1. Not finding help here? Sign up for a free 30min 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!

How much work is done by the Adiabatic Compression of Diatomic Gasses

  1. Feb 16, 2016 #1

    grandpa2390

    User Avatar
    Gold Member

    I apologize if my questions were in the wrong place. I am in "Advanced" Physics Classes. And so I post the questions from those classes in the Advanced Physics forum. And these are questions that weren't covered in Introductory Physics (1st three semesters). But they keep being moved to this forum. I don't understand the criteria. I guess that forum is for doctoral students and above? The "Really Advanced" stuff lol. regular graduate level classes and below go here? Want to make sure my questions are in the right place.


    1 liter of ideal diatomic gas (allow for rotation but not vibration) at a pressure of 1 atmosphere and a temperature of 300 K is compressed adiabatically to a pressure of 7 atm. What is the volume of the gas after compression?

    so I found the volume to be .249 Liters
    the final temperature after compression is 523 K

    what I can't find is the work that was done during compression on the gas.
    I drew a graph of the compression and found the equation of the line to be ##(-6/.751)##*v+8.989=p
    i then tried integrating that over v from 1 Liter to .249 Liters. but the answer I got of 303 J is wrong...

    What am I doing wrong?

    I apologize about not having the guidelines on here. I accidentally erased them, and then the forum saved the draft. and I can't get them back...
     
  2. jcsd
  3. Feb 17, 2016 #2

    DrClaude

    User Avatar

    Staff: Mentor

    One thing from the homework template that is missing is the "Relevant equations" part. Can you write the relevant equations?

    Also, it is very helpful for the helpers to see the details or your previous calculations.
     
  4. Feb 17, 2016 #3

    ehild

    User Avatar
    Homework Helper
    Gold Member

    What is the relation between pressure and volume during an adiabatic compression/expansion? It is not linear!
     
  5. Feb 17, 2016 #4

    grandpa2390

    User Avatar
    Gold Member

    You are right. In class he did an example that was not linear. I'm not sure what the relationship is though because in the example it wasn't necessary. We did get a formula using log(T/T2), maybe that one is it

    ln(T2/T1)^3/2 = ln (v1/v2)...


    or no, this one. W=-NKTln(V2/V1) is probably my best bet

    no, that didn't work. I realized that formula is for isothermic because it only takes one T into account.

    I found on another post where the helper asked if the person could find ##C_v## I found it using (f/2)NK but I don't understand the rest of his post to see how it is relevant. https://www.physicsforums.com/threads/work-required-for-adiabatic-compression.644227/
     
    Last edited: Feb 17, 2016
  6. Feb 17, 2016 #5

    DrClaude

    User Avatar

    Staff: Mentor

  7. Feb 17, 2016 #6

    grandpa2390

    User Avatar
    Gold Member

  8. Feb 17, 2016 #7

    ehild

    User Avatar
    Homework Helper
    Gold Member

    Read the problem text carefully.
    What is the relation between P and V during an adiabatic expansion/compression?
    You have to integrate pdV to get the work. Express P with V.
     
  9. Feb 17, 2016 #8

    grandpa2390

    User Avatar
    Gold Member

    they are inverses (as one rises, the other falls). And they are not linearly related...

    I know I have to integrate. I di that initially. the problem is I don't know the relationship. I'm going to try V^gamma = 1/P
     
  10. Feb 17, 2016 #9

    ehild

    User Avatar
    Homework Helper
    Gold Member

    It is close, but not correct. What is the relation exactly between P and V?
     
  11. Feb 17, 2016 #10

    grandpa2390

    User Avatar
    Gold Member

    I don't know. I have looked through my notes 5 times, and that's all I have got. :(

    he just said it was equal to a constant.

    I have another formula he said was equal to a constant. VT^3/2

    is VT^f/2 = PV^gamma?
     
  12. Feb 17, 2016 #11

    ehild

    User Avatar
    Homework Helper
    Gold Member

  13. Feb 17, 2016 #12

    grandpa2390

    User Avatar
    Gold Member

  14. Feb 17, 2016 #13

    grandpa2390

    User Avatar
    Gold Member

  15. Feb 17, 2016 #14

    vela

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    This is the kind of problem you do in a freshman/sophomore physics class. The advanced physics forum is for upper division and beyond. A good indicator is if the answer you're looking for is a numerical one, it's probably an intro physics question.

    You need to learn how to read the book. It's a skill like any other, and you have to develop and practice it. Don't expect to be able to simply read the book and have the material be clear to you on the first or second pass. If it were that easy, you wouldn't need to take a class.
     
  16. Feb 17, 2016 #15

    grandpa2390

    User Avatar
    Gold Member

    Exactly! That is why I am here.
    I have read the chapter several times and the linked articles. If I say it went over my head, then I am not lacking the skill, I am lacking the background.
     
    Last edited by a moderator: Feb 17, 2016
  17. Feb 17, 2016 #16

    DrClaude

    User Avatar

    Staff: Mentor

    You still haven't answered my question about the relevant equations. What equations to you have involving work?
     
  18. Feb 17, 2016 #17

    grandpa2390

    User Avatar
    Gold Member

    I did answer it. W= (integral of) -pdv
    You take the equation of the line and find the area beneath it. Then multiply that by 101 and that gives you work. My problem is I don't know how to find the equation of the line for adiabatic compression/expansion. I emailed my professor though. @ehild did the best he could for me. It sounds like it is a very simple formula or algorithm that I am missing. If I weren't 9 credit hours from graduating, I'd switch majors... :(
    once again, thanks for trying in a respectful manner.
     
  19. Feb 17, 2016 #18

    grandpa2390

    User Avatar
    Gold Member

    My professor explained to me what it is. W=-Pi*Vi^(7/5)*integral(Vi, Vf, V^(-7/5)*dV). I told you I was an idiot. Vi and Vf are constants. We know them. Vf is the variable. I was expecting, for some reason, a line that could be graphed without initial conditions. I'm an idiot. :)

    it's not an inability to read. I proposed that solution earlier, but a dumb flaw in my reasoning prevented me from pursuing it.
     
  20. Feb 17, 2016 #19

    ehild

    User Avatar
    Homework Helper
    Gold Member

    Remember:

    pVγ is constant along a reversible adiabat. (γ=Cp/Cv)

    which means that P2V2γ=P1V1γ. You know P1, V1 and P2. It is diatomic gas, from that you get γ, and you can calculate V2.
    From the equation for an adiabat, ##P=P_1\frac{V_1^\gamma}{V^\gamma}##, so
    ##W=-\int_{V_1}^{V_2}{P_1\frac{V_1^\gamma}{V^\gamma}dV}##
     
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: How much work is done by the Adiabatic Compression of Diatomic Gasses
  1. How much work is done (Replies: 8)

  2. How much work is done? (Replies: 4)

Loading...