Adiabatic Process: which is constant?

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Penemonie
Posts: 2
Joined: Wed Jul 06, 2016 4:28 pm

Adiabatic Process: which is constant?

Postby Penemonie » Wed Jul 06, 2016 4:37 pm

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/hbase/thermo/adiab.html, P*V is constant but not with T. So what is the derivation? Can I get the answer from PV=nRT? :| :|

bunnysid
Posts: 20
Joined: Mon Nov 23, 2015 9:55 am

Re: Adiabatic Process: which is constant?

Postby bunnysid » Thu Jul 07, 2016 1:19 am

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.

throwawayhehexD
Posts: 8
Joined: Sun Nov 06, 2016 12:06 pm

Re: Adiabatic Process: which is constant?

Postby throwawayhehexD » Sun Nov 06, 2016 12:09 pm

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.




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