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In
thermodynamics Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of th ...
, a reversible process is a
process A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic. Things called a process include: Business and management *Business process, activities that produce a specific se ...
, involving a
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 expres ...
and its
surroundings Surroundings are the area around a given physical or geographical point or place. The exact definition depends on the field. Surroundings can also be used in geography (when it is more precisely known as vicinity, or vicinage) and mathematics, ...
, whose direction can be reversed by infinitesimal changes in some
properties Property is the ownership of land, resources, improvements or other tangible objects, or intellectual property. Property may also refer to: Mathematics * Property (mathematics) Philosophy and science * Property (philosophy), in philosophy a ...
of the surroundings, such as pressure or temperature. Throughout an entire reversible process, the system is in
thermodynamic equilibrium Thermodynamic equilibrium is an axiomatic concept of thermodynamics. It is an internal state of a single thermodynamic system, or a relation between several thermodynamic systems connected by more or less permeable or impermeable walls. In therm ...
, both physical and chemical, and ''nearly'' in pressure and temperature equilibrium with its surroundings. This prevents unbalanced forces and acceleration of moving system boundaries, which in turn avoids friction and other dissipation. To maintain equilibrium, reversible processes are extremely slow ( ''quasistatic''). The process must occur slowly enough that after some small change in a thermodynamic parameter, the physical processes in the system have enough time for the other parameters to self-adjust to match the new, changed parameter value. For example, if a container of water has sat in a room long enough to match the steady temperature of the surrounding air, for a small change in the air temperature to be reversible, the whole system of air, water, and container must wait long enough for the container and air to settle into a new, matching temperature before the next small change can occur. While processes in
isolated system In physical science, an isolated system is either of the following: # a physical system so far removed from other systems that it does not interact with them. # a thermodynamic system enclosed by rigid immovable walls through which neither ...
s are never reversible, cyclical processes can be reversible or irreversible. Reversible processes are hypothetical or idealized but central to the
second law of thermodynamics The second law of thermodynamics is a physical law based on universal experience concerning heat and energy interconversions. One simple statement of the law is that heat always moves from hotter objects to colder objects (or "downhill"), unless ...
. Melting or freezing of ice in water is an example of a realistic process that is ''nearly'' reversible. Additionally, the system must be in (quasistatic) equilibrium with the surroundings at all time, and there must be no dissipative effects, such as friction, for a process to be considered reversible. Reversible processes are useful in thermodynamics because they are so idealized that the equations for
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 ...
and expansion/compression work are simple. This enables the analysis of model processes, which usually define the maximum efficiency attainable in corresponding real processes. Other applications exploit that entropy and internal energy are state functions whose change depends only on the initial and final states of the system, not on how the process occurred. Therefore, the entropy and internal-energy change in a real process can be calculated quite easily by analyzing a reversible process connecting the real initial and final system states. In addition, reversibility defines the thermodynamic condition for
chemical equilibrium In a chemical reaction, chemical equilibrium is the state in which both the reactants and products are present in concentrations which have no further tendency to change with time, so that there is no observable change in the properties of th ...
.

# Overview

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 can be carried out in one of two ways: reversibly or irreversibly. An
ideal Ideal may refer to: Philosophy * Ideal (ethics), values that one actively pursues as goals * Platonic ideal, a philosophical idea of trueness of form, associated with Plato Mathematics * Ideal (ring theory), special subsets of a ring considere ...
thermodynamically reversible process is free of dissipative losses and therefore the magnitude of
work Work may refer to: * Work (human activity), intentional activity people perform to support themselves, others, or the community ** Manual labour, physical work done by humans ** House work, housework, or homemaking ** Working animal, an animal ...
performed by or on the system would be maximized. The incomplete conversion of heat to work in a cyclic process, however, applies to both reversible and irreversible cycles. The dependence of work on the path of the thermodynamic process is also unrelated to reversibility, since expansion work, which can be visualized on a pressure–volume diagram as the area beneath the equilibrium curve, is different for different reversible expansion processes (e.g. adiabatic, then isothermal; vs. isothermal, then adiabatic) connecting the same initial and final states.

# Irreversibility

In an
irreversible process In science, a process that is not reversible is called irreversible. This concept arises frequently in thermodynamics. All complex natural processes are irreversible, although a phase transition at the coexistence temperature (e.g. melting of ...
, finite changes are made; therefore the system is not at equilibrium throughout the process. In a cyclic process, the difference between the reversible work $\left(\, W_\mathsf \,\right)$ and the actual work $\left(\, W_\mathsf \,\right)$ for a process as shown in the following equation: $\; I = W_\mathsf - W_\mathsf ~.$

# Boundaries and states

Simple reversible processes change the state of a system in such a way that the net change in the combined
entropy Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodyna ...
of the system and its surroundings is zero. (The entropy of the system alone is conserved only in reversible adiabatic processes.) Nevertheless, the
Carnot cycle A Carnot cycle is an ideal thermodynamic cycle proposed by French physicist Sadi Carnot in 1824 and expanded upon by others in the 1830s and 1840s. By Carnot's theorem, it provides an upper limit on the efficiency of any classical thermodynam ...
demonstrates that the state of the surroundings may change in a reversible process as the system returns to its initial state. Reversible processes define the boundaries of how efficient heat engines can be in thermodynamics and engineering: a reversible process is one where the machine has maximum efficiency (see
Carnot cycle A Carnot cycle is an ideal thermodynamic cycle proposed by French physicist Sadi Carnot in 1824 and expanded upon by others in the 1830s and 1840s. By Carnot's theorem, it provides an upper limit on the efficiency of any classical thermodynam ...
). In some cases, it may be important to distinguish between reversible and quasistatic processes. Reversible processes are always quasistatic, but the converse is not always true. For example, an infinitesimal compression of a gas in a cylinder where there is
friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of t ...
between the piston and the cylinder is a ''quasistatic'', but ''not reversible'' process. Although the system has been driven from its equilibrium state by only an infinitesimal amount, energy has been irreversibly lost to waste heat, due to
friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of t ...
, and cannot be recovered by simply moving the piston in the opposite direction by the infinitesimally same amount.

# Engineering archaisms

Historically History (derived ) is the systematic study and the documentation of the human activity. The time period of event before the invention of writing systems is considered prehistory. "History" is an umbrella term comprising past events as well ...
, the term ''Tesla principle'' was used to describe (among other things) certain reversible processes invented by
Nikola Tesla Nikola Tesla ( ; ,"Tesla"
'' alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whi ...
s where the current's magnitude and direction varied cyclically. During a demonstration of the
Tesla turbine Tesla turbine at Nikola Tesla Museum The Tesla turbine is a bladeless centripetal flow turbine patented by Nikola Tesla in 1913. It is referred to as a ''bladeless turbine''. The Tesla turbine also known as the ''boundary-layer turbine'', '' ...
, the disks revolved and machinery fastened to the shaft was operated by the engine. If the turbine's operation was reversed, the disks acted as a
pump A pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic energy. Pumps can be classified into three major groups according to the method they ...
.

# Footnotes

* Time reversibility *
Carnot cycle A Carnot cycle is an ideal thermodynamic cycle proposed by French physicist Sadi Carnot in 1824 and expanded upon by others in the 1830s and 1840s. By Carnot's theorem, it provides an upper limit on the efficiency of any classical thermodynam ...
*
Entropy production Entropy production (or generation) is the amount of entropy which is produced in any irreversible processes such as heat and mass transfer processes including motion of bodies, heat exchange, fluid flow, substances expanding or mixing, anelastic ...
*
Toffoli gate In logic circuits, the Toffoli gate (also CCNOT gate), invented by Tommaso Toffoli, is a universal reversible logic gate, which means that any classical reversible circuit can be constructed from Toffoli gates. It is also known as the "control ...
* Time evolution *
Quantum circuit In quantum information theory, a quantum circuit is a model for quantum computation, similar to classical circuits, in which a computation is a sequence of quantum gates, measurements, initializations of qubits to known values, and possibly ot ...
*
Reversible computing Reversible computing is any model of computation where the computational process, to some extent, is time-reversible. In a model of computation that uses deterministic transitions from one state of the abstract machine to another, a necessary ...
*
Maxwell's demon Maxwell's demon is a thought experiment that would hypothetically violate the second law of thermodynamics. It was proposed by the physicist James Clerk Maxwell in 1867. In his first letter Maxwell called the demon a "finite being", while the '' ...
*
Stirling engine A Stirling engine is a heat engine that is operated by the cyclic compression and expansion of air or other gas (the ''working fluid'') between different temperatures, resulting in a net conversion of heat energy to mechanical work. More spec ...

# References

{{reflist, 22em, refs= {{cite book , author1=Atkins, P. , author2=Jones, L. , author3=Laverman, L. , year=2016 , title=Chemical Principles , edition=7th , publisher=Freeman , ISBN=978-1-4641-8395-9 {{cite web , author=DeVoe, H. , year=2020 , title=Spontaneous reversible and irreversible processes , department=''Thermodynamics and Chemistry'' , series=Bookshelves , website=chem.libretexts.org , url=https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/DeVoes_Thermodynamics_and_Chemistry/03%3A_The_First_Law/3.02%3A_Spontaneous_Reversible_and_Irreversible_Processes {{cite magazine , title={{grey, o title cited} , date=January 1919 , magazine= Electrical Experimenter , page=615 , type=low-res. text photo , via=teslasociety.com , url=http://www.teslasociety.com/pictures/teslaxc4.jpg {{cite book , author=Giancoli, D.C. , year=2000 , title=Physics for Scientists and Engineers (with Modern Physics) , edition=3rd , publisher=Prentice-Hall {{cite web , last1=McGovern , first1=Judith , date=17 March 2020 , title=Reversible processes , website=PHYS20352 Thermal and Statistical Physics , publisher=University of Manchester , url=https://theory.physics.manchester.ac.uk/~judith/stat_therm/node23.html , access-date=2 November 2020 , quote=This is the hallmark of a reversible process: An infinitesimal change in the external conditions reverses the direction of the change. {{cite web , title = Tesla's new monarch of machines , work = The
New York Herald Tribune The ''New York Herald Tribune'' was a newspaper published between 1924 and 1966. It was created in 1924 when Ogden Mills Reid of the '' New-York Tribune'' acquired the '' New York Herald''. It was regarded as a "writer's newspaper" and compet ...
, date = 15 Oct 1911 , publisher = Tesla Engine Builders Association , url = http://www.teslaengine.org/page/te.html , url-status = live , archive-url = https://web.archive.org/web/20110928213127/http://www.teslaengine.org/page/te.html , archive-date = September 28, 2011
{{cite book , author1=Sears, F.W. , author2=Salinger, G.L. , name-list-style=amp , year=1986 , title=Thermodynamics, Kinetic Theory, and Statistical Thermodynamics , edition=3rd , publisher=Addison-Wesley {{cite book , author=Zumdahl, Steven S. , year=2005 , chapter=§ 10.2 The isothermal expansion and compression of an ideal gas , title=Chemical Principles , edition=5th , publisher=Houghton Mifflin Thermodynamic processes