thermodynamic equilibrium


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thermodynamic equilibrium

(ther-moh-dÿ-nam -ik) A condition existing in a system when all the atoms and molecules have an equal share in the available heat energy. The temperature is then the same in all parts of the system by whatever method of measurement. A system that is not in thermodynamic equilibrium is unstable and the state of the system will change until equilibrium is reached. When strict equilibrium does not exist throughout a region, such as a stellar atmosphere, each small volume of gas may act as though in equilibrium even though a neighboring volume may have a slightly different temperature; this is a situation of local thermodynamic equilibrium (LTE).
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Equilibrium, Thermodynamic

 

the state spontaneously reached by a thermodynamic system after a sufficiently long time when isolated from its surroundings; the parameters of state of the system then no longer vary with time. Isolation does not preclude the possibility of contacts of a certain type with the surroundings, such as thermal contact with a thermostat or the exchange of matter. The process by which the system reaches equilibrium is called relaxation. When the system is in thermodynamic equilibrium, all irreversible processes associated with energy dissipation—including thermal conduction, diffusion, and chemical reactions—come to a halt. The equilibrium state of the system is determined by the values of its external thermodynamic parameters, such as volume and electric or magnetic field strength, and by the value of the temperature. Strictly speaking, the parameters of state of a system in a state of equilibrium are not absolutely fixed, since in very small volumes they can experience small fluctuations about their average values. The isolation of a system is generally achieved by using stationary walls impermeable to matter. If, as in Dewar flasks, the stationary walls isolating the system are essentially non-heat-conducting—that is, if adiabatic walls are used—the energy of the system remains constant. When heat-conducting, or diathermal, walls are used between the system and the surroundings, heat exchange is possible until equilibrium is established. When such a system is in prolonged thermal contact with an environment having a very high heat capacity, the temperatures of the two systems are equalized, and thermodynamic equilibrium is reached. When the walls are semipermeable to matter, thermodynamic equilibrium is reached if the chemical potentials of the environment and the system are equalized through the exchange of matter between the system and the environment.

One of the conditions for thermodynamic equilibrium is mechanical equilibrium. In mechanical equilibrium, all macroscopic motions of parts of the system are impossible, but translational motion and rotation of the system as a whole are permissible. In the absence of external fields and rotation of the system, constant pressure throughout the entire system is a condition for mechanical equilibrium. Other necessary conditions for thermodynamic equilibrium are constant temperature and constant chemical potential throughout the system. The sufficient conditions for thermodynamic equilibrium (stability conditions) can be obtained from the second law of thermodynamics (the principle of maximum entropy). Examples of these conditions are an increase in pressure as the volume decreases at constant temperature and a positive value of the heat capacity at constant pressure. In the general case, a system is in thermodynamic equilibrium when a minimum value is had by the thermodynamic potential that corresponds to the variables that are independent under the conditions of the experiment. For example, if the volume and temperature are constant, the Helmholtz free energy must have a minimum value; if the pressure and temperature are constant, the Gibbs free energy must have a minimum value.

REFERENCES

Kubo, R. Termodinamika. Moscow, 1970. (Translated from English.)
Samoilovich, A. G. Termodinamika i statisticheskaia fizika, 2nd ed. Moscow, 1955.
Waals, J. D. van der, and F. Konstamm. Kurs termostatiki, part 1: Obshchaia termostatika. Moscow, 1936. (Translated from English.)

D. N. ZUBAREV

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

thermodynamic equilibrium

[¦thər·mō·dī′nam·ik ‚ē·kwə′lib·rē·əm]
(thermodynamics)
Property of a system which is in mechanical, chemical, and thermal equilibrium.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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Saha's equation under deviation from thermodynamic equilibrium. Bull.
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So Chaisson defines life as an "open, coherent, space-time structure maintained far from thermodynamic equilibrium by a flow of energy through it." This definition covers not only bacteria and people, but stars, galaxies, and planets as well.
What they have in common is that all operate and evolve far from thermodynamic equilibrium. This connection of operation and evolution is central to the thermodynamic paradigm.
Just where that energy came from and whether it will eventually dissipate to thermodynamic equilibrium is still being debated by the cosmologists.
To model the thermodynamic equilibrium the DECHEMA - UNIFAC model was used and for the mass transfer the AIChE model.
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If the clouds maintain thermodynamic equilibrium with their surroundings at the lower temperature inferred from the new data, the material must concentrate in thin sheet, Smith says.