- #1
swampwiz
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- TL;DR Summary
- What is the underlying reason that thermodynamic state has 2 degrees of freedom - i.e., that any 2 properties (saturated state excepted) completely determines the state?
I'm trying to delve into the reason why this is so. It seems that there are 5 fundamental properties:
P - Pressure
V - Volume (specific)
T - Temperature
S - Entropy (specific)
U - Internal Energy
(Yes, there are other types of energy, but they are fully determinable from these 5 - e.g., Enthalpy: H = U + PV)
Since there are 5 such possible domain variables, having a net of 2 DOF means that there must be 3 constraint equations. The 1st Law of Thermodynamics provides 1 of the equations:
dU = δQ - δW = T dS - P dV
so what are the other 2?
I can see for an ideal gas that the Ideal Gas law provides another:
p V = R T
but even for this model, there must be yet another.
P - Pressure
V - Volume (specific)
T - Temperature
S - Entropy (specific)
U - Internal Energy
(Yes, there are other types of energy, but they are fully determinable from these 5 - e.g., Enthalpy: H = U + PV)
Since there are 5 such possible domain variables, having a net of 2 DOF means that there must be 3 constraint equations. The 1st Law of Thermodynamics provides 1 of the equations:
dU = δQ - δW = T dS - P dV
so what are the other 2?
I can see for an ideal gas that the Ideal Gas law provides another:
p V = R T
but even for this model, there must be yet another.