Classical thermodynamics, heat transfer Definition

Isothermal process

Builds on Temperature Quasistatic process Ideal gas

Definition

A process of a thermodynamic system is isothermal if the temperature of the system is the same at all times of the process. In the usual case the process is quasistatic, and the temperature is constant along its curve in the state space.

Remarks

Let $n$ moles of ideal gas at temperature $T$ expand quasistatically from volume $V_1$ to $V_2$ . Then $pV = nRT$ is constant, and the work done by the gas is $nRT \ln(V_2 / V_1)$ . Since the internal energy of an ideal gas depends only on $T$ , the heat absorbed is also $nRT \ln(V_2 / V_1)$ . The expansion of a gas in an insulated cylinder is not isothermal: the gas does work without taking in heat, so it cools.

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Ideal gas concept
Quasistatic process concept
Temperature concept
Isothermal process definition

Tag	`ZE5VKP3BMQ`
URI	`m2://ZE5VKP3BMQ`
Kind	definition
Writing	W0 stub
MSC	80A05, 80A10
Defines	Isothermal process
Uses	Temperature, Quasistatic process, Ideal gas
Changed	3 November 2026

Isothermal process

Definition

Remarks

Reading path

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