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Change in entropy at constant temperature

by ricof
Tags: constant, entropy, temperature
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ricof
#1
Nov21-09, 12:00 PM
P: 14
1. The problem statement, all variables and given/known data

Calculate the entropy change when 0.011m3 of a perfect gas at a constant temperature of 273K is compressed from an initial pressure 1x10^5 N/m/m to a pressure of 1x10^6 N/m/m.

I know the equation but cannot work out Q!! Please help!

2. Relevant equations

dS=Q/T

3. The attempt at a solution
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Mapes
#2
Nov21-09, 12:57 PM
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Hi ricof, welcome to PF. Can you combine the First Law with what you know about the energy of an ideal gas at constant temperature?
cavalier
#3
Nov21-09, 02:21 PM
P: 29
Some more relevant equations.
The first law
[tex]\Delta E=Q+W[/tex]
Work for isothermal expansion
[tex]W=-nRTln\frac{V_{f}}{V_{i}}[/tex]

If you combine this with what you know about change in internal energy at constant temperature, like Mapes said, and that integral of yours, you can figure out the answer.

ricof
#4
Nov21-09, 03:48 PM
P: 14
Change in entropy at constant temperature

Because T is constant, there is no change in internal energy right?

And also, in the above equation for isothermal expansion, how is n calculated?
Mapes
#5
Nov21-09, 05:27 PM
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Quote Quote by ricof View Post
Because T is constant, there is no change in internal energy right?

And also, in the above equation for isothermal expansion, how is n calculated?
Agreed on the first question. For the second: apply the ideal gas law.
ricof
#6
Nov21-09, 08:08 PM
P: 14
Ok, so as dE = 0,

Q = -W

and as PV = nRT,

-W=PV ln (v1/v2) which is Q

Thanks guys, my answer is correct. Thanks for helping!


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