thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed b ...

, 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 s ...

, 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 open system (systems theory), environment, is described by its boundaries, str ...

and its

surroundings Surroundings, or environs is an 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 ...

, 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: Philosophy and science * Property (philosophy), in philosophy and logic, an abstraction characterizing an ...

of the surroundings, such as pressure or temperature. Throughout an entire reversible process, the system is in thermodynamic equilibrium, 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 systems 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 (metaphysics), universal empirical observation concerning heat and Energy transformation, energy interconversions. A simple statement of the law is that heat always flows spont ...

. 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 energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...

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.

Overview

Thermodynamic processes can be carried out in one of two ways: reversibly or irreversibly. An ideal thermodynamically reversible process is free of dissipative losses and therefore the magnitude of work 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 thermodynamics, an irreversible process is a thermodynamic processes, process that cannot be undone. All complex natural processes are irreversible, although a phase transition at the coexistence temperature (e.g. melting of ice cubes in wate ...

, finite changes are made; therefore the system is not at equilibrium throughout the process. In a cyclic process, the difference between the reversible work

(\, W_\mathsf \,)

and the actual work

(\, W_\mathsf \,)

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, most commonly associated with states of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynamics, where it was first recognized, to the micros ...

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 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). 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. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study 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

, and cannot be recovered by simply moving the piston in the opposite direction by the infinitesimally same amount.

Engineering archaisms

Historically, the term ''Tesla principle'' was used to describe (among other things) certain reversible processes invented by

Nikola Tesla Nikola Tesla (;"Tesla"
. ''Random House Webster's Unabridged Dictionary''. ; 10 July 1856 – 7 ...

. However, this phrase is no longer in conventional use. The principle stated that some systems could be reversed and operated in a complementary manner. It was developed during Tesla's research in

alternating current Alternating current (AC) is an electric current that 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 w ...

s where the current's magnitude and direction varied cyclically. During a demonstration of the Tesla 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 Slurry, slurries, by mechanical action, typically converted from electrical energy into hydraulic or pneumatic energy. Mechanical pumps serve in a wide range of application ...

Footnotes

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

Overview

Irreversibility

Boundaries and states

Engineering archaisms

Footnotes

See also

References