9.7 and 9.11 formula differences

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Emily Glaser 1F
Posts: 156
Joined: Thu Jul 27, 2017 3:01 am

9.7 and 9.11 formula differences

Postby Emily Glaser 1F » Sat Jan 27, 2018 5:13 pm

9.7 uses the formula, deltaS = n C ln(T2/T1)
9.11 uses the formula, deltaS = n R ln (T2/T1)
What's the difference? Why

Andres Reynoso 1J
Posts: 30
Joined: Fri Sep 29, 2017 7:06 am

Re: 9.7 and 9.11 formula differences

Postby Andres Reynoso 1J » Sat Jan 27, 2018 5:32 pm

The difference lies in what the question is asking.

For 9.7 part a, the question asks for the entropy change at a constant pressure with a change in temperature. For this we use the known equation where Cp is the specific heat of a gas at constant pressure.

For 9.11, the question asks for the entropy change at a constant temperature with a change in pressure. For this, we can use where R is the gas constant. As the solution manual states, we can rewrite that equation since P is the inverse of V for ideal gases.

To summarize, it all really depends on what the question asks. Hope I helped!

Timothy Kim 1B
Posts: 62
Joined: Fri Sep 29, 2017 7:04 am

Re: 9.7 and 9.11 formula differences

Postby Timothy Kim 1B » Sat Jan 27, 2018 9:19 pm

In the first question, we use the first formula because it is asking for entropy at a constant pressure. when you derive the equation, it comes out as that.

For the second problem, the conditions are different because it is isothermal. Thus, you end up using a different equation that keeps into account that the internal energy is zero, which leads to the second equation.

Rakhi Ratanjee 1D
Posts: 59
Joined: Fri Sep 29, 2017 7:04 am

Re: 9.7 and 9.11 formula differences

Postby Rakhi Ratanjee 1D » Sun Jan 28, 2018 6:12 pm

For 9.11, the solutions manual states that "since P is the inverse of V for ideal gases, we can write: delta S = n*R*ln(P1/P2).
How can you derive this?

zanekoch1A
Posts: 25
Joined: Thu Jul 13, 2017 3:00 am

Re: 9.7 and 9.11 formula differences

Postby zanekoch1A » Thu Feb 01, 2018 9:57 am

For 9.7 I understand why you use deltas=nCpln(t2/t1) but what do you use for Cp? Is there a way to solve it without knowing Cp because I don't think its given.


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