# Isothermal Process

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Related to Isothermal Process: Isentropic process

## Isothermal process

A thermodynamic process which occurs with a heat addition or removal rate just adequate to maintain constant temperature. The change in the internal energy per mole U accompanying a change in volume in an isothermal process is given by the equation below, where T is the

temperature, P is the pressure, and V is the volume per mole. The integral in the equation is zero for an ideal gas, and approximately zero for a condensed phase (solid or liquid) for which the volume changes vary little with pressure. Thus, in both these cases, U2 = U1. For real gases, the integral is nonzero, and the internal energy change is computed using the equation of state of the gas in the equation. See Thermodynamic processes

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

## Isothermal Process

a process that occurs in a physical system at constant temperature.

To create an isothermal process a system is usually placed in a thermostat (a massive body in thermoequilibrium) of high thermal conductivity, so that heat exchange with the system proceeds quite rapidly and the difference between the temperature of the system and that of the thermostat is negligible. Another method is to use heat sources or heat sinks while checking for constant temperature by means of thermometers. Boiling a fluid and melting a solid at constant pressure, for example, are isothermal processes. For an ideal gas, the product of pressure by volume is constant during an isothermal process (the Boyle-Mariotte law).

Generally speaking, a certain amount of heat is conveyed to (or given off from) the system in an isothermal process, and external work is performed. The work performed by an ideal gas in an isothermal process is equal to NkT log (V2/V1), where N is the number of particles of the gas, T is the temperature, and K2 are the volumes of the gas at the start and completion of the process, and k is Boltzmann’s constant.

In solids and in most liquids an isothermal process changes the volume of a body very little, as long as there is no phase transition.

V. L. POKROVSKII [10–291—4]

## isothermal process

[¦ī·sə¦thər·məl ′prä·səs]
(thermodynamics)
Any constant-temperature process, such as expansion or compression of a gas, accompanied by heat addition or removal from the system at a rate just adequate to maintain the constant temperature.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
As we saw in Section 8.2, it is not possible to generate an isothermal process simply by adding extra terms to the Lagrangian.
Completing the investigation of the isothermal process of pressure solidification involves the evaluation of the procedure itself.
This inapplicability could be attributed to the secondary crystallization process and to the basic assumption that nonisothermal crystallization is an infinite Isothermal process. However, Liu's approach, a combination of the Avrami and Ozawa equations, satisfactorily analyzes non-isothermal crystallization kinetic data for PET and PBT/VA blends.
The Avrami exponent for the nonisothermal crystallization process was strikingly different from that of the isothermal process, which indicates different crystallization behaviors.
Finally, since RAMP is based on 2 isothermal processes and does not require thermal cycling, it can operate with much simpler instrumentation than PCR.
(Sestak and Berggren, 1971) In order to describe the change of crystallization rate as a function of temperature, the Arrhenius equation was used for homogeneous kinetics in isothermal processes (ASTM, 2008).
Then, the Avrami model was used to describe the development of the relative degree of crystallization in isothermal processes accordingly with the following equation:
Incomplete re actions obtained during isothermal processes are caused by vitrification, which dramatically reduce the mobility of the molecules.

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