Thermo Dyna Ics

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Thermodynamics describes how systems change when they interact with one another or

with their surroundings. This can be applied to a wide variety of topics in science and

engineering, such as engines,phase transitions,chemical reactions, transport phenomena,and evenblack holes. The results of thermodynamics are essential for other fields of

physicsand forchemistry,chemical engineering, aerospace engineering,mechanical

engineering,cell biology,biomedical engineering, materials science, and are useful forother fields such aseconomics.[13][14]

Many of the empirical facts of thermodynamics are comprehended in its fourlaws. The

first law specifies that energy can be exchanged between physical systems as heat and

thermodynamic work.[15]The second law concerns a quantity calledentropy, that expresseslimitations, arising from what is known as irreversibility, on the amount of thermodynamic

work that can be delivered to an external system by a thermodynamic process. [16] Many

writers offer various axiomatic formulations of thermodynamics, as if it were a completedsubject, but non-equilibrium processes continue to make difficulties for it.

[edit] Introduction

The plain term 'thermodynamics' refers to macroscopic description of bodies and processes.[17] "Any reference to atomic constitution is foreign to ... thermodynamics".[18] The qualifiedterm 'statistical thermodynamics' refers to descriptions of bodies and processes in terms of

the atomic constitution of matter.

Thermodynamics is built on the study of energy transfers that can be strictly resolved into

two distinct components, heat andwork, specified by macroscopic variables.[19][20]

Thermodynamic equilibrium is one of the most important concepts for thermodynamics.

Thermodynamics is well understood and validated for systems in thermodynamic

equilibrium, but as the systems and processes of interest are taken further and further from

thermodynamic equilibrium, their thermodynamical study becomes more and moredifficult. Systems in thermodynamic equilibrium have very well experimentally

reproducible behaviour, and as interest moves further towards non-equilibrium systems,

experimental reproducibility becomes more difficult. The present article takes a gradualapproach to the subject, starting with a focus on cyclic processes and thermodynamic

equilibrium, and then gradually beginning to further consider non-equilibrium systems.

For thermodynamics and statistical thermodynamics to apply to a process in a body, it isnecessary that the atomic mechanisms of the process fall into just two classes: those sorapid that, in the time frame of the process of interest, the atomic states effectively visit all

of their accessible range, and those so slow that their effects can be neglected in the time

frame of the process of interest.[21]The rapid atomic mechanisms mediate the macroscopicchanges that are of interest for thermodynamics and statistical thermodynamics, because

they quickly bring the system near enough to thermodynamic equilibrium. "When

intermediate rates are present, thermodynamics and statistical mechanics cannot be
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applied."[21] The intermediate rate atomic processes do not bring the system near enough to

thermodynamic equilibrium in the time frame of the macroscopic process of interest. This

separation of time scales of atomic processes is a theme that recurs throughout the subject.

Basic for thermodynamics are the concepts ofsystem andsurroundings.[8][22]

There are two fundamental kinds of entity in thermodynamics, states of a system, and

processes of a system. This allows two fundamental approaches to thermodynamic

reasoning, that in terms of states of a system, and that in terms of cyclic processes of asystem.

A thermodynamic system can be defined in terms of its states. In this way, a

thermodynamic system is amacroscopic physical object, explicitly specified in terms of

macroscopic physical and chemical variables which describe its macroscopic properties.The macroscopic state variables of thermodynamics have been recognized in the course of

empirical work in physics and chemistry.[9]

A thermodynamic system can also be defined in terms of the processes which it can

undergo. Of particular interest are cyclic processes. This was the way of the founders ofthermodynamics in the first three quarters of the nineteenth century.

The surroundings of a thermodynamic system are other thermodynamic systems that can

interact with it. An example of a thermodynamic surrounding is a heat bath, which is

considered to be held at a prescribed temperature, regardless of the interactions it mighthave with the system.

The macroscopic variables of a thermodynamic system can under some conditions be

related to one another through equations of state. They express the constitutive peculiaritiesof the material of the system. Classical thermodynamics is characterized by its study ofmaterials that have equations of state that express relations between mechanical variables

and temperature that are reached much more rapidly than any changes in the surroundings.

A classical material can usually be described by a function that makes pressure dependenton volume and temperature, the resulting pressure being established much more rapidly

than any imposed change of volume or temperature.[23]

Thermodynamic facts can often be explained by viewing macroscopic objects as

assemblies of very many microscopic oratomic objects that obey Hamiltonian dynamics.[8][24][25] The microscopic or atomic objects exist in species, the objects of each species being

all alike. Because of this likeness, statistical methods can be used to account for themacroscopic properties of the thermodynamic system in terms of the properties of themicroscopic species. Such explanation is called statistical thermodynamics; also often it is

also referred to by the term 'statistical mechanics', though this term can have a wider

meaning, referring to 'microscopic objects', such as economic quantities, that do not obeyHamiltonian dynamics.[24]
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The thermodynamicists representative of the original eight founding schools of

thermodynamics. The schools with the most-lasting effect in founding the modern versionsof thermodynamics are the Berlin school, particularly as established in Rudolf Clausiuss

1865 textbookThe Mechanical Theory of Heat, the Vienna school, with the statistical

mechanics ofLudwig Boltzmann, and the Gibbsian school at Yale University, AmericanengineerWillard Gibbs' 1876 On the Equilibrium of Heterogeneous Substances launching

chemical thermodynamics.[26]

[edit] History

The history of thermodynamics as a scientific discipline generally begins with Otto vonGuericke who, in 1650, built and designed the world's first vacuum pump and

demonstrated avacuum using his Magdeburg hemispheres. Guericke was driven to make a

vacuum in order to disproveAristotle's long-held supposition that 'nature abhors a vacuum'.Shortly after Guericke, the English physicist and chemistRobert Boyle had learned of

Guericke's designs and, in 1656, in coordination with English scientist Robert Hooke, built

an air pump.[27] Using this pump, Boyle and Hooke noticed a correlation betweenpressure,

temperature, and volume. In time,Boyle's Lawwas formulated, which states that pressure

and volume areinversely proportional. Then, in 1679, based on these concepts, anassociate of Boyle's named Denis Papinbuilt a steam digester, which was a closed vessel

with a tightly fitting lid that confined steam until a high pressure was generated.

Later designs implemented a steam release valve that kept the machine from exploding. Bywatching the valve rhythmically move up and down, Papin conceived of the idea of a

piston and a cylinder engine. He did not, however, follow through with his design.

Nevertheless, in 1697, based on Papin's designs, engineerThomas Savery built the first
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engine, followed by Thomas Newcomen in 1712. Although these early engines were crude

and inefficient, they attracted the attention of the leading scientists of the time.

The fundamental concepts ofheat capacity andlatent heat, which were necessary for thedevelopment of thermodynamics, were developed by ProfessorJoseph Blackat the

University of Glasgow, where James Wattwas employed as an instrument maker. Wattconsulted with Black in order to conduct experiments on his steam engine, but it was Watt

who conceived the idea of the external condenserwhich resulted in a large increase insteam engine efficiency.[28] Drawing on all the previous work ledSadi Carnot, the "father of

thermodynamics", to publishReflections on the Motive Power of Fire (1824), a discourse

on heat, power, energy and engine efficiency. The paper outlined the basic energeticrelations between the Carnot engine, the Carnot cycle, and motive power. It marked the

start of thermodynamics as a modern science.[11]

The first thermodynamic textbook was written in 1859 by William Rankine, originally

trained as a physicist and a civil and mechanical engineering professor at theUniversity of

Glasgow.

[29]

The first and second laws of thermodynamics emerged simultaneously in the1850s, primarily out of the works ofWilliam Rankine, Rudolf Clausius, andWilliam

Thomson (Lord Kelvin).

The foundations of statistical thermodynamics were set out by physicists such as JamesClerk Maxwell, Ludwig Boltzmann, Max Planck,Rudolf Clausiusand J. Willard Gibbs.

During the years 187376 the American mathematical physicist Josiah Willard Gibbs

published a series of three papers, the most famous being On the Equilibrium ofHeterogeneous Substances,[4]in which he showed how thermodynamic processes,

includingchemical reactions, could be graphically analyzed, by studying the energy,

entropy, volume, temperature andpressureof the thermodynamic system in such a manner,one can determine if a process would occur spontaneously. [30] Also Pierre Duhem in the19th century wrote about chemical thermodynamics.[5] During the early 20th century,

chemists such asGilbert N. Lewis, Merle Randall,[6] and E. A. Guggenheim[7] [8] applied the

mathematical methods of Gibbs to the analysis of chemical processes.

[edit] Etymology

The etymology ofthermodynamics has an intricate history.[31] It was first spelled in ahyphenated form as an adjective (thermo-dynamic) and from 1854 to 1868 as the noun

thermo-dynamics to represent the science of generalized heat engines.[31]

The components of the word thermodynamics are derived from theGreekwords

therme, meaning heat, anddynamis, meaning power.[32][33][34]

Pierre Perrot claims that the term thermodynamics was coined byJames Joule in 1858 to

designate the science of relations between heat and power.[11] Joule, however, never used

that term, but used instead the termperfect thermo-dynamic engine in reference to

Thomsons 1849[35] phraseology.[31]
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rot-10%23cite_note-Perrot-10http://en.wikipedia.org/wiki/Thermodynamics#cite_note-kelvin1849-34%23cite_note-kelvin1849-34http://en.wikipedia.org/wiki/Thermodynamics#cite_note-eoht-30%23cite_note-eoht-30http://en.wikipedia.org/wiki/Thomas_Newcomenhttp://en.wikipedia.org/wiki/Heat_capacityhttp://en.wikipedia.org/wiki/Latent_heathttp://en.wikipedia.org/wiki/Joseph_Blackhttp://en.wikipedia.org/wiki/James_Watthttp://en.wikipedia.org/wiki/Watt_steam_engine#Separate_condenserhttp://en.wikipedia.org/wiki/Steam_enginehttp://en.wikipedia.org/wiki/Thermodynamics#cite_note-27%23cite_note-27http://en.wikipedia.org/wiki/Nicolas_L%C3%A9onard_Sadi_Carnothttp://en.wikipedia.org/wiki/Reflections_on_the_Motive_Power_of_Firehttp://en.wikipedia.org/wiki/Carnot_enginehttp://en.wikipedia.org/wiki/Carnot_cyclehttp://en.wikipedia.org/wiki/Motive_powerhttp://en.wikipedia.org/wiki/Thermodynamics#cite_note-Perrot-10%23cite_note-Perrot-10http://en.wikipedia.org/wiki/William_John_Macquorn_Rankinehttp://en.wikipedia.org/wiki/University_of_Glasgowhttp://en.wikipedia.org/wiki/University_of_Glasgowhttp://en.wikipedia.org/wiki/Thermodynamics#cite_note-28%23cite_note-28http://en.wikipedia.org/wiki/William_John_Macquorn_Rankinehttp://en.wikipedia.org/wiki/Rudolf_Clausiushttp://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvinhttp://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvinhttp://en.wikipedia.org/wiki/James_Clerk_Maxwellhttp://en.wikipedia.org/wiki/James_Clerk_Maxwellhttp://en.wikipedia.org/wiki/Ludwig_Boltzmannhttp://en.wikipedia.org/wiki/Max_Planckhttp://en.wikipedia.org/wiki/Rudolf_Clausiushttp://en.wikipedia.org/wiki/Josiah_Willard_Gibbshttp://en.wikipedia.org/wiki/Josiah_Willard_Gibbshttp://en.wikipedia.org/wiki/On_the_Equilibrium_of_Heterogeneous_Substanceshttp://en.wikipedia.org/wiki/On_the_Equilibrium_of_Heterogeneous_Substanceshttp://en.wikipedia.org/wiki/Thermodynamics#cite_note-Gibbs_1876-3%23cite_note-Gibbs_1876-3http://en.wikipedia.org/wiki/Thermodynamic_processeshttp://en.wikipedia.org/wiki/Chemical_reactionhttp://en.wikipedia.org/wiki/Energyhttp://en.wikipedia.org/wiki/Entropyhttp://en.wikipedia.org/wiki/Volume_(thermodynamics)http://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Pressurehttp://en.wikipedia.org/wiki/Thermodynamic_systemhttp://en.wikipedia.org/wiki/Thermodynamics#cite_note-29%23cite_note-29http://en.wikipedia.org/wiki/Pierre_Duhemhttp://en.wikipedia.org/wiki/Thermodynamics#cite_note-Duhem_1886-4%23cite_note-Duhem_1886-4http://en.wikipedia.org/wiki/Gilbert_N._Lewishttp://en.wikipedia.org/wiki/Merle_Randallhttp://en.wikipedia.org/wiki/Thermodynamics#cite_note-Lewis_Randall_1923-5%23cite_note-Lewis_Randall_1923-5http://en.wikipedia.org/wiki/E._A._Guggenheimhttp://en.wikipedia.org/wiki/Thermodynamics#cite_note-Guggenheim_1933-6%23cite_note-Guggenheim_1933-6http://en.wikipedia.org/wiki/Thermodynamics#cite_note-Guggenheim_1949.2F1967-7%23cite_note-Guggenheim_1949.2F1967-7http://en.wikipedia.org/w/index.php?title=Thermodynamics&action=edit&section=3http://en.wikipedia.org/wiki/Thermodynamics#cite_note-eoht-30%23cite_note-eoht-30http://en.wikipedia.org/wiki/Thermodynamics#cite_note-eoht-30%23cite_note-eoht-30http://en.wikipedia.org/wiki/Greek_languagehttp://en.wiktionary.org/wiki/%CE%B8%CE%AD%CF%81%CE%BC%CE%B7http://en.wiktionary.org/wiki/%CE%B4%CF%8D%CE%BD%CE%B1%CE%BC%CE%B9%CF%82http://en.wikipedia.org/wiki/Thermodynamics#cite_note-31%23cite_note-31http://en.wikipedia.org/wiki/Thermodynamics#cite_note-32%23cite_note-32http://en.wikipedia.org/wiki/Thermodynamics#cite_note-33%23cite_note-33http://en.wikipedia.org/wiki/James_Joulehttp://en.wikipedia.org/wiki/Thermodynamics#cite_note-Perrot-10%23cite_note-Perrot-10http://en.wikipedia.org/wiki/Thermodynamics#cite_note-kelvin1849-34%23cite_note-kelvin1849-34http://en.wikipedia.org/wiki/Thermodynamics#cite_note-eoht-30%23cite_note-eoht-30


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By 1858, thermo-dynamics, as a functional term, was used inWilliam Thomson's paperAn

Account of Carnot's Theory of the Motive Power of Heat.[35]

[edit] Branches of description

The study of thermodynamical systems has developed into several related branches, eachusing a different fundamental model as a theoretical or experimental basis, or applying the

principles to varying types of systems.

