserved<ref name="KK">{{Cite book|author=Kittel and Kroemer|title=Thermal Physics |year=1980|publisher=W. H. Freeman |location=New York| isbn=0-7167-1088-9}}</ref> and that heat flow is a form of energy transfer. For homogeneous systems, with a well-defined temperature and pressure, a commonly used corollary of the first law is that, for a system subject only to [[pressure]] forces and heat transfer (e.g., a cylinder-full of gas), the differential change in the internal energy of the system (with a ''gain'' in energy signified by a positive quantity) is given as
| |
| | |
| :<math>\mathrm{d}E = T\mathrm{d}S - P\mathrm{d}V\,</math>,
| |
| | |
| where the first term on the right is the heat transferred into the system, expressed in terms of [[temperature]] ''T'' and [[entropy]] ''S'' (in which entropy increases and the change d''S'' is positive when the system is heated), and the last term on the right hand side is identified as work done on the system, where pressure is ''P'' and volume ''V'' (the negative sign results since compression of the system requires work to be done on it and so the volume change, d''V'', is negative when work is done on the system).
| |
| | |
| This equation is highly specific, ignoring all chemical, electrical, nuclear, and gravitational forces, effects such as [[advection]] of any form of energy other than heat and pV-work. The general formulation of the first law (i.e., conservation of energy) is valid even in situations in which the system is not homogeneous. For these cases the change in internal energy of a ''closed'' system is expressed in a general form by
| |
| | |
| :<math>\mathrm{d}E=\delta Q+\delta W</math>
| |
| | |
| where <math>\delta Q</math> is the heat supplied to the system and <math>\delta W</math> is the work applied to the system.
| |
| | |
| ===Equipartition of energy===
| |
| The energy of a mechanical [[harmonic oscillator]] (a mass on a spring) is alternatively [[kinetic energy|kinetic]] and [[potential]]. At two points in the oscillation [[Frequency|cycle]] it is entirely kinetic, and alternatively at two other points it is entirely potential. Over the whole cycle, or over many cycles, net energy is thus equally split between kinetic and potential. This is called [[equipartition principle]]; total energy of a system with many degrees of freedom is equally split among all available degrees of freedom.
| |
| | |
| This principle is vitally important to understanding the behavior of a quantity closely related to energy, called [[entropy]]. Entropy is a measure of evenness of a [[distribution (mathematics)|distribution]] of energy between parts of a system. When an isolated system is given more degrees of freedom (i.e., given new available [[energy state]]s that are the same as existing states), then total energy spreads over all available degrees equally without distinction between "new" and "old" degrees. This mathematical result is called the [[second law of thermodynamics]].
| |
| | |
| ===Oscillators, phonons, and photons===
| |
| {{Original research|section|date=August 2009}}
| |
| In an ensemble (connected collection) of unsynchronized [[oscillator]]s, the average energy is spread equally between kinetic and potential types.
| |
| | |
| In a solid, [[thermal energy]] (often referred to loosely as heat content) can be accurately described by an ensemble of thermal [[phonon]]s that act as mechanical oscillators. In this model, thermal energy is equally kinetic and potential.
| |
| | |
| In an [[ideal gas]], the interaction potential between particles is essentially the [[Dirac delta function|delta function]] which stores no energy: thus, all of the thermal energy is kinetic.
| |
| | |
| Because an electric oscillator ([[LC circuit]]) is analogous to a mechanical oscillator, its energy must be, on average, equally kinetic and potential. It is entirely arbitrary whether the magnetic energy is considered kinetic and whether the electric energy is considered potential, or vice versa. That is, either the [[inductor]] is analogous to the mass while the [[capacitor]] is analogous to the spring, or vice versa.
| |
| | |
| 1. By extension of the previous line of thought, in [[free space]] the electromagnetic field can be considered an ensemble of oscillators, meaning that [[radiant energy|radiation energy]] can be considered equally potential and kinetic. This model is useful, for example, when the electromagnetic [[Lagrangian]] is of primary interest and is interpreted in terms of potential and kinetic energy.
