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Thermodynamics -- calculating fluid mass

  1. Nov 24, 2015 #1
    This is quite possibly very basic, but I have a horrible habit of forgetting basics as I move up the ladder in difficulty during the semester. This is a question regarding Thermodynamics, taken from Chapter 1; Introduction and the First Law of Thermodynamics of the book Applied Thermodynamics for Engineering Technologist (5th Edition).

    I am including the whole question just to avoid any misunderstandings.

    1. The problem statement, all variables and given/known data
    A fluid at 0.7 bar occupying 0.09m3 is compressed reversibly to a pressure of 3.5 bar according to law pvn = constant. The fluid is then heated reversibly at constant volume until the pressure is 4 bar; the spesific volume is then 0.05m3 / kg. A reversible expansion according to a law pv2 = constant restores the fluid to its initial state. Sketch the cycle to a p-v diagram and calculate:

    i) The mass of fluid present (THIS is the one I cannot seem to figure out.. basics be damned)
    ii) the value of n in the first process.
    iii) the net work of the cycle.

    2. Relevant equations
    It is a fluid, not explicitly stated to be gas or liquid, so the basic pv = nRT doesn't necessarily apply here.

    3. The attempt at a solution
    I'm stumped, really. I have the answer (0.0753 kg) but I cannot figure out how to get to it.
     
  2. jcsd
  3. Nov 24, 2015 #2

    DrClaude

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    Staff: Mentor

    Did you do that?

    To get the mass, you will need to use the specific volume given.
     
  4. Nov 24, 2015 #3

    DrClaude

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    Staff: Mentor

    I just calculated it myself, and I'm off by a factor of 10. Could you please check all the numerical values you have given in the problem?
     
  5. Nov 24, 2015 #4
    Okay, so I rechecked the values, and the Spesific volume is meant to be 0.5, NOT 0.05. One slight tap too much on the 0 there.

    Also, I did sketch the p-v diagram, yes. I'm pretty sure I got the understanding of how the process looks.

    And when you say that I need to use Spesific volume, it strikes me how stupid I have actually been trying to figure this out. With that in mind, which formula would you suggest looking into for calculating the mass when I don't actually have the current volume? It's a pvn = constant, so I can't figure it out with the p1V1 = p2V2
     
  6. Nov 24, 2015 #5

    DrClaude

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    Good, then I get the correct result.

    Then you should see that there are two processes that link the initial state to the unknown volume.
     
  7. Nov 24, 2015 #6
    Yes there is. The 1 -> 2 and 3 ->4 both link the process from the initial volume to the unknown. Does it have anything to do with the pv2 = constant that is the exponent for the revesible process happening in 3 -> 4?

    Or put in this way, I can figure out the volume using p1V12 = p3V32, where V3 = V2
     
  8. Nov 24, 2015 #7

    DrClaude

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    Staff: Mentor

    Correct.
     
  9. Nov 24, 2015 #8
    Then I have my answer and I am greatly appreciative of your help. Thank you very much!
     
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