# Polytropic Process for an Ideal Gas

Hi,

I have a general question about polytropic processes when working with an ideal gas.

A polytropic process is one for which PV$\gamma$ = constant.

What I am wondering is if we assume that n is constant, does that mean that the temperature does not change for the process?

I see that PV$\gamma$ = nrT = constant, but for some reason it doesn't seem right that a polytropic process would be equivalent to an isothermal process if the number of moles of the system doesn't change.

A little light on the subject would be great.

Thanks,

-Luna

DrClaude
Mentor
Hi,

I have a general question about polytropic processes when working with an ideal gas.

A polytropic process is one for which PV$\gamma$ = constant.

What I am wondering is if we assume that n is constant, does that mean that the temperature does not change for the process?
No. A good example of such a process is adiabatic expansion/compression, for which ##T## changes.

I see that PV$\gamma$ = nrT
That equation is only valid if ##\gamma = 1##. Otherwise, there is no simple relationship between ##PV^\gamma## and ##T##.

Ok this concept finally sunk in.

PV$\gamma$ does not equal nRT unless $\gamma$=1. So a polytropic process in no way implies that nRT is a constant (even when n is constant).

DrClaude, thanks for the input!

DrClaude
Mentor
Ok this concept finally sunk in.

PV$\gamma$ does not equal nRT unless $\gamma$=1. So a polytropic process in no way implies that nRT is a constant (even when n is constant).
\begin{align} P_iV_i^\gamma &= P_fV_f^\gamma \\ \left( \frac{n R T_i}{V_i} \right) V_i^\gamma &= \left( \frac{n R T_f}{V_f} \right) V_f^\gamma \\ T_i V_i^{\gamma - 1} &= T_f V_f^{\gamma - 1} \end{align}