Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Thermodynamics Problem with a Vacuum

  1. Sep 7, 2010 #1
    1. The problem statement, all variables and given/known data

    Calculate q, w, ΔH, ΔU, ΔS for the following problem: One mole of a monatomic ideal gas starting at 300K, and 10 atm, the gas freely expands into a vacuum to triple its volume.

    2. Relevant equations

    w = PΔV,
    ΔU = q - w or Cv dT,
    ΔH = ΔU + PΔV or Cp dT,
    ΔS = qrev/T,
    q = ???

    3. The attempt at a solution

    I'm having trouble understanding how the concept of the vacuum affects things such as temperature and pressure. Do temperature and/or pressure remain constant? I'm confused how to start this problem, since I can't identify the type of situation (adiabatic, isothermal, isobaric, etc.) this creates. Explanation on the effect a vacuum has would be very helpful.

  2. jcsd
  3. Sep 8, 2010 #2
    They're just saying vacuum to make it clear that there isn't anything else there. After the expansion, the initial gas has simply filled a larger volume.
  4. Sep 9, 2010 #3
    I'm not going to answer your question directly, but I'll give you some hints. Since it is in a vacuum, will there be any heat put into or taken away from the system? Is any work applied to or from the gas (actually this question can be complicated depending on how you look at it)?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook