Energy Balance in Closed Systems | Thermodynamics | (Solved examples)

Learn about energy balance in closed systems, and how internal energy (U) changes when heat or work is done on/by the system. We also solve a few examples step by step to cover the material learned.
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Intro (00:00)
A 0.5-m^3 rigid tank contains refrigerant-134a (03:23)
A rigid 10-L vessel initially contains a mixture of liquid water (06:43)
A rigid container equipped with a stirring device (09:22)
Books used:
📘Çengel Yunus A. and M. A. Boles, Thermodynamics: an engineering approach. New York, NY: McGraw-Hill Education, 2016.
📘C. Borgnakke and R. E. Sonntag, Fundamentals of thermodynamics. Hoboken, NJ: Wiley, 2019.

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[@QuestionSolutions]

I am really glad to hear that! Best wishes with your studies.

[@josephisrael8959]

Why was a potentially changing quality considered in the second example but not in the first one, considering both situations are identical (two fluid mixtures in a sealed rigid tank being heated up)?

[@QuestionSolutions]

You could do it that way, but we used a super heated vapor table to figure out the second value, so we didn't need the quality change in the first example. In the 2nd example, we used the same equation we used to find U1 to find U2 so we needed the new quality value. Two different methods were used.

[@josephisrael8959]

@@QuestionSolutions So essentially, you only consider a quality change when the value of U2 can be directly found from the table without having to extrapolate it like you did in the 1st example?

[@QuestionSolutions]

@@josephisrael8959 You can do it either way depending on whether you have the necessary table or not. So it boils down to what is given, and what method you find is easier.

[@josephisrael8959]

@@QuestionSolutions That clarifies things. Thank you very much.

[@QuestionSolutions]

@@josephisrael8959 You're very welcome!

[@muheezolamilekan9818]

Hello, in the second example you used the same equation to find the U1 and U2 by using the table and finding a new quality but did it differently in the first, is there a reason for that or both can be applied. Thank you very much for the videos

[@QuestionSolutions]

So you can actually use multiple methods to get to the same answer. I try to show as many as I can, sometimes, I focus on one method, other times, multiple ones. Usually, it's easier to use one over the other, but that doesn't mean it's not useable. Use whichever method you'd like :)

[@muheezolamilekan9818]

@@QuestionSolutions Oh alright, thanks a lot for the quick reply.

[@narendradc2402]

do you think in the last problem you missed showing a dot? it is about rate of change.

[@QuestionSolutions]

Yes, rate change should have a dot on top.

[@JGambrosia]

Just wondering, what is specifically is evaporation internal energy? Is it the internal energy of a liquid undergoing evaporation? Maybe the internal energy of a liquid already evaporated (gas)?

[@QuestionSolutions]

Sorry, where are you referring to? Please provide me a with a timestamp. Thanks!

[@JGambrosia]

@@QuestionSolutions i’m referring to 4:12 u_fg.

[@JGambrosia]

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[@QuestionSolutions]

@@JGambrosia It's the change in internal energy during evaporation. Easy way to remember is to see that it's actually just the difference between Ug and Uf, so Ug - Uf = Ufg.

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[@7mood3bdullah47]

how do you know q+w but you write q-w

[@QuestionSolutions]

I don't know where you're referring to. Please use timestamps.