Let's say I have an adiabatic, rigid, open container that has an amount of air at some pressure, $P_\text{cv}$, and some temperature, $T_\text{cv}$. I have heated pressurized air coming into the container. I am trying to calculate the change in specific entropy of the air in the container. I use the $T~ds$ equation, but I am not sure which pressure to use. The equation used is


Would the pressure term in the equation be the pressure of the incoming air, or would it be the pressure of the air in the control volume?

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    $\begingroup$ You're pumping air into your container. You either need an outlet in your system (and a way to describe the heat carried away convectively), or you're going to need to have a $\mu dN$ term if this is going to have any chance of being right. $\endgroup$ – Jerry Schirmer Sep 8 '13 at 21:08
  • $\begingroup$ We have never learned about $\mu dN$ in class and there is no outlet in the specified problem $\endgroup$ – Greg Harrington Sep 9 '13 at 4:15
  • $\begingroup$ you've learned about $PV = NkT$, right? Well, you're changing $N$ by pumping gas, in, so $P,V$ and $T$ are going to have to change in response. $\endgroup$ – Jerry Schirmer Sep 9 '13 at 12:14

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