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Homework Help: PV^lamba=constant in an isentropic Flow

  1. Jan 10, 2016 #1
    if a gas expands isentropically (consider the process reversible adiabatic), from point 1 in a nozzle to point 2 downstream of the nozzle. If you know the pressure and temperature at point 1, can you use the relationship pv=1/M RT to work out the specific volume at point 1? And similarly if you know pressure at point 2 and temp at point 2, can you use Pv= 1/M RT to work v2? Or do you have to use the equation Pv^lamba = constant. The thing is you don't know the constant here.
  2. jcsd
  3. Jan 10, 2016 #2
    If ax = b and you know a and b, do you have enough information to determine x?

    If Pv=MRT, and you know P, T, and R, do you have enough information to determine (v/M)?
  4. Jan 10, 2016 #3
    You were NOT helpful at all with that silly answer. I don't know why you bothered replying. All I am asking is can you use pv=(1/M)RT to determine specific volume at either points 1 or do you have to use the adiabatic expression Pv^lamba. Clearly this is not your area!
  5. Jan 10, 2016 #4
    I'm sending you a Warning with infraction points for insulting another member, namely me. I happen to be an expert in thermodynamics, but also in mathematics. You may not be familiar with how we do things here at Physics Forums. Let me fill you in. Our goal is to help you solve your own problem, not just give you the answer. We do this by giving you hints and asking you leading questions so that you can work through the answer by yourself. There was a reason I asked you these specific leading questions. Please have some patience and reconsider the questions I asked you.

    If I get another response like the previous one you sent, you will be banned from Physics Forums.
  6. Jan 10, 2016 #5


    Staff: Mentor

    In addition to what Chet already said, your post is deserving of a warning all by itself. Homework questions must be posted using the homework template, which you apparently deleted. Since Chet already dinged you with a heavy (and well deserved, IMO) infraction, I won't add to it this time...
  7. Jan 12, 2016 #6
    I apologise but this does not detract from the fact your response was not helpful. I would not consider it a leading question. I do appreciate the significance of leading question but that is not what it was. Anyway, I have figured it out, but thanks for at least posting a reply. I would also request you to read people's post carefully before answering, as then your answers would be more helpful. My question wasn't to do with being able to find vM by knowing P, R and T. You can have a look at it again if you want to, but it is not necessary for me at this point.
  8. Jan 12, 2016 #7
    Your main question was: If you know the pressure and temperature at point 1, can you use the relationship pv=1/M RT to work out the specific volume at point 1?

    It takes two intensive variables to establish the thermodynamic state of a system. If you know any two of these three (pressure, temperature, specific volume), you have enough information to uniquely determine the third one using the equation of state, in this case, the ideal gas law. My point was that if you know pressure and temperature of an ideal gas, you can always use the ideal gas law to uniquely determine the specific volume. You don't need another equation to do this. I was merely trying to help you figure this out on your own.

    Last edited: Jan 12, 2016
  9. Jan 18, 2016 #8
    Thank you, and again I am deeply sorry.
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