Can't understand what's thermal equilibrium

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Discussion Overview

The discussion revolves around the concept of thermal equilibrium, its definition, and its relationship with temperature. Participants explore theoretical aspects, definitions, and implications of thermal equilibrium in physical systems.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants note that thermal equilibrium is defined before temperature, suggesting a foundational relationship between the two concepts.
  • Others propose that temperature is relative, raising questions about its definition in the context of thermal equilibrium.
  • One participant explains that thermal equilibrium occurs when two systems can exchange energy without performing work, leading to no net energy transfer, which is characterized by equal temperatures.
  • Another participant discusses the relationship between entropy and thermal equilibrium, stating that thermal equilibrium is defined by the condition where energy exchange does not affect total entropy, and that temperature is defined by this condition.
  • There is mention of the uniqueness of thermal equilibrium being an assumption based on systems with large numbers of particles, indicating a potential limitation in the generality of the concept.

Areas of Agreement / Disagreement

Participants express differing views on the foundational definitions of thermal equilibrium and temperature, indicating that the discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Some limitations include the dependence on definitions of temperature and thermal equilibrium, as well as assumptions regarding the properties of large particle systems.

Tosh5457
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In wikipedia it says: "Thermal equilibrium is a theoretical physical concept, used especially in theoretical texts, that means that all temperatures of interest are unchanging in time and uniform in space."

But isn't temperature defined because of the thermal equilibrium concept? It's defined because of the zeroth law, and the zeroth law uses thermal equilibrium concept.
 
Science news on Phys.org
Yes, first we define thermal equilibrium, then we define temperature.
 
Because temperature is relative?
 
If you have two systems which can exchange energy, but neither can do work on the other, thermal equilibrium is the state in which no net energy transfer takes place. It turns out one quantity has to be equal in both systems for this to occur, and we call this quantity the temperature.
 
Given two systems in "thermal contact" (aka, two systems that can exchange energy) and constraints on the combined system (for example, a constant total energy = U) we know that the entropy, aka the number of states, must always increase ignoring slight fluctuations.

The condition that an exchange of energy between the two systems has no effect on the total entropy is DEFINED to be thermal equilibrium. Temperature is then DEFINED by this thermal equilibrium condition.

The uniqueness of thermal equilibrium (aka, a single peak in the combined multiplicity functions) and therefore temperature is an assumption based on the properties of systems with very large numbers of particles.
 

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