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!

Homework Help: Isothermal expansion

  1. Sep 19, 2005 #1
    If air has a pressure of 40 psig and a volume of 8 cu. ft. expands isothermally to a pressure of 10 psig, find the external work performed during the expansion. How do I do this, do I first have to change 40 psig to psia, and how do I do that? This question has me lost! Any help appreciated, even a formula for me to understand it would help. thank you
  2. jcsd
  3. Sep 20, 2005 #2

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    Use PV=nRT.

    If T is constant then [itex]P_iV_i = P_fV_f[/itex]. So you can work out what the final volume is.

    The work is:

    [tex]W = \int_{P_i}^{P_f} PdV = \int_{V_i}^{V_f} \frac{nRT}{V}dV [/tex]

    You have to work out that integral (hint: [itex]\frac{d}{dV}ln V = 1/V[/itex]) and plug in the initial and final volumes.

    You don't have to do any conversions. You just need the pressure ratio P_f/P_i.


    [edit: this last comment is not quite correct. You do have to work out nRT = P_iV_i which means you have to do a conversion. PSIA is absolute pressure in pounds/in^2, which means you have to include atmospheric pressure. PSIG is gauge pressure, which is 1 atm less than actual. It is easier to work in MKS. I would convert to MKS and then convert back.]

    Last edited: Sep 21, 2005
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook