# [Thermodynamics] Ideal Gas Properties of Air

1. Dec 29, 2008

### Altairs

I was studying the Ideal gas properties of air. Among other properties like $h$, $P_r$, $U$, $V_r$ I also found an $S^o$. What is this entropy like thing ?

2. Dec 29, 2008

### stewartcs

It is the standard state entropy:

$$S^0 = \int_{T_{ref}}^T \frac{C_p(T)}{T}dT$$

CS

3. Jan 3, 2009

### Altairs

Standard State Entropy ? What's the difference between Standard STate Entropy and the normal Entropy ?

4. Jan 3, 2009

### stewartcs

Standard state entropy is just referenced at absolute zero (t,ref in the integral above).

When the temperature change during a process is large and the specific heats of the ideal gas vary nonlinearly within the temperature range, the assumption of constant specific heats may lead to considerable errors in entropy-change calculations. For those cases, the variation of specific heats with temperature should be properly accounted for by utilizing accurate relations for the specific heats as a function of temperature. The entropy change during a process is then determined by substituting those Cv(T) and Cp(T) relations into the entropy equations and integrating.

Instead of performing the laborious integrals each time you have a new process, it is convenient to perform these integrals once and tabulate the results. For this purpose absolute zero is chosen as the reference temperature as defined in the equation above.

One may then simply refer to the tabulated values to determine the entropy change.

CS

5. Feb 17, 2009

### Darknes51986

what is Vr... I know how to get Vr from my text but not sure what it is and what its used for...Im trying to find values for internal energy for an otto cycle question...im working on EES but only know how to get the values for Vr from the text