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!

Work, heat, thermodynamics Cylindric Piston

  1. Feb 1, 2014 #1
    Consider 5.5 L of a gas at a pressure of 3.0 atm in a cylinder with a movable piston. The external pressure is changed so that the volume changes to 10.5 L.
    a) Calculate the work done, and indicate the correct sign.
    b) Use the preceding data but consider the process to occur in two steps. At the end of the first step, the volume is 7.0 L. The second step results in a final volume of 10.5 L. Calculate the work done, and indicate the correct sign
    c) Calculate the work done if after the first step the volume is 8.0 L and the second step leads to a volume of 10.5 L.

    a)w=-7.86J
    b)w=-9J
    c)w=-9.085J

    Could you please check my answers? I have a test on thermodynamics soon.

    Basically, I used w=-pdeltaV.
    For example,
    a)p=5.5L*3.0atm/10.5L
    deltaV=10.5L-5.5L
    w=-pdeltaV=-7.86J.

    However, I am not sure for b) and c). The textbook seems to convey that I should get different answers for b) and c). But, they are very similar!
     
  2. jcsd
  3. Feb 1, 2014 #2

    DrClaude

    User Avatar

    Staff: Mentor

    That equation is only valid for constant P, which is clearly not the case here.
     
  4. Feb 1, 2014 #3
    Then how do i solve it?
     
  5. Feb 1, 2014 #4

    DrClaude

    User Avatar

    Staff: Mentor

    You have to use
    $$
    w = - \int_{V_i}^{V_f} P \; \mathrm{d} V
    $$
    where ##P## will be a function of ##V##. For an ideal gas, you substitute ##P## using the ideal gas law (if the temperature is constant and can be taken out of the integral).

    I notice that no details are given about the kind of compression being done (isothermal, adiabatic, ...). If it's adiabatic compression, you have to use the first law to get ##\Delta U## from which you can deduce ##W##.

    Also, I don't understand why doing the compression in steps can change the total work done.
     
  6. Feb 1, 2014 #5
    This question comes from Chemistry, Zumdahl, edition 9 in the chapter 6, problem 6/7.(for those who may have the book)

    We never saw the types of compression and cal II is not prereq for my chemistry course.

    Here is an example of a similar problem we did in class:
    Find work done by gas in cylindrical piston:

    Vi=0.015L
    Vf=0.025L
    Pexternal=1atm
    W=-1atm*(0.025L-0.015L)

    Does this make sense?
     
  7. Feb 1, 2014 #6

    DrClaude

    User Avatar

    Staff: Mentor

    I don't have that book. If it's not too much work to post the full problem, it would help figure out what is asked, as there seems to be information missing.

    Yes. You see, here the external pressure is constant, therefore you can treat it as a constant (!) and look only at the change in volume.
     
  8. Feb 1, 2014 #7
    This is the whole question! It seems quite disappointing to be honest with you. It is definitely not clear.
     
  9. Feb 2, 2014 #8

    Redbelly98

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    I have this book -- though mine is 6th ed., but it has the same problem. I confirm that the problem statement was posted completely and accurately as given in the book. And I agree that we need information about the type of path taken to solve the problem. Isothermal? A sudden drop to the final pressure, followed by expansion to the final volume? Something else? We simply don't know.

    I will note that the question comes from a section called "In-Class Discussion Questions", which according to the book "are designed to be considered by groups of students in class." This is in contrast to questions and exercises that would typically be given as homework.

    Since integral calculus is not required for the class, I presume the work is intended to be calculated using simple geometric area formulas to get "the area under the curve" in a P-V diagram. But, no information is given on what shape to use for the curve.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Work, heat, thermodynamics Cylindric Piston
Loading...