# Volume in thermodynamics

1. Nov 18, 2012

### berra

I am confused over what the volume V stands for in thermodynamics.
Is it the smallest possible region of space containing all particles being studied? If so does it have to be connected?
Is it the volume enclosed by the surroundings of the system? If so, i thought the system was supposed to be fixed over time, but then why does one define quantities such as dV?
I hope you understand my confusion and can help me understand.

Last edited: Nov 18, 2012
2. Nov 18, 2012

### Staff: Mentor

Usually, it is assumed that the fluid is constrained to be in some fixed volume - the interior of a box, for example. That volume can change (the box can expand for example).
If you have two disconnected regions, they cannot exchange particles and pressure, so it is better to treat them as two different volumes V1, V2.

3. Nov 18, 2012

### berra

I want to explore the phase shift from gas to liquid along the wall of a container (its cold outside it). I guess I would have to have a semiopen system then, since the liquid only arises on the wall. But then what is the volume of the system so I can look at the phase diagram and see if it has a phase change? (I was thinking of getting T and p from Navier Stokes)

4. Nov 19, 2012

### Staff: Mentor

You can consider a large chunk of gas, where the volume change due to condensation is negligible.
Common phase diagrams are p and T only.

5. Nov 19, 2012

### Staff: Mentor

In the system you are considering, the temperature and water vapor partial pressure are not uniform within the container. Typically, there will be a thin boundary layer region near the wall in which the temperature varies rapidly from the bulk value for the chamber to the colder value at the wall, and in which the water vapor partial pressure varies rapidly from the bulk value for the chamber to the lower partial pressure value at the wall. Immediately at the wall, the water partial pressure is at the equilibrium value with the wall temperature. The rate of heat transfer from the bulk of the chamber to the wall depends on the thickness of the thermal boundary layer and temperature difference across the boundary layer, and the rate of water vapor mass transfer to the wall depends on the thickness of the concentration boundary layer and the vapor pressure difference across the boundary layer.