[edit] Classical thermodynamics

Classical thermodynamics is the description of the states (especially equilibrium states) and

processes of thermodynamical systems, using macroscopic, empirical properties directlymeasurable in the laboratory. It is used to model exchanges of energy, work, heat, and

matter, based on the laws of thermodynamics. The qualifierclassicalreflects the fact that it

represents the descriptive level in terms of macroscopic empirical parameters that can be

measured in the laboratory, that was the first level of understanding in the 19th century. Amicroscopic interpretation of these concepts was provided by the development of statistical

thermodynamics.

[edit] Statistical thermodynamics

Statistical thermodynamics, also called statistical mechanics, emerged with the

development of atomic and molecular theories in the second half of the 19th century andearly 20th century, supplementing thermodynamics with an interpretation of the

microscopic interactions between individual particles or quantum-mechanical states. This

field relates the microscopic properties of individual atoms and molecules to themacroscopic, bulk properties of materials that can be observed on the human scale, thereby

explaining thermodynamics as a natural result of statistics, classical mechanics, and

quantum theory at the microscopic level.

[edit] Chemical thermodynamics

Chemical thermodynamics is the study of the interrelation ofenergy with chemical

reactions and chemical transport and with physical changes ofstate within the confines ofthe laws of thermodynamics.

[edit] Thermodynamic equilibriumEquilibrium thermodynamics studies transformations of matter and energy in systems as

they approach equilibrium. The equilibrium means balance. In a thermodynamicequilibrium state there is no macroscopic flow and no macroscopic change is occurring or

can be triggered; within the system, every microscopic process is balanced by its opposite;

this is called the principle of detailed balance. A central aim in equilibriumthermodynamics is: given a system in a well-defined initial state, subject to accurately
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specified constraints, to calculate what the state of the system will be once it has reached

equilibrium. A thermodynamic system is said to be homogeneous when all its locally

defined intensive variables are spatially invariant. A simple system in thermodynamicequilibrium is homogeneous unless it is affected by a time-invariant externally imposed

field of force, such as gravity, electricity, or magnetism.

Most systems found in nature are not in thermodynamic equilibrium, exactly considered;

for they are changing or can be triggered to change over time, and are continuously anddiscontinuously subject to flux of matter and energy to and from other systems. [36] For

example, according to Callen, "in absolute thermodynamic equilibrium all radioactive

materials would have decayed completely and nuclear reactions would have transmuted allnuclei to the most stable isotopes. Such processes, which would take cosmic times to

complete, generally can be ignored.".[36] Such processes being ignored, many systems in

nature are nearly enough in thermodynamic equilibrium that for many purposes theirbehaviour can be nearly enough approximated by equilibrium calculations. This is an

example of the notion of separation of time scales mentioned above. Hystersis can

sometimes be explained in terms of more or less fixed more or less microscopicinhomogeneities in solid materials. According to Adkins: "It is precisely these

inhomogeneities which preven the attainment of the true equilibrium within any normal

period of time."[37]

For their thermodynamic study, systems that are further from thermodynamic equilibriumcall for more general ideas.Non-equilibrium thermodynamics is a branch of

thermodynamics that deals with systems that are not in thermodynamic equilibrium. Non-

equilibrium thermodynamics is distinct because it is concerned with transport processes

and with the rates of chemical reactions.[38] Non-equilibrium systems can be in stationarystates that are not homogeneous even when there is no externally imposed field of force; in

this case, the description of the internal state of the system requires a field theory.

[39][40][41]

One of the methods of dealing with non-equilibrium systems is to introduce so-called'internal variables'. These are quantities that express the local state of the system, besides

the usual local thermodynamic variables; in a sense such variables might be seen as

expressing the 'memory' of the materials. Hysteresis may sometimes be described in thisway. In contrast to the usual thermodynamic variables, 'internal variables' cannot be

controlled by external manipulations.[42] This approach is usually unnecessary for gases and

liquids, but may be useful for solids.[43]Many natural systems still today remain beyond thescope of currently known macroscopic thermodynamic methods.

[edit] Laws of thermodynamics

Main article: Laws of thermodynamics

Thermodynamics states a set of four laws which are valid for all systems that fall within the

constraints implied by each. In the various theoretical descriptions of thermodynamicsthese laws may be expressed in seemingly differing forms, but the most prominent

formulations are the following:
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Zeroth law of thermodynamics:If two systems are each in thermal equilibrium with

a third, they are also in thermal equilibrium with each other.

This statement implies that thermal equilibrium is anequivalence relation on the set ofthermodynamic systems under consideration. Systems are said to be in thermal equilibrium

with each other if spontaneous molecular thermal energy exchanges between them do notlead to a net exchange of energy. This law is tacitly assumed in every measurement of

temperature. For two bodies known to be at the sametemperature, if one seeks to decide ifthey will be in thermal equilibrium when put into thermal contact, it is not necessary to

actually bring them into contact and measure any changes of their observable properties in

time.[44]In traditional statements, the law provides an empirical definition of temperatureand justification for the construction of practical thermometers. In contrast to absolute

thermodynamic temperatures, empirical temperatures are measured just by the mechanical

properties of bodies, such as their volumes, without reliance on the concepts of energy,entropy or the first, second, or third laws of thermodynamics.[45][46] Empirical temperatures

lead tocalorimetry for heat transfer in terms of the mechanical properties of bodies,

without reliance on mechanical concepts of energy.

The zeroth law was not initially labelled as a law, as its basis in thermodynamicalequilibrium was implied in the other laws. By the time the desire arose to label it as a law,

the first, second, and third laws had already been explicitly stated. It was impracticable to

renumber them, and so it was numbered thezeroth law.

First law of thermodynamics:A change in the internal energy of a closedthermodynamic system is equal to the difference between theheatsupplied to the

system and the amount ofworkdone by the system on its surroundings.[47][48][49][50][51][52]

[53][54][55][56]

The first law of thermodynamics asserts the existence of a state variable for a system, theinternal energy, and tells how it changes in thermodynamic processes. The law allows a

given internal energy of a system to be reached by any combination of heat and work. It is

important that internal energy is a variable of state of the system (seeThermodynamicstate) whereas heat and work are variables that describe processes or changes of the state of

systems.

The first law observes that the internal energy of an isolated system obeys the principle of

conservation of energy, which states that energy can be transformed (changed from oneform to another), but cannot be created or destroyed.[57]

Second law of thermodynamics:Heat cannot spontaneously flow from a colder

location to a hotter location.

The second law of thermodynamics is an expression of the universal principle ofdissipation of kinetic and potential energy observable in nature. The second law is an

observation of the fact that over time, differences in temperature, pressure, and chemical

potential tend to even out in a physical system that is isolated from the outside world.
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Entropy is a measure of how much this process has progressed. The entropy of an isolated

system which is not in equilibrium will tend to increase over time, approaching a maximum

value at equilibrium.

In classical thermodynamics, the second law is a basic postulate applicable to any system

involving heat energy transfer; in statistical thermodynamics, the second law is aconsequence of the assumed randomness of molecular chaos. There are many versions of

the second law, but they all have the same effect, which is to explain the phenomenon ofirreversibility in nature.

Third law of thermodynamics:As a system approaches absolute zero, all processes

cease and the entropy of the system approaches a minimum value.

The third law of thermodynamics is a statistical law of nature regarding entropy and theimpossibility of reachingabsolute zero of temperature. This law provides an absolute

reference point for the determination of entropy. The entropy determined relative to this

point is the absolute entropy. Alternate definitions are, "the entropy of all systems and ofall states of a system is smallest at absolute zero," or equivalently "it is impossible to reach

the absolute zero of temperature by any finite number of processes".

Absolute zero, at which all activity (with the exception of that caused by zero point energy)

would stop is 273.15 C (degrees Celsius), or 459.67 F (degrees Fahrenheit) or 0 K(kelvin).

[edit] System models

A diagram of a generic thermodynamic system

An important concept in thermodynamics is the thermodynamic system, a precisely defined

region of the universe under study. Everything in the universe except the system is known

as thesurroundings. A system is separated from the remainder of the universe by aboundary which may be notional or not, but which by convention delimits a finite volume.

Exchanges ofwork, heat, ormatterbetween the system and the surroundings take place

across this boundary.
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In practice, the boundary is simply an imaginary dotted line drawn around a volume when

there is going to be a change in the internal energy of that volume. Anything that passes

across the boundary that effects a change in the internal energy needs to be accounted for inthe energy balance equation. The volume can be the region surrounding a single atom

resonating energy, such as Max Planckdefined in 1900; it can be a body of steam or air in

a steam engine, such as Sadi Carnot defined in 1824; it can be the body of a tropicalcyclone, such as Kerry Emanuel theorized in 1986 in the field ofatmospheric

thermodynamics; it could also be just one nuclide (i.e. a system ofquarks) as hypothesized

in quantum thermodynamics.

Boundaries are of four types: fixed, moveable, real, and imaginary. For example, in anengine, a fixed boundary means the piston is locked at its position; as such, a constant

volume process occurs. In that same engine, a moveable boundary allows the piston to

move in and out. For closed systems, boundaries are real while for open system boundariesare often imaginary.

Generally, thermodynamics distinguishes three classes of systems, defined in terms of whatis allowed to cross their boundaries:

Interactions of thermodynamic systems

Type of system Mass flow WorkHeat

Open

Closed

Isolated

As time passes in an isolated system, internal differences in the system tend to even out andpressures and temperatures tend to equalize, as do density differences. A system in which

all equalizing processes have gone to completion is considered to be in astate ofthermodynamic equilibrium.

In thermodynamic equilibrium, a system's properties are, by definition, unchanging in time.Systems in equilibrium are much simpler and easier to understand than systems which are

not in equilibrium. Often, when analyzing a thermodynamic process, it can be assumed that

each intermediate state in the process is at equilibrium. This will also considerably simplifythe situation. Thermodynamic processes which develop so slowly as to allow each

intermediate step to be an equilibrium state are said to bereversible processes.

[edit] States and processesThere are two fundamental kinds of entity in thermodynamics, states of a system, andprocesses of a system. This allows two fundamental approaches to thermodynamic

reasoning, that in terms of states of a system, and that in terms of cyclic processes of a

system.
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The approach through states of a system requires a full account of the state of the system as

well as a notion of process from one state to another of a system, but may require only a

partial account of the state of the surroundings of the system or of other systems.

The notion of a cyclic process does not require a full account of the state of the system, but

does require a full account of how the process occasions transfers of matter and energybetween the system and its surroundings, which must include at least two heat reservoirs at

different temperatures, one hotter than the other. In this approach, the notion of a properlynumerical scale of temperature is a presupposition of thermodynamics, not a notion

constructed by or derived from it.

[edit] Thermodynamic state variables

When a system is at thermodynamic equilibrium under a given set of conditions of its

surroundings, it is said to be in a definite thermodynamic state, which is fully described byits state variables.

Thermodynamic state variables are of two kinds,extensive and intensive.[8][24] Examples of

extensive thermodynamic variables are total mass and total volume. Examples of intensive

thermodynamic variables aretemperature, pressure, and chemical concentration; intensivethermodynamic variables are defined at each spatial point and each instant of time in a

system. Physical macroscopic variables can be mechanical or thermal.[24] Temperature is a

thermal variable; according to Guggenheim, "the most important conception inthermodynamics is temperature."[8]

If a system is in thermodynamic equilibrium and is not subject to an externally imposed

force field, such as gravity, electricity, or magnetism, then (subject to a proviso stated in

the following sentence) it is homogeneous, that is say, spatially uniform in all respects.[58]

There is a proviso here; a system in thermodynamic equilibrium can be inhomogeneous in

the following respect: it can consist of several so-called 'phases', each homogeneous in

itself, in immediate contiguity with other phases of the system, but distinguishable by theirhaving various respectively different physical characters; a mixture of different chemical

species is considered homogeneous for this purpose if it is physically homogeneous.[59] For

example, a vessel can contain a system consisting of water vapour overlying liquid water;then there is a vapour phase and a liquid phase, each homogeneous in itself, but still in

thermodynamic equilibrium with the other phase. For the immediately present account,

systems with multiple phases are not considered, though for many thermodynamic

questions, multiphase systems are important.

In a sense, a homogeneous system can be regarded as spatially zero-dimensional, because it

has no spatial variation.

If a system in thermodynamic equilibrium is homogeneous, then its state can be described

by a number ofintensive variablesand extensive variables.[41][60][61]
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Intensive variables are defined by the property that if any number of systems, each in its

own separate homogeneous thermodynamic equilibrium state, all with the same respective

values of all of their intensive variables, regardless of the values of their extensivevariables, are laid contiguously with no partition between them, so as to form a new

system, then the values of the intensive variables of the new system are the same as those

of the separate constituent systems. Such a composite system is in a homogeneousthermodynamic equilibrium. Examples of intensive variables are temperature, chemical

concentration, pressure, density of mass, density of internal energy, and, when it can be

properly defined, density of entropy.[62]

Extensive variables are defined by the property that if any number of systems, regardless oftheir possible separate thermodynamic equilibrium or non-equilibrium states or intensive

variables, are laid side by side with no partition between them so as to form a new system,

then the values of the extensive variables of the new system are the sums of the values ofthe respective extensive variables of the individual separate constituent systems.

Obviously, there is no reason to expect such a composite system to be in in a homogeneous

thermodynamic equilibrium. Examples of extensive variables are mass, volume, andinternal energy. They depend on the total quantity of mass in the system. [63]

Though, when it can be properly defined, density of entropy is an intensive variable,

entropy itself does not fit into this classification of state variables. [64] The reason is that

entropy is a property of a system as a whole, and not necessarily related simply to itsconstituents separately. It is true that for any number of systems each in its own separate

homogeneous thermodynamic equilibrium, all with the same values of intensive variables,

removal of the partitions between the separate systems results in a composite homogeneous

system in thermodynamic equilibrium, with all the values of its intensive variables thesame as those of the constituent systems, and it is reservedly or conditionally true that the

entropy of such a restrictively defined composite system is the sum of the entropies of theconstituent systems. But if the constituent systems do not satisfy these restrictiveconditions, the entropy of a composite system cannot be expected to be the sum of the

entropies of the constituent systems, because the entropy is a property of the composite

system as a whole. Therefore, though under these restrictive reservations, entropy satisfiessome requirements for extensivity defined just above, entropy in general does not fit the

above definition of an extensive variable.

Being neither an intensive variable nor an extensive variable according to the above

definition, entropy is thus a stand-out variable, because it is a state variable of a system as awhole.[64] A non-equilibrium system can have a very inhomogeneous dynamical structure.

This is one reason for distinguishing the study of equilibrium thermodynamics from the

study of non-equilibrium thermodynamics.

The physical reason for the existence of extensive variables is the time-invariance ofvolume in a given inertial reference frame, and the strictly local conservation of mass,

momentum, angular momentum, and energy. As noted by Gibbs, entropy is unlike energy

and mass, because it is not locally conserved. [64] The stand-out quantity entropy is neverconserved in real physical processes; all real physical processes are irreversible. [65]The
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motion of planets seems reversible on a short time scale (millions of years), but their

motion, according toNewton's laws, is mathematically an example ofdeterministic chaos.

Eventually a planet will suffer an unpredictable collision with an object from itssurroundings, outer space in this case, and consequently its future course will be radically

unpredictable. Theoretically this can be expressed by saying that every natural process

dissipates some information from the predictable part of its activity into the unpredictablepart. The predictable part is expressed in the generalized mechanical variables, and the

unpredictable part in heat.

There are other state variables which can be regarded as conditionally 'extensive' subject to

reservation as above, but not extensive as defined above. Examples are the Gibbs freeenergy, the Helmholtz free energy, and the enthalpy. Consequently, just because for some

systems under particular conditions of their surroundings such state variables are

conditionally conjugate to intensive variables, such conjugacy does not make such statevariables extensive as defined above. This is another reason for distinguishing the study of

equilibrium thermodynamics from the study of non-equilibrium thermodynamics. In

another way of thinking, this explains why heat is to be regarded as a quantity that refers toa process and not to a state of a system.

[edit] Equation of state

The properties of a system can under some conditions be described by an equation of state

which specifies the relationship between state variables. The equation of state expresses

constitutive properties of the material of the system. It must comply with some

thermodynamic constraints, but cannot be derived from the general principles ofthermodynamics alone.

[edit] Thermodynamic processes

Athermodynamic process is defined by changes of state internal to the system of interest,combined with transfers of matter and energy to and from the surroundings of the system

or to and from other systems. A system is demarcated from its surroundings or from other

systems by partitions which may more or less separate them, and may move as a piston tochange the volume of the system and thus transfer work.

[edit] Dependent and independent variables for a process

A process is described by changes in values of state variables of systems or by quantities of

exchange of matter and energy between systems and surroundings. The change must bespecified in terms of prescribed variables. The choice of which variables are to be used is

made in advance of consideration of the course of the process, and cannot be changed.

Certain of the variables chosen in advance are called the independent variables.[66] From

changes in independent variables may be derived changes in other variables calleddependent variables. For example a process may occur at constant pressure with pressure

prescribed as an independent variable, and temperature changed as another independent
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variable, and then changes in volume are considered as dependent. Careful attention to this

principle is necessary in thermodynamics.[67][68]

[edit] Changes of state of a system

In the approach through states of the system, a process can be described in two main ways.

In one way, the system is considered to be connected to the surroundings by some kind of

more or less separating partition, and allowed to reach equilibrium with the surroundings

with that partition in place. Then, while the separative character of the partition is kept

unchanged, the conditions of the surroundings are changed, and exert their influence on thesystem again through the separating partition, or the partition is moved so as to change the

volume of the system; and a new equilibrium is reached. For example, a system is allowed

to reach equilibrium with a heat bath at one temperature; then the temperature of the heatbath is changed and the system is allowed to reach a new equilibrium; if the partition

allows conduction of heat, the new equilibrium will be different from the old equilibrium.

In the other way, several systems are connected to one another by various kinds of more or

less separating partitions, and to reach equilibrium with each other, with those partitions inplace. In this way, one may speak of a 'compound system'. Then one or more partitions is

removed or changed in its separative properties or moved, and a new equilibrium is

reached. The Joule-Thomson experiment is an example of this; a tube of gas is separatedfrom another tube by a porous partition; the volume available in each of the tubes is

determined by respective pistons; equilibrium is established with an initial set of volumes;

the volumes are changed and a new equilibrium is established. [69][70][71][72][73] Anotherexample is in separation and mixing of gases, with use of chemically semi-permeable

membranes.[74]

[edit] Cyclic processes

Acyclic process[75] is a process that can be repeated indefinitely often without changing the

final state of the system in which the process occurs. The only traces of the effects of acyclic process are to be found in the surroundings of the system or in other systems. This is

the kind of process that concerned early thermodynamicists such as Carnot, and in terms of

which Kelvin defined absolute temperature,[76][77] before the use of the quantity of entropyby Rankine[78] and its clear identification byClausius.[79] For some systems, for example

with some plastic working substances, cyclic processes are practically nearly unfeasible

because the working substance undergoes practically irreversible changes.[40] This is why

mechanical devices are lubricated with oil and one of the reasons why electrical devices areoften useful.

