Math 2.0

Classical thermodynamics, heat transfer Definition

Thermodynamic equilibrium

Builds on Thermodynamic systemThermodynamic state

Definition

Let Σ\Sigma be a thermodynamic system. Σ\Sigma is in thermodynamic equilibrium when all of the following hold:

  1. Its macroscopic state does not change in time.
  2. There is no net macroscopic flow of matter or of energy inside Σ\Sigma or across its boundary.
  3. It is in thermal, mechanical and chemical equilibrium, so that temperature and pressure have no gradients and no net chemical reaction occurs.

Two systems in contact through a wall that lets heat pass are in thermal equilibrium with each other when no net heat flows between them.

Remarks

A gas at rest in a closed rigid box with insulating walls, after it has settled, is in thermodynamic equilibrium. Its temperature and pressure are the same everywhere, and nothing changes in time. A metal bar whose ends are held at two different temperatures can reach a steady state, but it is not in equilibrium. It fails clause 2, because heat flows through the bar all the time.

Reading path

To understand this page from the ground up, read these 3 pages in order. The foundations come first.

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Thermodynamic state, concept, no evidence levelThermodynamic stateconceptThermodynamic system, concept, no evidence levelThermodynamic systemconceptThermodynamic equilibrium, definition, no evidence levelThermodynamic equilib…definition
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  1. Thermodynamic state concept
  2. Thermodynamic system concept
  3. Thermodynamic equilibrium definition

Further reading

  1. Web https://en.wikipedia.org/wiki/Thermodynamic_equilibrium
About this page
TagCG915TGF65
URIm2://CG915TGF65
Kinddefinition
WritingW0 stub
MSC80A05, 80A10
DefinesThermodynamic equilibrium
UsesThermodynamic system, Thermodynamic state
Changed3 November 2026

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