Energy

This article is about the scalar physical quantity. For an overview of and topical guide to energy, seeOutline of energy.For other uses, seeEnergy (disambiguation).

"Energetic" redirects here. For other uses, seeEnergetic (disambiguation).

Inphysics,energy(fromAncient Greekἐνέργεια(enérgeia)'activity') is thequantitativepropertythat istransferredto abodyor to aphysical system,recognizable in the performance ofworkand in the form ofheatandlight.Energy is aconserved quantity—the law ofconservation of energystates that energy can beconvertedin form, but not created or destroyed;matterand energy may also be converted to one another. The unit ofmeasurementfor energy in theInternational System of Units(SI) is thejoule(J).

Energy
Aplasma globe,usingelectrical energyto createplasma,light,heat,movementand a faintsound
Common symbols	E
SI unit	joule
Other units	kW⋅h,BTU,calorie,eV,erg,foot-pound
InSI base units	J = kg⋅m2⋅s−2
Extensive?	yes
Conserved?	yes
Dimension	ML2T−2

Common forms of energy include thekinetic energyof a moving object, thepotential energystored by an object (for instance due to its position in afield), theelastic energystored in a solid object,chemical energyassociated withchemical reactions,theradiant energycarried byelectromagnetic radiation,and theinternal energycontained within athermodynamic system.All livingorganismsconstantly take in and release energy.

Due tomass–energy equivalence,any object that has mass when stationary (calledrest mass) also has an equivalent amount of energy whose form is calledrest energy,and any additional energy (of any form) acquired by the object above that rest energy will increase the object's total mass just as it increases its total energy.

Human civilization requires energy to function, which it gets fromenergy resourcessuch asfossil fuels,nuclear fuel,orrenewable energy.The Earth'sclimateandecosystemsprocesses are driven by the energy the planet receives from the Sun (although a small amount is also contributed bygeothermal energy).

Forms

In a typicallightningstrike, 500megajoulesofelectric potential energyis converted into the same amount of energy in other forms, mostlylight energy,sound energyandthermal energy.

Thermal energyis energy of microscopic constituents of matter, which may include bothkineticandpotential energy.

The total energy of asystemcan be subdivided and classified intopotential energy,kinetic energy,or combinations of the two in various ways. Kinetic energy is determined by themovementof an object – or thecomposite motionof the object's components – whilepotential energyreflects the potential of an object to have motion, generally being based upon the object's position within afieldor what is stored within the field itself.

While these two categories are sufficient to describe all forms of energy, it is often convenient to refer to particular combinations of potential and kinetic energy as its own form. For example, the sum of translational androtationalkinetic and potential energy within a system is referred to asmechanical energy,whereas nuclear energy refers to the combined potentials within an atomic nucleus from either thenuclear forceor theweak force,among other examples.

Some forms of energy (that an object or system can have as a measurable property)

Type of energy	Description
Mechanical	the sum ofmacroscopictranslational and rotational kinetic and potential energies
Electric	potential energy due to or stored in electric fields
Magnetic	potential energy due to or stored in magnetic fields
Gravitational	potential energy due to or stored in gravitational fields
Chemical	potential energy due to chemical bonds
Ionization	potential energy thatbindsan electron to its atom or molecule
Nuclear	potential energy thatbindsnucleonsto form theatomic nucleus(and nuclear reactions)
Chromodynamic	potential energy thatbindsquarksto formhadrons
Elastic	potential energy due to the deformation of a material (or its container) exhibiting a restorative force as it returns to its original shape
Mechanical wave	kinetic and potential energy in an elastic material due to a propagatingoscillationof matter
Sound wave	kinetic and potential energy in a material due to a sound propagated wave (a particular type of mechanical wave)
Radiant	potential energystored in the fields of waves propagated byelectromagnetic radiation,includinglight
Rest	potential energydue toan object'srest mass
Thermal	kinetic energy of themicroscopicmotion of particles, a kind of disordered equivalent of mechanical energy

History

Main articles:History of energyandtimeline of thermodynamics, statistical mechanics, and random processes

Thomas Young,the first person to use the term "energy" in the modern sense

The wordenergyderives from theAncient Greek:ἐνέργεια,romanized:energeia,lit. 'activity, operation',which possibly appears for the first time in the work ofAristotlein the 4th century BC. In contrast to the modern definition, energeia was a qualitative philosophical concept, broad enough to include ideas such as happiness and pleasure.

In the late 17th century,Gottfried Leibnizproposed the idea of theLatin:vis viva,or living force, which defined as the product of the mass of an object and its velocity squared; he believed that totalvis vivawas conserved. To account for slowing due to friction, Leibniz theorized that thermal energy consisted of the motions of the constituent parts of matter, although it would be more than a century until this was generally accepted. The modern analog of this property,kinetic energy,differs fromvis vivaonly by a factor of two. Writing in the early 18th century,Émilie du Châteletproposed the concept ofconservation of energyin the marginalia of her French language translation of Newton'sPrincipia Mathematica,which represented the first formulation of a conserved measurable quantity that was distinct frommomentum,and which would later be called "energy".

In 1807,Thomas Youngwas possibly the first to use the term "energy" instead ofvis viva,in its modern sense.Gustave-Gaspard Coriolisdescribed "kinetic energy"in 1829 in its modern sense, and in 1853,William Rankinecoined the term "potential energy".The law ofconservation of energywas also first postulated in the early 19th century, and applies to anyisolated system.It was argued for some years whether heat was a physical substance, dubbed thecaloric,or merely a physical quantity, such asmomentum.In 1845James Prescott Joulediscovered the link between mechanical work and the generation of heat.

These developments led to the theory of conservation of energy, formalized largely by William Thomson (Lord Kelvin) as the field ofthermodynamics.Thermodynamics aided the rapid development of explanations of chemical processes byRudolf Clausius,Josiah Willard Gibbs,andWalther Nernst.It also led to a mathematical formulation of the concept ofentropyby Clausius and to the introduction of laws ofradiant energybyJožef Stefan.According toNoether's theorem,the conservation of energy is a consequence of the fact that the laws of physics do not change over time.Thus, since 1918, theorists have understood that the law ofconservation of energyis the direct mathematical consequence of thetranslational symmetryof the quantityconjugateto energy, namely time.

Units of measure

Joule's apparatus for measuring the mechanical equivalent of heat. A descending weight attached to a string causes a paddle immersed in water to rotate.

Main article:Units of energy

In 1843, James Prescott Joule independently discovered the mechanical equivalent in a series of experiments. The most famous of them used the "Joule apparatus": a descending weight, attached to a string, caused rotation of a paddle immersed in water, practically insulated from heat transfer. It showed that the gravitationalpotential energylost by the weight in descending was equal to theinternal energygained by the water throughfrictionwith the paddle.

In theInternational System of Units(SI), the unit of energy is the joule, named after Joule. It is aderived unitthat is equal to the energy expended (orworkdone) in applying a force of one newton through a distance of one metre. However energy is also expressed in many other units not part of the SI, such asergs,calories,British thermal units,kilowatt-hoursandkilocalories,which require a conversion factor when expressed in SI units.

The SI unit of energy rate (energy per unit time) is thewatt,which is a joule per second. Thus, one joule is one watt-second, and 3600 joules equal one watt-hour. TheCGSenergy unit is theergand theimperial and US customaryunit is thefoot pound.Other energy units such as theelectronvolt,food calorieor thermodynamickcal(based on the temperature change of water in a heating process), andBTUare used in specific areas of science and commerce.

Scientific use

Classical mechanics

Main articles:Mechanics,Mechanical work,andThermodynamics

In classical mechanics, energy is a conceptually and mathematically useful property, as it is aconserved quantity.Several formulations of mechanics have been developed using energy as a core concept.

Work,a function of energy, is force times distance.

$W=\int _{C}\mathbf {F} \cdot \mathrm {d} \mathbf {s}$

This says that the work ($W$) is equal to theline integralof theforceFalong a pathC;for details see themechanical workarticle. Work and thus energy isframe 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.

The total energy of a system is sometimes called theHamiltonian,afterWilliam 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 direct analogs in nonrelativistic quantum mechanics.

Another energy-related concept is called theLagrangian,afterJoseph-Louis Lagrange.This formalism is as fundamental as the Hamiltonian, and both can be used to derive the equations of motion or be derived from them. It was invented in the context ofclassical mechanics,but is generally useful in modern physics. The Lagrangian is defined as the kinetic energyminusthe 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(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 generalisation 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.

Chemistry

In the context ofchemistry,energyis 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 usually accompanied by a decrease, and sometimes an increase, of the total energy of the substances involved. Some energy may be transferred between the surroundings and the reactants in the form of heat or light; thus the products of a reaction have sometimes more but usually less energy than the reactants. A reaction is said to beexothermicorexergonicif the final state is lower on the energy scale than the initial state; in the less common case ofendothermicreactions the situation is the reverse.Chemical reactionsare usually not possible unless the reactants surmount an energy barrier known as theactivation energy.Thespeedof a chemical reaction (at a given temperatureT) is related to the activation energyEby the Boltzmann's population factor e^−E/kT;that is, the probability of a molecule to have energy greater than or equal toEat a given temperatureT.This exponential dependence of a reaction rate on temperature is known as theArrhenius equation.The activation energy necessary for a chemical reaction can be provided in the form of thermal energy.

Biology

Main articles:BioenergeticsandFood energy

Basic overview ofenergy and human life

Inbiology,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 biologicalcellororganelleof a biological organism. Energy used inrespirationis stored in substances such ascarbohydrates(including sugars),lipids,andproteinsstored bycells.In human terms, thehuman equivalent(H-e) (Human energy conversion) indicates, for a given amount of energy expenditure, the relative quantity of energy needed for humanmetabolism,using as a standard an average human energy expenditure of 12,500 kJ per day and abasal metabolic rateof 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.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.

Sunlight's radiant energy is also captured by plants aschemical potential energyinphotosynthesis,when carbon dioxide and water (two low-energy compounds) are converted into carbohydrates, lipids, proteins and oxygen. 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 organic molecules are ingested andcatabolismis triggered byenzymeaction.

All living creatures rely on an external source of energy to be able to grow and reproduce – radiant energy from the Sun in the case of green plants and chemical energy (in some form) in the case of animals. The daily 1500–2000Calories(6–8 MJ) recommended for a human adult are taken as food molecules, mostly carbohydrates and fats, of whichglucose(C₆H₁₂O₆) andstearin(C₅₇H₁₁₀O₆) are convenient examples. The food molecules are oxidized tocarbon dioxideandwaterin themitochondria $${\ce {C6H12O6 + 6O2 -> 6CO2 + 6H2O}}$$ $${\ce {C57H110O6 + (81 1/2) O2 -> 57CO2 + 55H2O}}$$ and some of the energy is used to convertADPintoATP:

ADP + HPO₄²⁻→ ATP + H₂O

The rest of the chemical energy of the carbohydrate or fat are converted into heat: the ATP is used as a sort of "energy currency", and some of the chemical energy it contains is used for othermetabolismwhen ATP reacts with OH groups and eventually splits into ADP and phosphate (at each stage of ametabolic pathway,some chemical energy is converted into heat). Only a tiny fraction of the original chemical energy is used forwork:

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: 3 kJ

Daily food intake of a normal adult: 6–8 MJ

It would appear that living organisms are remarkablyinefficient (in the physical sense)in their use of the energy they receive (chemical or radiant energy); mostmachinesmanage 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. Thesecond law of thermodynamicsstates 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" ).Simpler organisms can achieve higher energy efficiencies than more complex ones, but the complex organisms can occupyecological nichesthat 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 inecology.As an example, to take just the first step in thefood chain:of the estimated 124.7 Pg/a of carbon that isfixedbyphotosynthesis,64.3 Pg/a (52%) are used for the metabolism of green plants,i.e. reconverted into carbon dioxide and heat.

Earth sciences

Ingeology,continental drift,mountain ranges,volcanoes,andearthquakesare phenomena that can be explained in terms of energy transformations in the Earth's interior,whilemeteorologicalphenomena like wind, rain,hail,snow, lightning,tornadoesandhurricanesare all a result of energy transformations in ouratmospherebrought about bysolar energy.

Sunlight is the main input toEarth's energy budgetwhich accounts for its temperature and climate stability. Sunlight may be stored as gravitational potential energy after it strikes the Earth, as (for example when) 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 most weather phenomena, save a few exceptions, like those generated by volcanic events for example. An example of a solar-mediated weather event is a hurricane, which occurs when large unstable areas of warm ocean, heated over months, suddenly give up some of their thermal energy to power a few days of violent air movement.

In a slower process,radioactive decayof atoms in the core of the Earth releases heat. This thermal energy drivesplate tectonicsand may lift mountains, viaorogenesis.This slow lifting represents a kind of gravitational potentialenergy storageof the thermal energy, which may later be transformed into active kinetic energy during 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 (which created these atoms).

Cosmology

Incosmology and astronomythe phenomena ofstars,nova,supernova,quasarsandgamma-ray burstsare the universe's highest-output energy transformations of matter. Allstellarphenomena (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). Thenuclear fusionof hydrogen in the Sun also releases another store of potential energy which was created at the time of theBig 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 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.

Quantum mechanics

Main article:Energy operator

Inquantum mechanics,energy is defined in terms of theenergy operator (Hamiltonian) as a time derivative of thewave function.TheSchrödinger equationequates the energy operator to the full energy of a particle or a system. Its results can be considered as a definition of measurement of energy in quantum mechanics. The Schrödinger equation describes the space- and time-dependence of a slowly changing (non-relativistic)wave functionof quantum systems. The solution of this equation for a bound system is discrete (a set of permitted states, each characterized by anenergy level) which results in the concept ofquanta.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 byPlanck's relation:$E=h\nu$(where$h$is thePlanck constantand$\nu$the frequency). In the case of an electromagnetic wave these energy states are called quanta of light orphotons.

Relativity

When calculating kinetic energy (workto accelerate amassive bodyfrom zerospeedto some finite speed) relativistically – usingLorentz transformationsinstead ofNewtonian mechanics– Einstein discovered an unexpected by-product of these calculations to be an energy term which does not vanish at zero speed. He called itrest energy:energy which every massive body must possess even when being at rest. The amount of energy is directly proportional to the mass of the body:

$$E_{0}=m_{0}c^{2},$$ where

• m₀is therest massof the body,
• cis thespeed of lightin vacuum,
• $E_{0}$is the rest energy.

For example, considerelectron–positronannihilation, in which the rest energy of these two individual particles (equivalent to their rest mass) is converted to the radiant energy of the photons produced in the process. In this system thematterandantimatter(electrons and positrons) are destroyed and changed to non-matter (the photons). However, the total mass and total energy do not change during this interaction. The photons each have no rest mass but nonetheless have radiant energy which exhibits the same inertia as did the two original particles. This is a reversible process – the inverse process is calledpair creation– in which the rest mass of particles is created from the radiant energy of two (or more) annihilating photons.

In general relativity, thestress–energy tensorserves 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.

Energy and mass are manifestations of one and the same underlying physical property of a system. This property is responsible for the inertia and strength of gravitational interaction of the system ( "mass manifestations" ), and is also responsible for the potential ability of the system to perform work or heating ( "energy manifestations" ), subject to the limitations of other physical laws.

Inclassical physics,energy is a scalar quantity, thecanonical conjugateto time. Inspecial relativityenergy is also a scalar (although not aLorentz scalarbut a time component of theenergy–momentum 4-vector).In other words, energy is invariant with respect to rotations ofspace,but not invariant with respect to rotations ofspacetime(=boosts).

Transformation

Main article:Energy transformation

Some forms oftransferof energy ( "energy in transit" ) from one object or system to another

Type of transferprocess	Description
Heat	equal amount ofthermal energyin transit spontaneously towards a lower-temperatureobject
Work	equal amount of energy in transit due to a displacement in the direction of an appliedforce
Transfer of material	equal amount of energy carried bymatterthat is moving from one system to another

Aturbo generatortransforms the energy of pressurized steam into electrical energy.

Energy may betransformedbetween different forms at variousefficiencies.Items that transform between these forms are calledtransducers.Examples of transducers include abattery(fromchemical energytoelectric energy), a dam (fromgravitational potential energytokinetic energyof moving water (and the blades of aturbine) and ultimately toelectric energythrough anelectric generator), and aheat engine(from heat to work).

Examples of energy transformation include generatingelectric energyfrom heat energy via a steam turbine, or lifting an object against gravity using electrical energy driving a crane motor. Lifting against gravity performs mechanical work on the object and stores gravitational potential energy in the object. If the object falls to the ground, gravity does mechanical work on the object which transforms the potential energy in the gravitational field to the kinetic energy released as heat on impact with the ground. The Sun transformsnuclear potential energyto other forms of energy; its total mass does not decrease due to that itself (since it still contains the same total energy even in different forms) but its mass does decrease when the energy escapes out to its surroundings, largely asradiant energy.

There are strict limits to how efficiently heat can be converted intoworkin a cyclic process, e.g. in a heat engine, as described byCarnot's theoremand thesecond law of thermodynamics.However, some energy transformations can be quite efficient. The direction of transformations in energy (what kind of energy is transformed to what other kind) is often determined byentropy(equal energy spread among all availabledegrees of freedom) considerations. 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 theBig Bang,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 includenucleosynthesis,a process ultimately using the gravitational potential energy released from thegravitational collapseofsupernovaeto "store" energy in the creation of heavy isotopes (such asuraniumandthorium), andnuclear decay,a process in which energy is released that was originally stored in these heavy elements, before they were incorporated into the Solar System and the Earth. This energy is triggered and released in nuclearfission bombsor in civil nuclear power generation. Similarly, in the case of achemical explosion,chemical potentialenergy is transformed tokineticandthermal energyin a very short time.

Yet another example is that of apendulum.At its highest points thekinetic energyis zero and thegravitational potential energyis at its maximum. At its lowest point thekinetic energyis at its maximum and is equal to the decrease inpotential energy.If one (unrealistically) assumes that there is nofrictionor other losses, the conversion of energy between these processes would be perfect, and thependulumwould continue swinging forever.

Energy is also transferred from potential energy ($E_{p}$) to kinetic energy ($E_{k}$) and then back to potential energy constantly. This is referred to as conservation of energy. In thisisolated 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:

$$E_{pi}+E_{ki}=E_{pF}+E_{kF}$$(4)

The equation can then be simplified further since$E_{p}=mgh$(mass times acceleration due to gravity times the height) and${\textstyle E_{k}={\frac {1}{2}}mv^{2}}$(half mass times velocity squared). Then the total amount of energy can be found by adding$E_{p}+E_{k}=E_{\text{total}}$.

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 inmass–energy equivalence.The formulaE=mc², derived byAlbert Einstein(1905) quantifies the relationship betweenrelativistic massand energy within the concept of special relativity. In different theoretical frameworks, similar formulas were derived byJ.J. Thomson(1881),Henri Poincaré(1900),Friedrich Hasenöhrl(1904) and others (seeMass–energy equivalence#Historyfor further information).

Part of the rest energy (equivalent to rest mass) ofmattermay be converted to other forms of energy (still exhibiting mass), but neither energy nor mass can be destroyed; rather, both remain constant during any process. However, since$c^{2}$is extremely large relative to ordinary human scales, the conversion of an everyday amount of rest mass (for example, 1 kg) from rest energy to other forms of energy (such as kinetic energy, thermal energy, or the radiant energy carried by light and other radiation) can liberate tremendous amounts of energy (~$9\times 10^{16}$joules = 21 megatons of TNT), as can be seen innuclear reactorsand nuclear weapons. Conversely, the mass equivalent of an everyday amount energy is minuscule, which is why a loss of energy (loss of mass) from most systems is difficult to measure on a weighing scale, unless the energy loss is very large. Examples of large transformations between rest energy (of matter) and other forms of energy (e.g., kinetic energy into particles with rest mass) are found innuclear physicsandparticle physics.Often, however, the complete conversion of matter (such as atoms) to non-matter (such as photons) is forbidden byconservation laws.

Reversible and non-reversible transformations

Thermodynamics divides energy transformation into two kinds:reversible processesandirreversible processes.An irreversible process is one in which 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 thermal energy 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).

As the universe evolves with time, more and more of its energy becomes trapped in irreversible states (i.e., as heat or as other kinds of increases in disorder). This has led to the hypothesis of the inevitable thermodynamicheat 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 aheat engine,or be transformed to other usable forms of energy (through the use of generators attached to heat engines), continues to decrease.

Conservation of energy

Main article:Conservation of energy

The fact that energy can be neither created nor destroyed is called the law ofconservation of energy.In the form of thefirst law of thermodynamics,this states that aclosed system's energy is constant unless energy is transferred in or out asworkorheat,and that no energy is lost in transfer. 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. 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.

While heat can always be fully converted into work in a reversible isothermal expansion of an ideal gas, for cyclic processes of practical interest inheat enginesthesecond law of thermodynamicsstates that the system doing work always loses some energy aswaste heat.This creates a limit to the amount of heat energy that can do work in a cyclic process, a limit called theavailable energy.Mechanical and other forms of energy can be transformed in the other direction intothermal energywithout such limitations.The total energy of a system can be calculated by adding up all forms of energy in the system.

Richard Feynmansaid during a 1961 lecture:

There is a fact, or if you wish, alaw,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 theconservation 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

Most kinds of energy (with gravitational energy being a notable exception)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.

This law is a fundamental principle of physics. As shown rigorously byNoether's theorem,the conservation of energy is a mathematical consequence oftranslational symmetryof 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 iscanonical conjugateto 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 (though this is practically significant only for very short time intervals). The uncertainty principle should not be confused withenergy 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) appear as systemmass,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.

Inquantum mechanicsenergy is expressed using theHamiltonian operator.On any time scales, the uncertainty in the energy is by

$\Delta E\Delta t\geq {\frac {\hbar }{2}}$

which is similar in form to theHeisenberg Uncertainty Principle(but not really mathematically equivalent thereto, sinceHandtare not dynamically conjugate variables, neither in classical nor in quantum mechanics).

Inparticle physics,this inequality permits a qualitative understanding ofvirtual particles,which carrymomentum.The exchange of virtual particles with real particles is responsible for the creation of all knownfundamental forces(more accurately known asfundamental interactions).Virtual photonsare also responsible for the electrostatic interaction betweenelectric charges(which results inCoulomb's law), forspontaneousradiative decay of excited atomic and nuclear states, for theCasimir force,for theVan der Waals forceand some other observable phenomena.

Energy transfer

"Energy transfer" redirects here. For the pipeline company, seeEnergy Transfer Partners.

Closed systems

Energy transfer can be considered for the special case of systems which areclosedto transfers of matter. The portion of the energy which is transferred byconservative forcesover a distance is measured as theworkthe source system does on the receiving system. The portion of the energy which does not do work during the transfer is calledheat.Energy can be transferred between systems in a variety of ways. Examples include the transmission ofelectromagnetic energyvia photons, physical collisions which transferkinetic energy,tidal interactions,and the conductive transfer ofthermal energy.

Energy is strictly conserved and is also locally conserved wherever it can be defined. In thermodynamics, for closed systems, the process of energy transfer is described by thefirst law:

$\Delta {}E=W+Q$(1)

where$E$is the amount of energy transferred,$W$represents the work done on or by the system, and$Q$represents the heat flow into or out of the system. As a simplification, the heat term,$Q$,can sometimes be ignored, especially for fast processes involving gases, which are poor conductors of heat, or when thethermal efficiencyof the transfer is high. For suchadiabatic processes,

$\Delta {}E=W$(2)

This simplified equation is the one used to define thejoule,for example.

Open systems

Beyond the constraints of closed systems,open systemscan gain or lose energy in association with matter transfer (this process is illustrated by injection of an air-fuel mixture into a car engine, a system which gains in energy thereby, without addition of either work or heat). Denoting this energy by$E_{\text{matter}}$,one may write

$\Delta E=W+Q+E_{\text{matter}}.$(3)

Thermodynamics

Internal energy

Internal energyis 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.

