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 help on the first law please

  1. Mar 14, 2012 #1
    Hi all I am struggling with this textbook example :( I have applied the formula but the numbers I am getting are really wrong!

    A pump compressing hydrogen from a low pressure to a high pressure tank where the tanks pressure is constant and H2 is an ideal gas.
    Hydrogen: (H2: MW = 2, Cv = 10 kJ/kgK)
    Low pressure tank: (P = 4 bar, T = 30oC)
    High pressure tank: (P = 100 bar).

    1) Apply the first law of thermodynamics eliminating terms not needed
    2) Find the compressor outlet temp when the compression goes along an adiabatic and polytrophic process
  2. jcsd
  3. Mar 14, 2012 #2
    For the first law I have used Q-W = difference in energy and attempted to find the temp via finding the internal energy and enthalpy using deltau = cv x delta T and delta h = cp x delta T where I got 8520 KJ and 12063 KJ but now I am not sure how if I should integrate and remove pressure from the first law and find the temp now.
  4. Mar 14, 2012 #3
    They are asking you to apply two laws.
    Ideal gas law and 1st law.

    Clearly till the process is not know you cannot eliminate Q and W from the first law.

    Also how did you find value of change in internal energy.(to be precise delta of temperature).

    Is the final temp given?
  5. Mar 14, 2012 #4
    firstly for the first law does one simply remove pressure from the closed loop first law of thermodynamics?
  6. Mar 14, 2012 #5
    I don't understand your question completely.Can you add some more light?

    If you want to ask if you can take P out of the integral for calculating work, then it is not allowed unless P is a constant
  7. Mar 14, 2012 #6
    From the ideal gas law
    PV/T is constant.

    So P(1)V(1)/T(1) =P(2)V(2)/T(2)

    You know only P(1) P(2) and T (1)
    So this wont help.

    However, if you know the process like PV^x is constant

    Then you can apply ideal gas law and find a relationship between initial amd final volumes and thus initial and final temperature.
    (which is how you will solve your 2nd question)

    You cannot eliminate any term from the first law for this question as no procesa is stated.You can only substitute relations between variable.
    Like you can substitute P with nRT/V

    And how did you find delta U and delta H?
  8. Mar 14, 2012 #7
    Initially the question is worded quite loosely which is confusing (see top). For the first part of applying the first law of thermodynamics (assuming its a closed system) Q2 = U2 - U1 + 1W2 and this needs to be applied to the system eliminating parts not needed. P cannot be moved as you said
  9. Mar 14, 2012 #8
    (re read my previous post.I edited a few parts)

    From what I can catch The question is equivalent to stating that an ideal gas H2 is being compressed from State A to B. you have pressure and temp at A and pressure at B.
    They want you to find temp at B for a polytropic process which by convention is take as PV^x is constant

    Now according to me you cannot eliminate any term from first law.As Q and W are process dependent

    Also please amswer how did you find delta U in your 2nd post?

    Recheck if you have posted all the data stated in the question
    Last edited: Mar 14, 2012
  10. Mar 15, 2012 #9

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    The compressor takes a volume Vi at Pi and compresses it adiabatically to volume Vf and pressure Pf. Pf=100 and Pi = 4 .

    For 1) write out the first law. Since it is adiabatic, what is Q? What does that tell you about ΔU and W?

    For 2) apply the adiabatic condition: [itex]PV^\gamma = K[/itex] to find the ratio of initial to final volume. Use the volume and pressure ratios to find the temperature ratio Tf/Ti.

  11. Mar 15, 2012 #10
    For adiabatic process solve by equation:
    w= (Cv+R)(T2-T1)
    is this giving correct answer?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook