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


by fractal01
Tags: graphs, misc, thermodynamics
fractal01
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#1
Aug14-11, 11:34 AM
P: 14
Quote Quote by ReaverKS View Post
I've attached a picture that is essentially the setup for the problem. There's really only one other major obstacle to overcome and that is that the states are not fixed. I.e. state 1 all you know that is obvious is that P1=0.1Mpa=100kpa but you need to know something else to be able to start doing some calculations. My advice is to open your book or notes and look for the basic definition of quality. Once you get the quality, with knowing the pressure the state is fixed and you can easily calculate anything you could want (including internal energy, u1). State 2 also needs to be fixed, they give you x2=1, you have to read the problem carefully to gather this information. You need to know more information about state 2 in order to fix it, so that you can calculate other information about state 2, such as u2. You can actually calculate v1 and you know that v1=v2 because it's a closed system, and I assume the vessel to be rigid, therefore V1=V2 and v1=v2. Now you know x2=1 and v2=v1 which you can calculate v1 if you know x1. Now you've got state 2 fixed and you can calculate the quality at state 2, then you can calculate the internal energy at state 2. Then by utilizing the first law, you just need to determine what the work is, look at my post above for the general formula for a closed system for work. Notice that this is an isometric system (constant volume) so what does that mean about dV?
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?
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ReaverKS
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#2
Aug14-11, 12:08 PM
P: 16
Quote Quote by fractal01 View Post
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?
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.
fractal01
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#3
Aug14-11, 07:23 PM
P: 14
Quote Quote by ReaverKS View Post
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.
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!!

fractal01
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#4
Aug14-11, 07:30 PM
P: 14

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


Quote Quote by ReaverKS View Post
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.
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!!
ReaverKS
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#5
Aug14-11, 11:23 PM
P: 16
Quote Quote by fractal01 View Post
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!!
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.
Redbelly98
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Aug15-11, 08:05 PM
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