| |
| | |
| 2. On the other hand, in the key equation <math>m^2 c^4 = E^2 - p^2 c^2</math>, the contribution <math>mc^2</math> is called the rest energy, and all other contributions to the energy are called kinetic energy. For a particle that has mass, this implies that the kinetic energy is <math>0.5 p^2/m</math> at speeds much smaller than ''c'', as can be proved by writing <math>E = mc^2 </math> √<math>(1 + p^2 m^{-2}c^{-2})</math> and expanding the square root to lowest order. By this line of reasoning, the energy of a photon is entirely kinetic, because the photon is massless and has no rest energy. This expression is useful, for example, when the energy-versus-momentum relationship is of primary interest.
| |
| | |
| The two analyses are entirely consistent. The electric and magnetic degrees of freedom in item 1 are ''transverse'' to the direction of motion, while the speed in item 2 is ''along'' the direction of motion. For non-relativistic particles these two notions of potential versus kinetic energy are numerically equal, so the ambiguity is harmless, but not so for relativistic particles.<!-- confusing -->
| |
| | |
| ===Quantum mechanics===
| |
| {{Main|Energy operator}}
| |
| In quantum mechanics energy is defined in terms of the [[Hamiltonian (quantum mechanics)|energy operator]]
| |
| as a time derivative of the [[wave function]]. The [[Schrödinger equation]] equates the energy operator to the full energy of a particle or a system. In results can be considered as a definition of measurement of energy in quantum mechanics. The Schrödinger equation describes the space- and time-dependence of slow changing (non-relativistic) [[wave function]] of quantum systems. The solution of this equation for bound system is discrete (a set of permitted states, each characterized by an [[energy level]]) which results in the concept of [[quantum|quanta]]. In the solution of the Schrödinger equation for any oscillator (vibrator) and for electromagnetic waves in a vacuum, the resulting energy states are related to the frequency by the [[Planck]] equation <math>E = h\nu</math> (where <math>h</math> is the [[Planck's constant]] and <math>\nu</math> the frequency). In the case of electromagnetic wave these energy states are called quanta of [[light]] or [[photon]]s.
| |
| | |
| ===Relativity===
| |
| When calculating kinetic energy ([[Mechanical work|work]] to accelerate a [[mass]] from zero [[speed]] to some finite speed) relativistically - using [[Lorentz transformations]] instead of [[Newtonian mechanics]], Einstein discovered an unexpected by-product of these calculations to be an energy term which does not vanish at zero speed. He called it [[rest mass energy]] - energy which every mass must possess even when being at rest. The amount of energy is directly proportional to the mass of body:
| |
| | |
| :<math> E = m c^2 </math>,
| |
| where
| |
| :''m'' is the mass,
| |
| :''c'' is the [[speed of light]] in vacuum,
| |
| :''E'' is the rest mass energy.
| |
| | |
| For example, consider [[electron]]-[[positron]] annihilation, in which the rest mass of individual particles is destroyed, but the inertia equivalent of the system of the two particles (its [[invariant mass]]) remains (since all energy is associated with mass), and this inertia and invariant mass is carried off by photons which individually are massless, but as a system retain their mass. This is a reversible process - the inverse process is called [[pair creation]] - in which the rest mass of particles is created from energy of two (or more) annihilating photons. In this system the [[matter]] (electrons and positrons) is destroyed and changed to non-matter energy (the photons). However, the total system mass and energy do not change during this interaction.
| |
| | |
| In general relativity, the [[stress-energy tensor]] serves as the source term for the gravitational field, in rough analogy to the way mass serves as the source term in the non-relativistic Newtonian approximation.<ref name="MTW"/>
| |
| | |
| It is not uncommon to hear that energy is "equivalent" to mass. It would be more accurate to state that every energy has an inertia and gravity equivalent, and because mass is a form of energy, then mass too has inertia and gravity associated with it.
| |
| | |
| ==Measurement==
| |
| {{expand section|date=January 2013}}
| |
| [[Image:X-ray microcalorimeter diagram.jpg|thumb|A [[Schematic|schematic diagram]] of a [[Calorimeter]] - An instrument used by physicists to measure energy. In this example is it is X-Rays.]]
