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Adiabatic Process: which is constant?

Posted: Wed Jul 06, 2016 4:37 pm
by Penemonie
Question #21 on the official GRE Physics Prep book: For an adiabatic process involving an ideal gas having volume V and temperature T , which of the following is constant? ( γ = CP/CV ) The answer is T*V^γ-1.

According to this website: http://hyperphysics.phy-astr.gsu.edu/hb ... adiab.html, P*V is constant but not with T. So what is the derivation? Can I get the answer from PV=nRT? :| :|

Re: Adiabatic Process: which is constant?

Posted: Thu Jul 07, 2016 1:19 am
by bunnysid
For an adiabatic process PV^γ = constant. Now use PV=nRT to find P in terms of T and put it in the equation. You will get the answer straight away.

Re: Adiabatic Process: which is constant?

Posted: Sun Nov 06, 2016 12:09 pm
by throwawayhehexD
So for irreversible adiabatic, PV=constant. For reversible adiabatic, PV^gamma=constant.

The GRE doesn't specify, but I do believe that almost all of the time they mean "reversible adiabatic" when they say "adiabatic". In my opinion, it's sloppy and they really should add "reversible" but they don't and that's just how it is.

So when you read "adiabatic", just assume they mean "reversible adiabatic" unless they tell you some other stuff that signals that they mean irreversible.