I have just started re-reading Thermal Physics, by Kittel and Kroemer. They state the fundamental assumption of thermal physics as:
...a closed system is equally likely to be in any of the quantum states accessible to it. All accessible quantum states are assumed to be equally probable - there is no reason to prefer some accessible states over other accessible states.
I previously thought I understood the fundamental assumption, but apparently I do not. For an isolated system (where the number of particles and total internal energy are constant), this makes sense, but how can it be true for a closed system?
I think the issue is that the author is defining what is meant by a "closed system" differently to every other textbook I've read. Indeed, he goes on to write:
A closed system will have constant energy, a constant number of particles....
I always thought that such a system was thermally isolated, and that a closed system could have variable energy but has constant particles.
Is Kittel defining the concept of a closed system to be different to other authors, or am I simply misunderstanding something vital?