## When to use Cv vs Cp

Volume: $\Delta S = nR\ln \frac{V_{2}}{V_{1}}$
Temperature: $\Delta S = nC\ln \frac{T_{2}}{T_{1}}$

LaurenDinh_3L
Posts: 23
Joined: Fri Sep 26, 2014 2:02 pm

### When to use Cv vs Cp

For Midterm 2010 4A, I don't understand why we need to use Cv when the volume is obviously changing due to compression. I read in a post from last year that we can just assume that since pressure isn't mentioned in the problem that we don't know if pressure is constant. I still can't wrap my head around the fact that we use Cv even though the volume is changing. And also, isn't nitrogen gas (N2) a linear molecule rather than atomic? So why do we use the C value for a monoatomic species?

Quynh Nguyen 2O
Posts: 27
Joined: Fri Sep 26, 2014 2:02 pm

### Re: When to use Cv vs Cp

I asked the same thing in a different post but it wasn't really answered! I'm also wondering the same thing.

LaurenDinh_3L
Posts: 23
Joined: Fri Sep 26, 2014 2:02 pm

### Re: When to use Cv vs Cp

Bumppp

Victoria Liang 3L
Posts: 11
Joined: Fri Sep 26, 2014 2:02 pm

### Re: When to use Cv vs Cp

Kayla Denton 1A
Posts: 106
Joined: Fri Sep 26, 2014 2:02 pm

### Re: When to use Cv vs Cp

You're right about the N2 thing; I think that was a mistake.
As for the constant volume, you're separating the problem into two processes since entropy is a state function.
1. reversible, isothermal compression (edit: NOT at constant volume here)
2. temperature change at constant volume.

Hope that helps!
Last edited by Kayla Denton 1A on Tue Feb 10, 2015 3:04 pm, edited 1 time in total.

LaurenDinh_3L
Posts: 23
Joined: Fri Sep 26, 2014 2:02 pm

### Re: When to use Cv vs Cp

I still don't quite understand how it can compress but still have the same volume? If it's compressing its volume is supposed to decrease. Therefore there is a change in volume.

Kayla Denton 1A
Posts: 106
Joined: Fri Sep 26, 2014 2:02 pm

### Re: When to use Cv vs Cp

Oh sorry, I meant: reversible, isothermal compression. THEN temperature change at constant volume. You're totally right, I have no idea why I wrote compression at constant volume.