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Thermodynamics: P,V,T graphs and other misc. questions.

  1. Aug 14, 2011 #1
    Hey! You seem to kn ow what youre talking about here and im stuck on a similar question, so is it ok if i ask you something?
    Basically I found your tips on things that you need to include when solving the question really helpful, and I was wondering, should you sketch graphs of specific volume against pressure, temperature, etc, or should you just use regular volume, or both?
    And I was wondering, how do you identify a isentropic problem?
  2. jcsd
  3. Aug 14, 2011 #2
    Re: Mechanical Thermodynamics Help for a Mechanical Freshie!

    Hello! I certainly don't mind helping you, feel free to ask me anything thermo I, I truly loved the course and it challenged me at the same time. Generally speaking you're always going to want to sketch P-v (specific volume) or T-v (specific volume) graphs because specific volume is in your thermodynamic tables, volume is not, and although you can calculate the total volume if you know the mass, there's a lot of problems where you don't know the mass. What's also important on those graphs is to put your vapor dome in there (if it belongs, don't put the vapor dome in if you're graphing an ideal gas!) and put your isobars and isotherms. You have isobars on a T-v diagram and isotherms on a P-v diagram. They're lines of constant temperature and pressure and I could possibly dig up a picture for you if need be.

    An isentropic problem can be identified in a couple ways: 1) they tell you it's isentropic 2) they tell you its reversible AND adiabatic (need both!) 3) they tell you it's an ideal gas with n=k.
  4. Aug 14, 2011 #3
    Re: Mechanical Thermodynamics Help for a Mechanical Freshie!

    Oh thank you! If you actually enjoy being asked thermo questions then I've got a few more for you! :) I've been trying to do question 2 above sticking to your tips and I couldnt tell what type of process it was so I just put "Process:not isobaric, isometric, isentropic or reversible. Ideal gas compression". Would this be right? It feels like it should be wrong...
    Also, I've been trying to work out how you would see that, in the last part of all of the questions the value would always be positive and I got to the point where it must be that (T1+T2)/2 > sqrt(T1 T2). I think that there must be some mathematical proof that you should be able to use to prove that this is true, but I've been trying for hours now and I can't think of what it would be. Any ideas appreciated!!
  5. Aug 14, 2011 #4
    Re: Mechanical Thermodynamics Help for a Mechanical Freshie!

    If you wrote a book with these things in you could probably sell quite a few copies because I think a lot of people have problems getting clear on some parts, like, containing the graphs for each process. I'm just saying cause there arent many clear concise ones out there, and I've seen quite a few now! Alas none have made me a thermo pro yet!!
  6. Aug 14, 2011 #5
    Re: Mechanical Thermodynamics Help for a Mechanical Freshie!

    I think it's my background, I currently work as a math tutor while I go to school and I find myself trying to find different ways to help students understand what's going on. Right now it's mostly highschool kids so it's fairly simple math, but I've managed to help quite a few students that were struggling with calculus. Everybody learns differently though so what might seem good to you is harder for others to grasp. I'm actually in the process of getting a blog up and running that's going to be general help/information for engineering students (not ready to be unveiled though) but I love thermo so maybe something will be made for that specifically.
    Last edited by a moderator: Aug 15, 2011
  7. Aug 15, 2011 #6


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