Thermodynamics: Where exactly does microscopic turn to macroscopic?

[mikeph]

Looking for a rough distance scale over which statistical methods begin to work, and macroscopic thermodynamic variables begin to make sense. Thanks

 

[xts]

It is rather not distance scale, but number of particles involved.

It depends on how subtle effects you want to describe.
Sometimes 100 particles may be successfully treated as a thermodynamical continuum, sometimes 10¹² or so still produce visible effects (Brownian moves) beyond continuous thermodynamics.