1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Entropy of gas

  1. Feb 23, 2016 #1
    Can anyone please answer this question? I have read that increased temperature increases entropy and increased pressure decreases entropy ,for a gas.And vice versa.decreased temperature decreases entropy and decreased pressure increases entropy.Can any one please tell me for a gas under pressure that is compressed further does the increase in temperature increase the entropy more than the increase in pressure reduces it and for a gas under pressure that expands does the decrease in temperature decrease the entropy more than the decrease in pressure increases it.In other words does temperature trump pressure or vice versa.
    And does it make any difference whether the gas (which is already under pressure) is compressed or expanded?
    Thanx, RanCam
  2. jcsd
  3. Feb 23, 2016 #2


    User Avatar
    Science Advisor
    Homework Helper

  4. Feb 23, 2016 #3


    User Avatar
    Science Advisor

    Try looking at the Sackur-Tetrode equation, which gives an expression for the entropy of an ideal gas. That, together with the ideal gas law (PV = N kT) should allow you to express the entropy in any terms you want.
  5. Feb 24, 2016 #4
    If the compression or expansion is done adiabatically and reversibly, the entropy remains constant. If the compression or expansion is done adiabatically and irreversibly, the entropy increases.

    In general, for an ideal gas, $$dS=nC_p\frac{dT}{T}-nR\frac{dP}{P}$$
  6. Feb 26, 2016 #5
    Could you elaborate on how this was obtained? Just the first step or two. I will try the rest.
  7. Feb 26, 2016 #6
    For a change between two closely neighboring equilibrium states of an ideal gas, the changes in enthalpy, entropy, and volume are related by
    This is the first step.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook