Thermodynamics is a branch of
physics that deals with
heat
In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
,
work, and
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied on ...
, and their relation to
energy,
entropy, and the physical properties of
matter and
radiation
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes:
* ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
. The behavior of these quantities is governed by the four
laws of thermodynamics which convey a quantitative description using measurable macroscopic
physical quantities, but may be explained in terms of
microscopic constituents by
statistical mechanics
In physics, statistical mechanics is a mathematical framework that applies statistical methods and probability theory to large assemblies of microscopic entities. It does not assume or postulate any natural laws, but explains the macroscopic ...
. Thermodynamics applies to a wide variety of topics in
science
Science is a systematic endeavor that Scientific method, builds and organizes knowledge in the form of Testability, testable explanations and predictions about the universe.
Science may be as old as the human species, and some of the earli ...
and
engineering, especially
physical chemistry,
biochemistry
Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology ...
,
chemical engineering and
mechanical engineering, but also in other complex fields such as
meteorology.
Historically, thermodynamics developed out of a desire to increase the
efficiency
Efficiency is the often measurable ability to avoid wasting materials, energy, efforts, money, and time in doing something or in producing a desired result. In a more general sense, it is the ability to do things well, successfully, and without ...
of early
steam engine
A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be trans ...
s, particularly through the work of French physicist
Sadi Carnot (1824) who believed that engine efficiency was the key that could help France win the
Napoleonic Wars. Scots-Irish physicist
Lord Kelvin was the first to formulate a concise definition of thermodynamics in 1854
which stated, "Thermo-dynamics is the subject of the relation of heat to forces acting between contiguous parts of bodies, and the relation of heat to electrical agency." German physicist and mathematician
Rudolf Clausius restated Carnot's principle known as the
Carnot cycle and gave so the
theory of heat a truer and sounder basis. His most important paper, "On the Moving Force of Heat",
[ Contains English translations of many of his other works.] published in 1850, first stated the
second law of thermodynamics. In 1865 he introduced the concept of
entropy. In 1870 he introduced the
virial theorem, which applied to
heat
In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
.
The initial application of thermodynamics to
mechanical heat engines was quickly extended to the study of chemical compounds and chemical reactions.
Chemical thermodynamics studies the nature of the role of
entropy in the process of
chemical reactions and has provided the bulk of expansion and knowledge of the field. Other formulations of thermodynamics emerged.
Statistical thermodynamics, or statistical mechanics, concerns itself with
statistical
Statistics (from German: ''Statistik'', "description of a state, a country") is the discipline that concerns the collection, organization, analysis, interpretation, and presentation of data. In applying statistics to a scientific, industria ...
predictions of the collective motion of particles from their microscopic behavior. In 1909,
Constantin Carathéodory presented a purely mathematical approach in an
axiomatic formulation, a description often referred to as ''geometrical thermodynamics''.
Introduction
A description of any thermodynamic system employs the four
laws of thermodynamics that form an axiomatic basis.
The first law specifies that energy can be transferred between physical systems as
heat
In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
, as
work, and with transfer of matter.
The second law defines the existence of a quantity called
entropy, that describes the direction, thermodynamically, that a system can evolve and quantifies the state of order of a system and that can be used to quantify the useful work that can be extracted from the system.
In thermodynamics, interactions between large ensembles of objects are studied and categorized. Central to this are the concepts of the thermodynamic ''
system
A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environment, is described by its boundaries, structure and purpose and expresse ...
'' and its ''
surroundings''. A system is composed of particles, whose average motions define its properties, and those properties are in turn related to one another through
equations of state
In physics, chemistry, and thermodynamics, an equation of state is a thermodynamic equation relating state variables, which describe the state of matter under a given set of physical conditions, such as pressure, volume, temperature, or intern ...
. Properties can be combined to express
internal energy
The internal energy of a thermodynamic system is the total energy contained within it. It is the energy necessary to create or prepare the system in its given internal state, and includes the contributions of potential energy and internal kinet ...
and
thermodynamic potential
A thermodynamic potential (or more accurately, a thermodynamic potential energy)ISO/IEC 80000-5, Quantities an units, Part 5 - Thermodynamics, item 5-20.4 Helmholtz energy, Helmholtz functionISO/IEC 80000-5, Quantities an units, Part 5 - Thermod ...
s, which are useful for determining conditions for
equilibrium and
spontaneous processes.
With these tools, thermodynamics can be used to describe how systems respond to changes in their environment. This can be applied to a wide variety of topics in
science
Science is a systematic endeavor that Scientific method, builds and organizes knowledge in the form of Testability, testable explanations and predictions about the universe.
Science may be as old as the human species, and some of the earli ...
and
engineering, such as
engines,
phase transitions,
chemical reactions,
transport phenomena, and even
black holes. The results of thermodynamics are essential for other fields of
physics and for
chemistry,
chemical engineering,
corrosion engineering,
aerospace engineering,
mechanical engineering,
cell biology,
biomedical engineering,
materials science, and
economics
Economics () is the social science that studies the production, distribution, and consumption of goods and services.
Economics focuses on the behaviour and interactions of economic agents and how economies work. Microeconomics analy ...
, to name a few.
This article is focused mainly on classical thermodynamics which primarily studies systems in
thermodynamic equilibrium.
Non-equilibrium thermodynamics is often treated as an extension of the classical treatment, but statistical mechanics has brought many advances to that field.
History

The
history of thermodynamics as a scientific discipline generally begins with
Otto von Guericke
Otto von Guericke ( , , ; spelled Gericke until 1666; November 20, 1602 – May 11, 1686 ; November 30, 1602 – May 21, 1686 ) was a German scientist, inventor, and politician. His pioneering scientific work, the development of experimental me ...
who, in 1650, built and designed the world's first
vacuum pump and demonstrated a
vacuum using his
Magdeburg hemispheres. Guericke was driven to make a vacuum in order to disprove
Aristotle
Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical Greece, Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatet ...
's long-held supposition that 'nature abhors a vacuum'. Shortly after Guericke, the Anglo-Irish physicist and chemist
Robert Boyle had learned of Guericke's designs and, in 1656, in coordination with English scientist
Robert Hooke
Robert Hooke FRS (; 18 July 16353 March 1703) was an English polymath active as a scientist, natural philosopher and architect, who is credited to be one of two scientists to discover microorganisms in 1665 using a compound microscope that ...
, built an air pump. Using this pump, Boyle and Hooke noticed a correlation between
pressure,
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied on ...
, and
volume
Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). Th ...
. In time,
Boyle's Law was formulated, which states that pressure and volume are
inversely proportional. Then, in 1679, based on these concepts, an associate of Boyle's named
Denis Papin built 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. By watching the valve rhythmically move up and down, Papin conceived of the idea of a
piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tig ...
and a cylinder engine. He did not, however, follow through with his design. Nevertheless, in 1697, based on Papin's designs, engineer
Thomas Savery built the first 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 of
heat capacity and
latent heat, which were necessary for the development of thermodynamics, were developed by Professor
Joseph Black at the University of Glasgow, where
James Watt
James Watt (; 30 January 1736 (19 January 1736 Old Style and New Style dates, OS) – 25 August 1819) was a Scottish people, Scottish invention, inventor, mechanical engineer, and chemist who improved on Thomas Newcomen's 1712 Newcomen steam ...
was employed as an instrument maker. Black and Watt performed experiments together, but it was Watt who conceived the idea of the
external condenser which resulted in a large increase in
steam engine
A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be trans ...
efficiency. Drawing on all the previous work led
Sadi Carnot, the "father of thermodynamics", to publish ''
Reflections on the Motive Power of Fire'' (1824), a discourse on heat, power, energy and engine efficiency. The book outlined the basic energetic relations between the
Carnot engine, the
Carnot cycle, and motive power. It marked the start of thermodynamics as a modern science.
The first thermodynamic textbook was written in 1859 by
William Rankine, originally trained as a physicist and a civil and mechanical engineering professor at the
University of Glasgow. The first and second laws of thermodynamics emerged simultaneously in the 1850s, primarily out of the works of
William Rankine,
Rudolf Clausius, and
William Thomson (Lord Kelvin).
The foundations of statistical thermodynamics were set out by physicists such as
James Clerk Maxwell
James Clerk Maxwell (13 June 1831 – 5 November 1879) was a Scottish mathematician and scientist responsible for the classical theory of electromagnetic radiation, which was the first theory to describe electricity, magnetism and ligh ...
,
Ludwig Boltzmann,
Max Planck,
Rudolf Clausius and
J. Willard Gibbs
Josiah Willard Gibbs (; February 11, 1839 – April 28, 1903) was an American scientist who made significant theoretical contributions to physics, chemistry, and mathematics. His work on the applications of thermodynamics was instrumental in t ...
.
Clausius, who first stated the basic ideas of the second law in his paper "On the Moving Force of Heat"
, published in 1850, and is called "one of the founding fathers of thermodynamics", introduced the concept of
entropy in 1865.
During the years 1873–76 the American mathematical physicist
Josiah Willard Gibbs published a series of three papers, the most famous being ''
On the Equilibrium of Heterogeneous Substances'',
in which he showed how
thermodynamic processes, including
chemical reactions, could be graphically analyzed, by studying the
energy,
entropy,
volume
Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). Th ...
,
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied on ...
and
pressure of the
thermodynamic system in such a manner, one can determine if a process would occur spontaneously. Also
Pierre Duhem in the 19th century wrote about chemical thermodynamics.
[Duhem, P.M.M. (1886). ''Le Potential Thermodynamique et ses Applications'', Hermann, Paris.] During the early 20th century, chemists such as
Gilbert N. Lewis
Gilbert Newton Lewis (October 23 or October 25, 1875 – March 23, 1946) was an American physical chemist and a Dean of the College of Chemistry at University of California, Berkeley. Lewis was best known for his discovery of the covalent bond a ...
,
Merle Randall,
and
E. A. Guggenheim[Guggenheim, E.A. (1933). ''Modern Thermodynamics by the Methods of J.W. Gibbs'', Methuen, London.][Guggenheim, E.A. (1949/1967). ''Thermodynamics. An Advanced Treatment for Chemists and Physicists'', 1st edition 1949, 5th edition 1967, North-Holland, Amsterdam.] applied the mathematical methods of Gibbs to the analysis of chemical processes.
Etymology
The etymology of ''thermodynamics'' has an intricate history.
It was first spelled in a hyphenated form as an adjective (''thermo-dynamic'') and from 1854 to 1868 as the noun ''thermo-dynamics'' to represent the science of generalized heat engines.
American
biophysicist Donald Haynie claims that ''thermodynamics'' was coined in 1840 from the
Greek root
θέρμη ''therme,'' meaning “heat”, and
δύναμις ''dynamis,'' meaning “power”.
Pierre Perrot claims that the term ''thermodynamics'' was coined by
James Joule in 1858 to designate the science of relations between heat and power,
however, Joule never used that term, but used instead the term ''perfect thermo-dynamic engine'' in reference to Thomson's 1849
phraseology.
By 1858, ''thermo-dynamics'', as a functional term, was used in
William Thomson's paper "An Account of Carnot's Theory of the Motive Power of Heat."
[Kelvin, William T. (1849) "An Account of Carnot's Theory of the Motive Power of Heat – with Numerical Results Deduced from Regnault's Experiments on Steam." ''Transactions of the Edinburg Royal Society, XVI. January 2.]
Scanned Copy
Branches of thermodynamics
The study of thermodynamical systems has developed into several related branches, each using a different fundamental model as a theoretical or experimental basis, or applying the principles to varying types of systems.
Classical thermodynamics
Classical thermodynamics is the description of the states of thermodynamic systems at near-equilibrium, that uses macroscopic, measurable properties. It is used to model exchanges of energy, work and heat based on the
laws of thermodynamics. The qualifier ''classical'' reflects the fact that it represents the first level of understanding of the subject as it developed in the 19th century and describes the changes of a system in terms of macroscopic empirical (large scale, and measurable) parameters. A microscopic interpretation of these concepts was later provided by the development of ''statistical mechanics''.
Statistical mechanics
Statistical mechanics
In physics, statistical mechanics is a mathematical framework that applies statistical methods and probability theory to large assemblies of microscopic entities. It does not assume or postulate any natural laws, but explains the macroscopic ...
, also known as statistical thermodynamics, emerged with the development of atomic and molecular theories in the late 19th century and early 20th century, and supplemented classical 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 the macroscopic, bulk properties of materials that can be observed on the human scale, thereby explaining classical thermodynamics as a natural result of statistics, classical mechanics, and
quantum theory at the microscopic level.
Chemical thermodynamics
Chemical thermodynamics is the study of the interrelation of
energy with
chemical reactions or with a physical change of
state
State may refer to:
Arts, entertainment, and media Literature
* ''State Magazine'', a monthly magazine published by the U.S. Department of State
* ''The State'' (newspaper), a daily newspaper in Columbia, South Carolina, United States
* '' Our ...
within the confines of the
laws of thermodynamics. The primary objective of chemical thermodynamics is determining the spontaneity of a given transformation.
Equilibrium thermodynamics
Equilibrium thermodynamics is the study of transfers of matter and energy in systems or bodies that, by agencies in their surroundings, can be driven from one state of thermodynamic equilibrium to another. The term 'thermodynamic equilibrium' indicates a state of balance, in which all macroscopic flows are zero; in the case of the simplest systems or bodies, their intensive properties are homogeneous, and their pressures are perpendicular to their boundaries. In an equilibrium state there are no unbalanced potentials, or driving forces, between macroscopically distinct parts of the system. A central aim in equilibrium thermodynamics is: given a system in a well-defined initial equilibrium state, and given its surroundings, and given its constitutive walls, to calculate what will be the final equilibrium state of the system after a specified thermodynamic operation has changed its walls or surroundings.
Non-equilibrium thermodynamics
Non-equilibrium thermodynamics is a branch of thermodynamics that deals with systems that are not in
thermodynamic equilibrium. Most systems found in nature are not in thermodynamic equilibrium because they are not in stationary states, and are continuously and discontinuously subject to flux of matter and energy to and from other systems. The thermodynamic study of non-equilibrium systems requires more general concepts than are dealt with by equilibrium thermodynamics. Many natural systems still today remain beyond the scope of currently known macroscopic thermodynamic methods.
Laws of thermodynamics

Thermodynamics is principally based on a set of four laws which are universally valid when applied to systems that fall within the constraints implied by each. In the various theoretical descriptions of thermodynamics these laws may be expressed in seemingly differing forms, but the most prominent formulations are the following.
Zeroth law
The
zeroth law of thermodynamics states: ''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 an
equivalence relation
In mathematics, an equivalence relation is a binary relation that is reflexive, symmetric and transitive. The equipollence relation between line segments in geometry is a common example of an equivalence relation.
Each equivalence relation ...
on the set of
thermodynamic systems under consideration. Systems are said to be in equilibrium if the small, random exchanges between them (e.g.
Brownian motion) do not lead to a net change in energy. This law is tacitly assumed in every measurement of temperature. Thus, if one seeks to decide whether two bodies are at the same
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied on ...
, it is not necessary to bring them into contact and measure any changes of their observable properties in time. The law provides an empirical definition of temperature, and justification for the construction of practical thermometers.
