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Ideal Gases. Query

  • Thread starter Brewer
  • Start date
  • #1
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I have found the following question on a previous exam paper whilst I've been doing my revision, and have a few questions about how to set up my answer.

Question:
Calculate the entropy change of 3 moles of an ideal gas when they are isothermally compressed (T = 30° C) from a volume 5V to a volume V.

My initial thought was to use PV = nRT, but because both n and T stay constant I then started thinking that I could just make P1V1 = P2V2.

However from my notes I have that PV^gamma = constant which confused me as there is no mention of a value for gamma in the question.

From here I would use the forumla W=-integralP(V)dV and then use the S=W/T formula.

Is this the correct way to go with this question?

Thanks for any feedback
 

Answers and Replies

  • #2
Tom Mattson
Staff Emeritus
Science Advisor
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It's simpler than that. Try to answer the following questions in order.

1.) For an isothermal process, what is [itex]dU[/itex]?
2.) Can you find an expression for [itex]dU[/itex] in terms of [itex]dS[/itex] and [itex]dV[/itex]? (since after all you were given 2 volumes and you were asked for the change in entropy).
3.) Can you integrate the expression found in 2.)?

That should get you there.
 
  • #3
lightgrav
Homework Helper
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PV^1 = constant for an isothermal process (treating it as an ideal gas ...)
PV^gamma = constant for an adiabatic process.
 

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