Thermodynamics help (1 Viewer)

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I'm taking a 300 level introductory course to thermodynamics (i am a mechanical engineering major). I am having trouble using the tables, such as knowing when to go by pressure and when to go by temp, etc. i know a lot of this just comes with experience. but i was wondering if there was a flowchart or something on the internet that laid out the process. our exams are open-notes so it would really help. Thanks!
 

Q_Goest

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Hey Alberto. Can you be a bit more specific? Are you refering to the steam tables?

We can help with specific problems, though...
 
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russ_watters

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Generally, it is dictated by the problem. Knowing what information you have and what information you don't and knowing which cycle you are working with and what changes in each step in the cycle is about the only way to go. For me, thermo was a "light-bulb" subject. It's one that you either understand or you don't and in the beginning, you start off not understanding it. You just have to follow the procedures and learn the definitions and the cycles, and that light bulb will eventially go on. Then you'll be able to "see" how the problems work. We can help you with specific problems...
 
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i can do alright on the homework problems because i have a lot of time to think about them. the exams are open book, open notes, but not a whole lot of extra time. i always end up feeling like there is one too many unknowns in the problem. since i have not been able to find a flowchart like i mentioned on the internet, i think i will try to make one myself. For example, if it is a liquid, you go by temp and not pressure (if they don't have corresponding values). That is the sort of thing i will put on my flowchart. i think i will start it tomorrow. if anyone has any advice, let me know, thanks!
 
Just ask yourself. Is the fluid superheated gas saturated or liquid. Once you know that the tables speak for themselfs. Remeber you need to properties thus intesive properties such as density and pressure and you know the rest
 
Hi there:
Be careful when reading the exam problems.
Write down on a piece of paper what is given and what needs to be calculated or provided by the student.
Then go to the steam tables and just go back and forth -- on a T-s diagram draw the state points as well as the cycle by itself. This should give you a good idea of where you are.
If you are good with homework problems, then you should be fine with you exam type problems.
Thanks,
Gordan
Good Luck!
 
i have a question to add... i under stand the charts and the T-V graphs that are made from them... but i don't get how to take information from the question and get those graphs from it...
 

stewartcs

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i have a question to add... i under stand the charts and the T-V graphs that are made from them... but i don't get how to take information from the question and get those graphs from it...
The property tables give the state(s) of various substances under specific conditions. One would use them to solve the question, not use the question to create the tables.

Do you have a specific example of a problem you are having trouble with? Examples generally help students to grasp the concepts better.

CS
 
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This is always very difficult for new thermo students and I even had a lot of problems with it myself. Ultimately what it comes down to is the states at which the substance is in regardless of the problem. I usually first determine the phase (saturated liquid, liquid/vapor mix, etc) which will narrow the tables down a bit. Then I look at the information I'm given. Problems usually only give either the temperature or pressure for a given state, usually not both. By using those two criteria you can usually nail it down to one. To get good at it, you really just need to practice, a lot. Eventually you have will have click in your brain and it will become second nature.
 
Hey i want to tell u something that these tables are used only when u have 2 Known s, and You want to find the third unknown.
 

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