The zeroth law was not initially recognized as a separate law of thermodynamics, as its basis in thermodynamical equilibrium was implied in the other laws. The first, second, and third laws had been explicitly stated already, and found common acceptance in the physics community before the importance of the zeroth law for the definition of temperature was realized. As it was impractical to renumber the other laws, it was named the ''zeroth law''.
First law
The
first law of thermodynamics states: ''In a process without transfer of matter, the change in
internal energy
The internal energy of a thermodynamic system is the total energy contained within it. It is the energy necessary to create or prepare the system in its given internal state, and includes the contributions of potential energy and internal kinet ...
,''
'', of a
thermodynamic system is equal to the energy gained as heat,''
'', less the thermodynamic work,''
'', done by the system on its surroundings.''
[The sign convention (Q is heat supplied ''to'' the system as, W is work done ''by'' the system) is that of Rudolf Clausius. The opposite sign convention is customary in chemical thermodynamics.]
:
.
where
denotes the change in the internal energy of a
closed system (for which heat or work through the system boundary are possible, but matter transfer is not possible),
denotes the quantity of energy supplied ''to'' the system as heat, and
denotes the amount of thermodynamic work done ''by'' the system ''on'' its surroundings. An equivalent statement is that
perpetual motion machines of the first kind are impossible; work
done by a system on its surrounding requires that the system's internal energy
decrease or be consumed, so that the amount of internal energy lost by that work must be resupplied as heat
by an external energy source or as work by an external machine acting on the system (so that
is recovered) to make the system work continuously.
For processes that include transfer of matter, a further statement is needed: ''With due account of the respective fiducial reference states of the systems, when two systems, which may be of different chemical compositions, initially separated only by an impermeable wall, and otherwise isolated, are combined into a new system by the thermodynamic operation of removal of the wall, then''
:
,
''where'' ''denotes the internal energy of the combined system, and'' ''and'' ''denote the internal energies of the respective separated systems.''
Adapted for thermodynamics, this law is an expression of the principle of
conservation of energy, which states that energy can be transformed (changed from one form to another), but cannot be created or destroyed.
Internal energy is a principal property of the
thermodynamic state
In thermodynamics, a thermodynamic state of a system is its condition at a specific time; that is, fully identified by values of a suitable set of parameters known as state variables, state parameters or thermodynamic variables. Once such a set o ...
, while heat and work are modes of energy transfer by which a process may change this state. A change of internal energy of a system may be achieved by any combination of heat added or removed and work performed on or by the system. As a
function of state, the internal energy does not depend on the manner, or on the path through intermediate steps, by which the system arrived at its state.
Second law
A traditional version of the
second law of thermodynamics states: ''Heat does not spontaneously flow from a colder body to a hotter body.''
The second law refers to a system of matter and radiation, initially with inhomogeneities in temperature, pressure, chemical potential, and other
intensive properties, that are due to internal 'constraints', or impermeable rigid walls, within it, or to externally imposed forces. The law observes that, when the system is isolated from the outside world and from those forces, there is a definite thermodynamic quantity, its
entropy, that increases as the constraints are removed, eventually reaching a maximum value at thermodynamic equilibrium, when the inhomogeneities practically vanish. For systems that are initially far from thermodynamic equilibrium, though several have been proposed, there is known no general physical principle that determines the rates of approach to thermodynamic equilibrium, and thermodynamics does not deal with such rates. The many versions of the second law all express the
irreversibility of such approach to thermodynamic equilibrium.
In macroscopic thermodynamics, the second law is a basic observation applicable to any actual thermodynamic process; in statistical thermodynamics, the second law is postulated to be a consequence of molecular chaos.
Third law
The
third law of thermodynamics states: ''As the temperature of a system approaches absolute zero, all processes cease and the entropy of the system approaches a minimum value.''