First law of thermodynamics

Thefirst law of thermodynamicsasserts that the total energy of a system and its surroundings (but not necessarilythermodynamic free energy) is always conservedand 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 topressureforces and heat transfer (e.g., a cylinder-full of gas) without chemical changes, the differential change in the internal energy of the system (with againin energy signified by a positive quantity) is given as

$\mathrm {d} E=T\mathrm {d} S-P\mathrm {d} V\,$,

where the first term on the right is the heat transferred into the system, expressed in terms oftemperatureTandentropyS(in which entropy increases and its change dSis positive when heat is added to the system), and the last term on the right hand side is identified as work done on the system, where pressure isPand volumeV(the negative sign results since compression of the system requires work to be done on it and so the volume change, dV,is negative when work is done on the system).

This equation is highly specific, ignoring all chemical, electrical, nuclear, and gravitational forces, effects such asadvectionof any form of energy other than heat andPV-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 aclosedsystem is expressed in a general form by

$\mathrm {d} E=\delta Q+\delta W$

where$\delta Q$is the heat supplied to the system and$\delta W$is the work applied to the system.

Equipartition of energy

The energy of a mechanicalharmonic oscillator(a mass on a spring) is alternatelykineticandpotential energy.At two points in the oscillationcycleit is entirely kinetic, and at two points it is entirely potential. Over a whole cycle, or over many cycles, average energy is equally split between kinetic and potential. This is an example of theequipartition principle:the total energy of a system with many degrees of freedom is equally split among all available degrees of freedom, on average.

This principle is vitally important to understanding the behavior of a quantity closely related to energy, calledentropy.Entropy is a measure of evenness of adistributionof energy between parts of a system. When an isolated system is given more degrees of freedom (i.e., given new availableenergy statesthat 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 part of thesecond law of thermodynamics.The second law of thermodynamics is simple only for systems which are near or in a physicalequilibrium state.For non-equilibrium systems, the laws governing the systems' behavior are still debatable. One of the guiding principles for these systems is the principle ofmaximum entropy production.It states that nonequilibrium systems behave in such a way as to maximize their entropy production.

See also

• Combustion
• Efficient energy use
• Energy democracy
• Energy crisis
• Energy recovery
• Energy recycling
• Index of energy articles
• Index of wave articles
• List of low-energy building techniques
• Orders of magnitude (energy)
• Power station
• Sustainable energy
• Transfer energy
• Waste-to-energy
• Waste-to-energy plant
• Zero-energy building

Further reading

• Alekseev, G.N. (1986).Energy and Entropy.Moscow: Mir Publishers.
• TheBiosphere(AScientific AmericanBook), San Francisco, W.H. Freeman and Co., 1970,ISBN0-7167-0945-7.This book, originally a 1970Scientific Americanissue, covers virtually every major concern and concept since debated regarding materials andenergy resources,populationtrends, andenvironmental degradation.
• Crowell, Benjamin (2011)."ch. 11".Light and Matter.Fullerton, California: Light and Matter.Archivedfrom the original on 2011-05-19.Retrieved2017-04-12.
• Energy and Power(AScientific AmericanBook), San Francisco, W.H. Freeman and Co., 1971,ISBN0-7167-0938-4.
• Ross, John S. (23 April 2002)."Work, Power, Kinetic Energy"(PDF).Project PHYSNET.Michigan State University.Archived(PDF)from the original on 26 April 2011.Retrieved10 April2009.
• Santos, Gildo M. "Energy in Brazil: a historical overview,"The Journal of Energy History(2018),onlineArchived2019-02-09 at theWayback Machine
• Smil, Vaclav (2008).Energy in nature and society: general energetics of complex systems.Cambridge, US: MIT Press.ISBN978-0-262-19565-2.
• Walding, Richard; Rapkins, Greg; Rossiter, Glenn (1999).New Century Senior Physics.Melbourne, Australia: Oxford University Press.ISBN978-0-19-551084-3.

Journals

• The Journal of Energy History / Revue d'histoire de l'énergie(JEHRHE), 2018–

External links

• EnergyatCurlie
• Differences between Heat and Thermal energy(Archived2016-08-27 at theWayback Machine) – BioCab