Reversible process (thermodynamics)
Thermodynamics |
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In thermodynamics, a reversible process is a process, involving a system and its surroundings, whose direction can be reversed by infinitesimal changes in some properties of the surroundings, such as pressure or temperature.[1][2][3]
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.[a] While processes in isolated systems are never reversible,[3] cyclical processes can be reversible or irreversible.[4] Reversible processes are hypothetical or idealized but central to the second law of thermodynamics.[3] 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.[5]
Reversible processes are useful in thermodynamics because they are so idealized that the equations for heat and expansion/compression work are simple.[6] 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.[6] 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, finite changes are made; therefore the system is not at equilibrium throughout the process. In a cyclic process, the difference between the reversible work and the actual work for a process as shown in the following equation:
Boundaries and states
Simple
In some cases, it may be important to distinguish between reversible and
Engineering archaisms
Historically, the term Tesla principle was used to describe (among other things) certain reversible processes invented by Nikola Tesla.[8] 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 currents 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.[9]
Footnotes
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The absolute standard for "fast" and "slow" thermodynamic change is the maximum amount of time required for a temperature change (and the consequential changes in pressure, etc.) to travel across each of the parts of the whole system.
However, depending on the system or the process considered, thermodynamically "slow" might sometimes seem "fast" in human terms: In the example of the container and room air, if the container is just a porcelain coffee cup, heat can flow fairly quickly between the small object and the larger room.
In a different version of the same process where the container is a 40 gallon metal tank of water, one might intuitively expect rematching of temperatures ("equilibration") of the coffee cup to only require a few minutes, which is fast by comparison to the hours one could expect for a 40 gallon tank of water.
evaporative cooling could speed up its equilibration even more, compared to an almost-sealed tank with only an open, narrow spigot. If the spigot is closed so the tank is sealed, how "springy" its walls are for adapting to consequent pressure change affects the speed of equilibration. Further issues involve whether the room air is stagnant or has forced air circulation (a fan); if the tank nearly fills the room, the smaller amount of heat in the air relative to the heat in the tank may speed up the temperatures settling out; radiative coolingrates depend even on what color the tank is; and so on.
See also
References
- ^
McGovern, Judith (17 March 2020). "Reversible processes". PHYS20352 Thermal and Statistical Physics. University of Manchester. Retrieved 2 November 2020.
This is the hallmark of a reversible process: An infinitesimal change in the external conditions reverses the direction of the change.
- ^ a b Sears, F.W. & Salinger, G.L. (1986). Thermodynamics, Kinetic Theory, and Statistical Thermodynamics (3rd ed.). Addison-Wesley.
- ^ a b c d DeVoe, H. (2020). "Spontaneous reversible and irreversible processes". Thermodynamics and Chemistry. chem.libretexts.org. Bookshelves.
- ^ Zumdahl, Steven S. (2005). "§ 10.2 The isothermal expansion and compression of an ideal gas". Chemical Principles (5th ed.). Houghton Mifflin.
- ISBN 978-0070606593. Retrieved 8 November 2022.
- ^ a b
Atkins, P.; Jones, L.; Laverman, L. (2016). Chemical Principles (7th ed.). Freeman. ISBN 978-1-4641-8395-9.
- ^ Giancoli, D.C. (2000). Physics for Scientists and Engineers (with Modern Physics) (3rd ed.). Prentice-Hall.
- ^
"[no title cited]". Electrical Experimenter(low-res. text photo). January 1919. p. 615 – via teslasociety.com.
- ^ "Tesla's new monarch of machines". The New York Herald Tribune. Tesla Engine Builders Association. 15 Oct 1911. Archived from the original on September 28, 2011.