A cyclic process of a system requires in its surroundings at least two heat reservoirs at

different temperatures, one at a higher temperature that supplies heat to the system, theother at a lower temperature that accepts heat from the system. The early work on

thermodynamics tended to use the cyclic process approach, because it was interested in

machines which would convert some of the heat from the surroundings into mechanical
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power delivered to the surroundings, without too much concern about the internal workings

of the machine. Such a machine, while receiving an amount of heat from a higher

temperature reservoir, always needs a lower temperature reservoir that accepts some lesseramount of heat, the difference in amounts of heat being converted to work. [80][81] Later, the

internal workings of a system became of interest, and they are described by the states of the

system. Nowadays, instead of arguing in terms of cyclic processes, some writers areinclined to derive the concept of absolute temperature from the concept of entropy, a

variable of state.

[edit] Commonly considered thermodynamic processes

It is often convenient to study a thermodynamic process in which a single variable, such astemperature, pressure, or volume, etc., is held fixed. Furthermore, it is useful to group these

processes into pairs, in which each variable held constant is one member of aconjugate

pair.

Several commonly studied thermodynamic processes are:

Isobaric process: occurs at constantpressure

Isochoric process: occurs at constantvolume (also called isometric/isovolumetric)

Isothermal process: occurs at a constant temperature

Adiabatic process: occurs without loss or gain of energy by heat

Isentropic process: a reversible adiabatic process, occurs at a constant entropy

Isenthalpic process: occurs at a constantenthalpy

Isolated process: occurs at constant internal energy and elementary chemicalcomposition

It is sometimes of interest to study a process in which several variables are controlled,subject to some specified constraint. In a system in which a chemical reaction can occur,

for example, in which the pressure and temperature can affect the equilibrium composition,a process might occur in which temperature is held constant but pressure is slowly altered,

just so that chemical equilibrium is maintained all the way. There will be a corresponding

process at constant temperature in which the final pressure is the same but is reached by a

rapid jump. Then it can be shown that the volume change resulting from the rapid jumpprocess is smaller than that from the slow equilibrium process.[82] The work transferred

differs between the two processes.

[edit] Instrumentation

There are two types ofthermodynamic instruments, the meter and the reservoir. A

thermodynamic meter is any device which measures any parameter of a thermodynamic

system. In some cases, the thermodynamic parameter is actually defined in terms of an

idealized measuring instrument. For example, the zeroth law states that if two bodies are inthermal equilibrium with a third body, they are also in thermal equilibrium with each other.

This principle, as noted by James Maxwell in 1872, asserts that it is possible to measure

temperature. An idealized thermometeris a sample of an ideal gas at constant pressure.
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From the ideal gas lawPV=nRT, the volume of such a sample can be used as an indicator

of temperature; in this manner it defines temperature. Although pressure is defined

mechanically, a pressure-measuring device, called abarometermay also be constructedfrom a sample of an ideal gas held at a constant temperature. A calorimeteris a device

which is used to measure and define the internal energy of a system.

A thermodynamic reservoir is a system which is so large that it does not appreciably alter

its state parameters when brought into contact with the test system. It is used to impose aparticular value of a state parameter upon the system. For example, a pressure reservoir is a

system at a particular pressure, which imposes that pressure upon any test system that it is

mechanically connected to. The Earth's atmosphere is often used as a pressure reservoir.

[edit] Conjugate variables

Main article: Conjugate variables

A central concept of thermodynamics is that ofenergy. By the First Law, the total energyof a system and its surroundings is conserved. Energy may be transferred into a system byheating, compression, or addition of matter, and extracted from a system by cooling,

expansion, or extraction of matter. In mechanics, for example, energy transfer equals the

product of the force applied to a body and the resulting displacement.

Conjugate variables are pairs of thermodynamic concepts, with the first being akin to a"force" applied to some thermodynamic system, the second being akin to the resulting

"displacement," and the product of the two equalling the amount of energy transferred. The

common conjugate variables are:

Pressure-volume (the mechanical parameters); Temperature-entropy (thermal parameters);

Chemical potential-particle number(material parameters).

[edit] Potentials

Thermodynamic potentials are different quantitative measures of the stored energy in asystem. Potentials are used to measure energy changes in systems as they evolve from an

initial state to a final state. The potential used depends on the constraints of the system,

such as constant temperature or pressure. For example, the Helmholtz and Gibbs energies

are the energies available in a system to do useful work when the temperature and volumeor the pressure and temperature are fixed, respectively.

The five most well known potentials are:

Name Symbol FormulaNatural

variables
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