Thermodynamic equilibrium
A system is in thermodynamic equilibrium when its large-scale properties, such as temperature, pressure, and composition, have stopped changing and there is no net flow of heat or matter within it or across its boundary. The microscopic parts are still in violent motion; what has settled is the statistics of that motion. Temperature is only strictly defined for a system at equilibrium, which is why the concept has to come before the rest of thermodynamics. Real systems are almost never exactly at equilibrium, so the framework is applied to processes slow enough that each stage is close to it.
Why it matters
It is the state in which thermodynamic quantities are even defined, so every calculation in the subject starts by assuming or approximating it.
Also written: equilibrium
Where it comes up
Read about Thermodynamic equilibrium on Wikipedia
Sources
- Thermodynamic equilibrium English Wikipedia
- Thermodynamics Britannica
