Confusions about reversible process and state variable in nonquasistatic process.
Hello.
I have uncleared concepts about a reversible process and a state variable in a nonquasistatic (or nonequilibrium) process.
First, I would like to ask you what is true definition of the reversible process? I've searched information about it and what I found is the following: the reversible process is the process which doesn't change a total entropy of the system and its surrounding or simply the entropy of the universe. Is it right definition? And some articles say that the reversible process is the process which can be always reversed after it is done and if the process is reversed so that the system goes back to the original state, then the thermodynamic state of the universe is also brought back to its original state. By refering this reversible process concept, I summarized an irreversible process as following: the irreversible process is the process that can not be reversed after it is done so even if the system is brought back to the original state by taking some other path, the universe can not be brought back to its original state. Is this summary right?
And Why the slow process speed is a necessary condition but not a sufficient condition for the reversible process? Why a nonquasistatic process can not be the reverisble process?
This is the last question. When the system evolves from one equilibrium state A to other equlibrium state B, a change of the state variable must be independent of a path taken between these two state, it means the differential of the state variable must be an exact differential. So I thought "if f is the state variable, then its differential is the exact differential". Is its conserve true? I mean "if df is the exact differential, then f is the state variable""? I think it is not always guaranteed since the differential is not defined on the nonquasistatic process so whether or not df is the exact differential is not fully tested for alll possible path (in other words, the independance of the change of f on the choice among all possible paths including nonquasistatic paths can not be fully tested).