| |
| Because energy is defined as the ability to do work on objects, there is no absolute measure of energy. Only the transition of a system from one state into another can be defined and thus energy is measured in relative terms. The choice of a baseline or zero point is often arbitrary and can be made in whatever way is most convenient for a problem.
| |
| For example in the case of measuring the energy deposited by X-rays as shown in the accompanying diagram, conventionally the technique most often employed is [[calorimetry]]. This is a [[thermodynamic]] technique that relies on the measurement of temperature using a [[thermometer]] or of intensity of radiation using a [[bolometer]].
| |
| | |
| ===Energy density===
| |
| {{Main|Energy density}}
| |
| ''Energy density'' is a term used for the amount of useful energy stored in a given system or region of space per unit volume.
| |
| | |
| For [[fuel]]s, the energy per unit volume is sometimes a useful parameter. In a few applications, comparing, for example, the effectiveness of [[hydrogen fuel]] to [[gasoline]] it turns out that hydrogen has a higher [[specific energy]] than does gasoline, but, even in liquid form, a much lower energy ''density''.
| |
| | |
| ==See also==
| |
| {{Wikipedia books}}
| |
| {{Portal|Energy|Physics}}
| |
| *[[Combustion]]
| |
| *[[Index of energy articles]]
| |
| *[[Index of wave articles]]
| |
| *[[Orders of magnitude (energy)]]
| |
| {{clear}}
| |
| | |
| ==Notes and references==
| |
| {{Reflist|colwidth=30em}}
| |
| | |
| ==Further reading==
| |
| {{refbegin}}
| |
| * {{Cite book|first=G. N. |last=Alekseev|title=Energy and Entropy |year=1986 |publisher=Mir Publishers |location=Moscow }}
| |
| * {{cite book |title=Light and Matter|last= Crowell|first= Benjamin|year=2011|origyear=2003|publisher= Light and Matter|location=Fullerton, CA|url=http://www.lightandmatter.com/html_books/lm/ch11/ch11.html}}
| |
| * {{cite web|last=Ross|first=John S. |title=Work, Power, Kinetic Energy |url=http://www.physnet.org/modules/pdf_modules/m20.pdf|work=Project PHYSNET|publisher=Michigan State University|date=23 April 2002}}
| |
| * {{Cite book|author=Smil, Vaclav|title=Energy in nature and society: general energetics of complex systems |year=2008 |publisher=[[MIT Press]] |location=Cambridge, USA |isbn=0-262-19565-8}}
| |
| * {{Cite book|author=Walding, Richard, Rapkins, Greg, Rossiter, Glenn |title=New Century Senior Physics |date=1999-11-01 |publisher=[[Oxford University Press]] |location=Melbourne, Australia |isbn=0-19-551084-4}}
| |
| {{refend}}
| |
| | |
| ==External links==
| |
| {{Sister project links|Energy}}
| |
| {{Prone to spam|date=July 2013}}
| |
| {{Z148}}<!-- {{No more links}}
| |
| | |
| Please be cautious adding more external links.
| |
| | |
| Wikipedia is not a collection of links and should not be used for advertising.
| |
| | |
| Excessive or inappropriate links will be removed.
| |
| | |
| See [[Wikipedia:External links]] and [[Wikipedia:Spam]] for details.
| |
| | |
| If there are already suitable links, propose additions or replacements on
| |
| the article's talk page, or submit your link to the relevant category at
| |
| the Open Directory Project (dmoz.org) and link there using {{Dmoz}}.
| |
| | |
| -->
| |
| *{{Dmoz|Science/Technology/Energy|Energy}}
| |
| {{Nature nav}}
| |
| {{technology}}
| |
| {{Footer energy}}
| |
| {{Natural resources}}
| |
| | |
| [[Category:Energy (physics)| ]]
| |
| [[Category:Energy| ]]
| |
| [[Category:State functions]]
| |
| | |
| {{Link GA|is}}
| |
| {{Link GA|frr}}
| |