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: Thermodynamics gases

  1. May 11, 2016 #1
    1. The problem statement, all variables and given/known data
    two cubic ft, of air at 85 f and 13.9 psi, compress to 115 psi.
    What is the final temp, the work, the internal energy "delta u", and Q

    2. Relevant equations
    well i dont have any equations, i am asking for the equations

    3. The attempt at a solution
    i dont know how to aproach to this problem because i dont know if this is
    or adiabatic process,
    i am sufing the web, but there is a ton of equations out there and i feel overwhelmed about it, so i coul use some help to make a solid base to start in thermodynamics
    Last edited: May 11, 2016
  2. jcsd
  3. May 11, 2016 #2


    User Avatar

    Staff: Mentor

    Isobaric means "at constant pressure" - can you compress something in an isobaric process?
  4. May 11, 2016 #3
    sorry i made a mistake it was
  5. May 11, 2016 #4


    User Avatar

    Staff: Mentor

    If it was isothermal - does it make sense to ask about the final temperature?
  6. May 11, 2016 #5
    I think they meant for you to assume that the compression is adiabatic and reversible, although this certainly isn't clear from the problem statement. Have you learned the equation for the first law of thermodynamics? If so, why didn't you list that as one of your equations? What about the ideal gas law; did you not learn about that?
  7. May 11, 2016 #6
    Yes i know these equations, the ideal gas is pv=nrt and firts law of thermpfynamics its w= Q-internal energy
  8. May 11, 2016 #7
    you are right, it was so obvious, sorry its adiabatic because everything changes
    how i should use this kind of info "one lb of air"? i dont see any weight or mass unit in equations, just "n" but if its air how i can know the moles of "air" i can if its two gases or one, for this problem i made the calculatins to find temperature final, and volume final, but i am stuck at the work calculation i have all exept the "n"

    i convert all to non english units

    V1=58.6337 lt
    T1=302 k
    P1=0.9458389 ATM
    P2=7.8252 ATM

    using that i found****
    T2=552.34039 K
    V2=12.9617 LT
    now i just need the W and internal energy because the problem ask for Q but in adiabatic Q=0 so i need a litle litle help

    i have this formulas for work***

    whic one its correct? and how i can know n?
    Last edited: May 11, 2016
  9. May 11, 2016 #8
    To get n, use PV=nRT together with the initial conditions.
  10. May 11, 2016 #9
    Both equations are correct, and both should give the same answer.
  11. May 11, 2016 #10
    should i use R=(0.0820574)ltatm/kmol?
    and what p and v, should work for p1v1=nRT1 and for p2v2=nRT2, i mean those two equations are valids or just whit initial valules, also if W=-internal energy any value of work its equal to a loss of internal energy and there is no need to calculate it_? also another question what if the problem says "1 lb of air" how i should consider the mass in equations?
    Last edited: May 11, 2016
  12. May 11, 2016 #11
    I'll leave it to you to work this out, considering the units that are needed.
    Try both equations, and see what you get.
    The first equation you gave actually is that change in internal energy.
    I don't understand this question. Are you asking how to convert lbs to kg, grams, or gram moles?
  13. May 11, 2016 #12
    Well my other problem are the "same" exept for the mass thing, this is an example:
    "one diesel machine have 159.09 grams of air, compress from 14.7 psi to 465 psi, whith 23.8 c of initial temp and 525.55 of final temp, calculate w and Q" its the same but the grams or lb, thing i dont know how to handle in the calculations
  14. May 11, 2016 #13
    You don't know how to convert from grans to gram-moles, correct? The molecular weight of a material is equal to the number of grams per gram mole of material. So, to convert from grams to gram-moles, you divide by the molecular weight.
  15. May 11, 2016 #14
    thanks for the response, but i still have doubts, the "air its a mix" i get the idea of gramss/mols of x given gas or two, tree but what gases i should consider for air and proportions?
  16. May 11, 2016 #15
    Use 21 mole percent for oxygen and 79 mole percent for nitrogen. This gives a molecular weight of (0.21)(32)+(0.79)(28)=29
  17. May 11, 2016 #16
    thank you very much i was really piss of about this because the teacher dint explain nothing at all, many thanks
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted