Timeline for What equation of state is needed for liquid states?
Current License: CC BY-SA 3.0
8 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Oct 13, 2012 at 21:15 | vote | accept | Paul | ||
| Oct 13, 2012 at 21:14 | comment | added | Ron Maimon | @Paul: Yeah, you got it. The incompressible approximation is good enough for most day-to-day fluids, away from the critical point. | |
| Oct 13, 2012 at 16:04 | comment | added | Paul | @RonMaimon: Ok... I think I understand now... If the fluid is incompressible, then pressure and temperature do not affect the volume occupied by a constant quanitity of mass. Compressibility necessarily implies that pressure and temperature affect the volume occupied by a constant quanitity of mass. Am I stating this correctly? | |
| Oct 13, 2012 at 15:30 | comment | added | Ron Maimon | @Paul: The equation of state is what I said V=constant, no dependence of V on P or T. So for this description, you need a Gibbs ensemble, P and T are independently varied and V is constant. If you use another ensemble and want to take the constant V limit, you have to give V a tiny dependence on pressure/temperature. | |
| Oct 13, 2012 at 15:28 | comment | added | Ron Maimon | @Prathyush: no change in volume with either temperature or pressure--- this is the approximation which works for 99% of heat engines. You only have non-negligible volume changes in gasses. The reason is that liquids have touching atoms, and don't compress or decompress well. They do compress a little bit, but this is not a significant amount of work during heating and cooling cycles, or pressure increase/decrease cycles. It's not like in gasses, where the work and the heat are always comparable. | |
| Oct 13, 2012 at 13:06 | comment | added | Paul | That makes a lot of sense. In this case, does the equation of state remain as the ideal gas law? | |
| Oct 13, 2012 at 12:26 | comment | added | Prathyush | what about change in volume with temprature? | |
| Oct 13, 2012 at 4:54 | history | answered | Ron Maimon | CC BY-SA 3.0 |