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Microstates and macrostates
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Many microscopic arrangements give the same macroscopic state.
A macrostate (P, V, T) is realised by an enormous number of microstates (positions and momenta). The most probable macrostate is the one with the most microstates.
Coin tosses: 50% heads is more likely than 100% heads.
Equilibrium is overwhelmingly probable.
In more detail
A macrostate describes a system by macroscopic quantities such as pressure and temperature, while a microstate specifies the position and momentum of every particle. Many microstates correspond to one macrostate.
Real-world uses
- Statistical mechanics counts microstates to compute entropy.
- Simulation of gases uses microstates to compute averages.
- Physicists explain why a macroscopic state such as a warm room is so much more probable than an unusual pattern of molecules.
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Frequently asked questions
How many microstates does a gas have?
An astronomically large number, which is why statistical methods are needed.
Are all microstates equally likely?
In an isolated system in equilibrium, yes, by the postulate of equal a priori probabilities.
What is a microstate?
A specific arrangement of the positions and velocities of every particle in a system.
Related topics in Kinetic Theory & Statistical Physics
Van der Waals equation · Compressibility factor · Liquefaction of gases · Boltzmann distribution · Partition function · Canonical ensemble