# Thermo: Isentropic expansion

1. Sep 7, 2009

1. The problem statement, all variables and given/known data

I have a system u =Av-2exp(s/R)

N moles of this substance initially at T0 and P0 are expanded isentropically until the pressure is halved. What is the final temperature?

2. Relevant equations

du = $$\partial$$u$$\partial$$s ds + $$\partial$$u$$\partial$$v dv
du = Tds - Pdv

3. The attempt at a solution

If this function is changing isentropically, ds would have to be zero, correct? The derivative of v would have to be such as to multiply P times 1/2, correct? But, if ds --> zero, how do I find my final temperature as a multiple of my initial temperature? Do I have to start plugging values which would satisfy these conditions back into the original equation?

2. Sep 7, 2009

### Mapes

How about expressing T as $(\partial U/\partial S)_V$ and P as $-(\partial U/\partial V)_S$ and looking at how the volume changes during the process?