Thermodynamic equilibrium ↩

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aliases: [Equilibrium]
shorthands: {}
created: 2022-03-07 20:35:41
modified: 2022-05-29 15:05:06

If an isolated system is found with equal probability in each accessible microstate, then it is in equilibrium.

A closed macroscopic system reaches thermodynamic equilibrium after a long time of being left alone that is, its macroscopic properties (state functions) become time-independent.

An isolated system is in equilibrium if and only if all accessible microstates are equally probable.

Choosing the timescale

This definition is an idealization, since for real physical systems, time-independence is only applicable for a chosen time scale.
Example: we take a cup of hot tea, and we mix a drop of milk in it.

The mixing takes a couple of seconds, but the tea keeps swirling in the cup for some more time
After around a minute, the macroscopic flow of the tea stops
After an hour, the tea cools to room temperature
After some days, the tea evaporates

The state of the tea changes continuously, but when looking at it a smaller timescale, it seems like it's in equilibrium (constant temperature, volume)