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Using Cp when pressure does not stay constant? 
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#1
Apr612, 05:14 PM

P: 6

I am working on some intro thermodynamic problems and am having difficulty understanding one point. In some control volume questions, ideal gases are used and an input pressure is given. The question may then ask for the exit pressure. The solution is then found using specific heat for constant pressure Cp in the calculations, and the exit pressure is different than the input pressure.
Isn't the condition to use Cp=dH/dT that pressure stays constant (the VdP term equals zero)? So if the pressure stays constant why isn't the exit pressure equal to the input pressure of the gas? 


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