## deltaS=nCvln(t2/t1) what is Cv?

Volume:
Temperature:

Josephine Lu 4L
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Joined: Fri Sep 28, 2018 12:18 am

### deltaS=nCvln(t2/t1) what is Cv?

what does the Cv mean in the equation deltaS=nCvln(t2/t1) ? what does it stand for, and when should i use it as opposed to deltaS=nRln(V2/v1)?

Zenita Leang 2K
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Joined: Fri Sep 28, 2018 12:28 am

### Re: deltaS=nCvln(t2/t1) what is Cv?

Cv is constant volume, which is times the constant R. You use this in deltaS=nCvln(t2/t1) instead of deltaS=nRln(V2/v1) when you have a monatomic ideal gas at constant volume to calculate for entropy.

Danny Zhang 4L
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### Re: deltaS=nCvln(t2/t1) what is Cv?

Cv is the heat capacity of the system when the volume is constant.

Emily Kennedy 4L
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### Re: deltaS=nCvln(t2/t1) what is Cv?

where does the 3/2 come from?

Sheridan Slaterbeck 1J
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Joined: Fri Sep 28, 2018 12:27 am

### Re: deltaS=nCvln(t2/t1) what is Cv?

when would you use deltaS=nRln(V2/v1) ?

Kayla Vo 1B
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### Re: deltaS=nCvln(t2/t1) what is Cv?

If the problem gives you temperature and the system is not isothermal, deltaS=nCvln(t2/t1) is used to determine the change in entropy. To calculate the change in entropy of an ideal gas in a reversible isothermal expansion, use deltaS=nRln(V2/V1).

Linh Vo 2J
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### Re: deltaS=nCvln(t2/t1) what is Cv?

Emily Kennedy 4L wrote:where does the 3/2 come from?

For heat capacities of ideal gases, we use Cv=3/2R for a monoatomic gas and Cv=5/2R for a diatomic gas such as N2 or H2. The constant being multiplied to R is determined by the greater number of degrees of freedom in diatomic as opposed to monoatomic gases, so 5/2 (diatomic) is much bigger than 3/2 (monoatomic).

Toru Fiberesima 1L
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### Re: deltaS=nCvln(t2/t1) what is Cv?

If a gas is compressed suddenly and irreversibly, treat it as a two step process where the dS from dT and dS from dV need to be accounted for. In this case, you would do dS=nCvln(T2/T1) if v was constant + deltaS=nRln(V2/V1).