## [Thermodynamics] Ideal Gas Properties of Air

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 ?

 PhysOrg.com physics news on PhysOrg.com >> Promising doped zirconia>> New X-ray method shows how frog embryos could help thwart disease>> Bringing life into focus

Recognitions:
Gold Member
 Quote by 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 ?
It is the standard state entropy:

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

CS

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

Recognitions:
Gold Member

## [Thermodynamics] Ideal Gas Properties of Air

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

 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