This law of thermodynamics is a statistical law of nature regarding entropy and the impossibility of reaching
absolute 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 include "the entropy of all systems and of all 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 would stop if it were possible to achieve, is −273.15 °C (degrees Celsius), or −459.67 °F (degrees Fahrenheit), or 0 K (kelvin), or 0° R (degrees
Rankine Rankine is a surname. Notable people with the surname include:
* William Rankine (1820–1872), Scottish engineer and physicist
** Rankine body an elliptical shape of significance in fluid dynamics, named for Rankine
** Rankine scale, an absolute-te ...
).
System models

An important concept in thermodynamics is the
thermodynamic system, which is a precisely defined region of the universe under study. Everything in the universe except the system is called the
''surroundings''. A system is separated from the remainder of the universe by a
''boundary'' which may be a physical or notional, but serve to confine the system to a finite volume. Segments of the ''boundary'' are often described as ''walls''; they have respective defined 'permeabilities'. Transfers of energy as
work, or as
heat
In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
, or of
matter, between the system and the surroundings, take place through the walls, according to their respective permeabilities.
Matter or energy that pass across the boundary so as to effect a change in the internal energy of the system need to be accounted for in the energy balance equation. The volume contained by the walls can be the region surrounding a single atom resonating energy, such as
Max Planck defined in 1900; it can be a body of steam or air in a
steam engine
A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be trans ...
, such as
Sadi Carnot defined in 1824. The system could also be just one
nuclide
A nuclide (or nucleide, from nucleus, also known as nuclear species) is a class of atoms characterized by their number of protons, ''Z'', their number of neutrons, ''N'', and their nuclear energy state.
The word ''nuclide'' was coined by Truman ...
(i.e. a system of
quark
A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. All commonly o ...
s) as hypothesized in
quantum thermodynamics. When a looser viewpoint is adopted, and the requirement of thermodynamic equilibrium is dropped, the system can be the body of a
tropical cyclone
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Dep ...
, such as
Kerry Emanuel theorized in 1986 in the field of
atmospheric thermodynamics, or the
event horizon of a
black hole.
Boundaries are of four types: fixed, movable, real, and imaginary. For example, in an engine, a fixed boundary means the piston is locked at its position, within which a constant volume process might occur. If the piston is allowed to move that boundary is movable while the cylinder and cylinder head boundaries are fixed. For closed systems, boundaries are real while for open systems boundaries are often imaginary. In the case of a jet engine, a fixed imaginary boundary might be assumed at the intake of the engine, fixed boundaries along the surface of the case and a second fixed imaginary boundary across the exhaust nozzle.
Generally, thermodynamics distinguishes three classes of systems, defined in terms of what is allowed to cross their boundaries:
As time passes in an isolated system, internal differences of pressures, densities, and temperatures tend to even out. A system in which all equalizing processes have gone to completion is said to be in a
state
State may refer to:
Arts, entertainment, and media Literature
* ''State Magazine'', a monthly magazine published by the U.S. Department of State
* ''The State'' (newspaper), a daily newspaper in Columbia, South Carolina, United States
* '' Our ...
of
thermodynamic equilibrium.
Once in thermodynamic equilibrium, a system's properties are, by definition, unchanging in time. Systems in equilibrium are much simpler and easier to understand than are systems which are not in equilibrium. Often, when analysing a dynamic thermodynamic process, the simplifying assumption is made that each intermediate state in the process is at equilibrium, producing thermodynamic processes which develop so slowly as to allow each intermediate step to be an equilibrium state and are said to be
reversible processes.
States and processes
When a system is at equilibrium under a given set of conditions, it is said to be in a definite
thermodynamic state
In thermodynamics, a thermodynamic state of a system is its condition at a specific time; that is, fully identified by values of a suitable set of parameters known as state variables, state parameters or thermodynamic variables. Once such a set o ...
. The state of the system can be described by a number of
state quantities that do not depend on the process by which the system arrived at its state. They are called
intensive variable
Physical properties of materials and systems can often be categorized as being either intensive or extensive, according to how the property changes when the size (or extent) of the system changes. According to IUPAC, an intensive quantity is one ...
s or
extensive variables according to how they change when the size of the system changes. The properties of the system can be described by an
equation of state which specifies the relationship between these variables. State may be thought of as the instantaneous quantitative description of a system with a set number of variables held constant.
A
thermodynamic process
Classical thermodynamics considers three main kinds of thermodynamic process: (1) changes in a system, (2) cycles in a system, and (3) flow processes.
(1)A Thermodynamic process is a process in which the thermodynamic state of a system is change ...
may be defined as the energetic evolution of a thermodynamic system proceeding from an initial state to a final state. It can be described by
process quantities
In thermodynamics, a quantity that is well defined so as to describe the path of a process through the equilibrium state space of a thermodynamic system is termed a process function, or, alternatively, a process quantity, or a path function. As ...
. Typically, each thermodynamic process is distinguished from other processes in energetic character according to what parameters, such as temperature, pressure, or volume, etc., are held fixed; Furthermore, it is useful to group these processes into pairs, in which each variable held constant is one member of a
conjugate pair.
Several commonly studied thermodynamic processes are:
*
Adiabatic process: occurs without loss or gain of energy by
heat
In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
*
Isenthalpic process: occurs at a constant
enthalpy
*
Isentropic process
In thermodynamics, an isentropic process is an idealized thermodynamic process that is both adiabatic and reversible. The work transfers of the system are frictionless, and there is no net transfer of heat or matter. Such an idealized process ...
: a reversible adiabatic process, occurs at a constant
entropy
*
Isobaric process: occurs at constant
pressure
*
Isochoric process: occurs at constant
volume
Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). Th ...
(also called isometric/isovolumetric)
*
Isothermal process
In thermodynamics, an isothermal process is a type of thermodynamic process in which the temperature ''T'' of a system remains constant: Δ''T'' = 0. This typically occurs when a system is in contact with an outside thermal reservoir, and ...
: occurs at a constant
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied on ...
*
Steady state process: occurs without a change in the
internal energy
The internal energy of a thermodynamic system is the total energy contained within it. It is the energy necessary to create or prepare the system in its given internal state, and includes the contributions of potential energy and internal kinet ...
Instrumentation
There are two types of
thermodynamic instruments
A thermodynamic instrument is any device which facilitates the quantitative measurement of thermodynamic systems. In order for a thermodynamic parameter to be truly defined, a technique for its measurement must be specified. For example, the ultim ...
, 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 in thermal 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
thermometer is a sample of an ideal gas at constant pressure. From the
ideal gas law ''pV=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 a
barometer
A barometer is a scientific instrument that is used to measure air pressure in a certain environment. Pressure tendency can forecast short term changes in the weather. Many measurements of air pressure are used within surface weather analysis ...
may also be constructed from a sample of an ideal gas held at a constant temperature. A
calorimeter is 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 its state parameters are not appreciably altered when it is brought into contact with the system of interest. When the reservoir is brought into contact with the system, the system is brought into equilibrium with the reservoir. For example, a pressure reservoir is a system at a particular pressure, which imposes that pressure upon the system to which it is mechanically connected. The Earth's atmosphere is often used as a pressure reservoir. The ocean can act as temperature reservoir when used to cool power plants.
Conjugate variables
The central concept of thermodynamics is that of
energy, the ability to do
work. By the
First Law
"First Law" is a science fiction short story by American writer Isaac Asimov, first published in the October 1956 issue of ''Fantastic Universe'' magazine and later collected in ''The Rest of the Robots'' (1964) and ''The Complete Robot'' (1982). ...
, the total energy of a system and its surroundings is conserved. Energy may be transferred into a system by heating, 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 equaling the amount of energy transferred. The common conjugate variables are:
*
Pressure-
volume
Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). Th ...
(the
mechanical parameters);
*
Temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied on ...
-
entropy (thermal parameters);
*
Chemical potential-
particle number
The particle number (or number of particles) of a thermodynamic system, conventionally indicated with the letter ''N'', is the number of constituent particles in that system. The particle number is a fundamental parameter in thermodynamics which is ...
(material parameters).
Potentials
Thermodynamic potential
A thermodynamic potential (or more accurately, a thermodynamic potential energy)ISO/IEC 80000-5, Quantities an units, Part 5 - Thermodynamics, item 5-20.4 Helmholtz energy, Helmholtz functionISO/IEC 80000-5, Quantities an units, Part 5 - Thermod ...
s are different quantitative measures of the stored energy in a system. Potentials are used to measure the 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 volume or the pressure and temperature are fixed, respectively.
The five most well known potentials are:
where
is the
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied on ...
,
the
entropy,
the
pressure,
the
volume
Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). Th ...
,
the
chemical potential,
the number of particles in the system, and
is the count of particles types in the system.
Thermodynamic potentials can be derived from the energy balance equation applied to a thermodynamic system. Other thermodynamic potentials can also be obtained through
Legendre transformation.
Axiomatic thermodynamics
Axiomatic thermodynamics is a
mathematical discipline
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...
that aims to describe thermodynamics in terms of rigorous
axioms, for example by finding a mathematically rigorous way to express the familiar
laws of thermodynamics.
The first attempt at an axiomatic theory of thermodynamics was
Constantin Carathéodory's 1909 work ''Investigations on the Foundations of Thermodynamics'', which made use of
Pfaffian systems and the concept of
adiabatic accessibility
Adiabatic accessibility denotes a certain relation between two equilibrium states of a thermodynamic system (or of different such systems). The concept was coined by Constantin Carathéodory in 1909 ("adiabatische Erreichbarkeit") and taken up 90 ...
, a notion that was introduced by Carathéodory himself.
In this formulation, thermodynamic concepts such as
heat
In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
,
entropy, and
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied on ...
are derived from quantities that are more directly measurable.
Theories that came after, differed in the sense that they made assumptions regarding
thermodynamic process
Classical thermodynamics considers three main kinds of thermodynamic process: (1) changes in a system, (2) cycles in a system, and (3) flow processes.
(1)A Thermodynamic process is a process in which the thermodynamic state of a system is change ...
es with arbitrary initial and final states, as opposed to considering only neighboring states.
Applied fields
See also
*
Thermodynamic process path
Thermodynamic diagrams are diagrams used to represent the thermodynamic states of a material (typically fluid) and the consequences of manipulating this material. For instance, a temperature–entropy diagram (Temperature–entropy diagram, T–s ...
Lists and timelines
*
List of important publications in thermodynamics
*
List of textbooks on thermodynamics and statistical mechanics
*
List of thermal conductivities
*
List of thermodynamic properties
In thermodynamics, a physical property is any property that is measurable, and whose value describes a state of a physical system. Thermodynamic properties are defined as characteristic features of a system, capable of specifying the system's stat ...
*
Table of thermodynamic equations
This article is a summary of common equations and quantities in thermodynamics (see thermodynamic equations for more elaboration).
Definitions
Many of the definitions below are also used in the thermodynamics of chemical reactions.
General ...
*
Timeline of thermodynamics
*
Thermodynamic equations
Notes
References
Further reading
* A nontechnical introduction, good on historical and interpretive matters.
*
* Vol. 1, pp. 55–349.
*
*
*
* 5th ed. (in Russian)
*
*
*
*
*
*
The following titles are more technical:
*
*
*
*
External links
*
*
Thermodynamics Data & Property Calculation WebsitesThermodynamics Educational WebsitesEngineering Thermodynamics – A Graphical ApproachThermodynamics and Statistical Mechanicsby Richard Fitzpatrick
{{Authority control
Energy
Chemical engineering