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: Calculate work from PV diagram

  1. Nov 19, 2013 #1
    1. Problem Statement

    The problem asks to find the net work from one complete cycle A-B-C. Problem 13, starting on page 66 of the slide show here:

    https://engineering.purdue.edu/~xe/Files/FE%20Review/Thermo/FEThermoReview2013.pdf [Broken]

    2. Relevant equations
    ideal gas law: PV=mRT
    Boyle's Law: W=RTln(v2/v1)

    3. Attempt at solution

    I understand most of the solution, but on page 67, says that W=R*T*ln(Vb/Va) is equivalent to W=p*Va*ln(Vb/Va), assuming ideal gas and constant mass. I don't understand why you can use volume here instead of specific volume.

    From the ideal gas law: PV=mRT and RT=PV/m. So I thought it should be W=p*Va/m*ln(Vb/Va). Why can we eliminate m here?

    Thank you!
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Nov 19, 2013 #2
    There should be an m in the equation W=R*T*ln(Vb/Va). It should read W=m*R*T*ln(Vb/Va).
  4. Nov 19, 2013 #3

    rude man

    User Avatar
    Homework Helper
    Gold Member

    What's wrong is "W = RT ln(Vb/Va)". It should be W = mRT ln(Vb/Va).
    Notice that the wrong formula is never used in the computations.

    P.S. why do they put a slash thru the V's?
  5. Nov 19, 2013 #4
    One equation for isothermal work is W = m*R*T* ln(Vb/Vb). He has either made a mistake by omitting the number of moles or he has calculated the heat for 1 mole.
  6. Nov 19, 2013 #5
    Thanks for the responses. Good to know I'm not crazy.

    In the reference material for the fundamentals of engineering exam, the formula also does not include mass. I guess its a typo. Maybe that's where the author of the presentation got the formula from.

    I think it is to differentiate between volume and specific volume. It confused me at first though.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted