## Delta U = n*Cv,m*deltaT

Samuel G Rivera - Discussion 4I
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### Delta U = n*Cv,m*deltaT

Why does Delta U = n*Cv,m*deltaT apply for both constant pressure and constant volume?

Fatemah Yacoub 1F
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### Re: Delta U = n*Cv,m*deltaT

For constant pressure the Cv would actually be switched to CP.

Sean Cheah 1E
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Joined: Wed Sep 18, 2019 12:20 am

### Re: Delta U = n*Cv,m*deltaT

Fatemah Yacoub 1F wrote:For constant pressure the Cv would actually be switched to CP.

Is this actually true? I thought that this equation for delta U only works with Cv. For example, in an isobaric process, q = nCp(delta T) and w = -P(delta V) = -nR(delta T). Thus delta U = q + w = n(Cp - R)(delta T). Since Cv = Cp - R, delta U = nCv(delta T).

Posts: 113
Joined: Thu Jul 11, 2019 12:16 am

### Re: Delta U = n*Cv,m*deltaT

Sean Cheah 1E wrote:
Fatemah Yacoub 1F wrote:For constant pressure the Cv would actually be switched to CP.

Is this actually true? I thought that this equation for delta U only works with Cv. For example, in an isobaric process, q = nCp(delta T) and w = -P(delta V) = -nR(delta T). Thus delta U = q + w = n(Cp - R)(delta T). Since Cv = Cp - R, delta U = nCv(delta T).

I'm pretty sure it works for systems under both conditions.

Samuel G Rivera - Discussion 4I
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### Re: Delta U = n*Cv,m*deltaT

My TA sent us a list of equations and Cv wasn't changed to Cp when pressure is constant so Delta U = n*Cv,m*deltaT under constant pressure and constant volume. I was just wondering why since I thought you would need to use Cp under constant pressure.

Jarrett Peyrefitte 2K
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### Re: Delta U = n*Cv,m*deltaT

For constant volume, Cv is used.
For constant pressure, Cp is used.

Jaci Glassick 2G
Posts: 104
Joined: Sat Jul 20, 2019 12:16 am

### Re: Delta U = n*Cv,m*deltaT

For constant pressure Cv is used and for constant volume Cp is used. My TA kind of mentioned that most of the time you are just going to end up using Cv but they both should work.

AniP_2D
Posts: 95
Joined: Sat Aug 17, 2019 12:17 am

### Re: Delta U = n*Cv,m*deltaT

From information I gained from the book, discussion, and videos, I also believe that when talking about constant pressure, you use Cp, but when talking about constant volume, you use Cv.