Calculating Change in Internal Energy: Work and Heat in an Insulated System

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Homework Help Overview

The discussion revolves around calculating the change in internal energy for an insulated system, involving the concepts of work and heat transfer. The original poster expresses uncertainty about applying the formula U=Q-W correctly, particularly regarding the signs and units involved.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the relationship between work done and heat absorbed, questioning the correct interpretation of the variables and units. There is uncertainty about whether the heat value is correctly assigned and how to handle unit conversions.

Discussion Status

Some participants have provided guidance on the signs of the variables and the need for unit conversion. There is an ongoing exploration of the calculations, with no clear consensus on the correctness of the final result yet.

Contextual Notes

Participants note the potential complexity of the problem compared to previous ones, indicating that there may be additional factors or assumptions at play that have not been fully addressed.

lilkrazyrae
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930.J of work are done by an insulated system that absorbs 3.35Kcal of heat. What is the change in the internal energy of the system?

Ok so I don't quite understand this. I know that U=Q-W. but I do I just do 3.35Kcal minus W? This just doesn't seem right.
 
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I can't see why it could be wrong. Why do you suspect so?
 
You don't know W?
Or is the doubt about signs.
Well is it that the book gives some different answer than what you got?
 
I wasn't sure if Q=3.35Kcal. Does it equal only that? And I don't even know what the answer is supposed to be, its just usually all our problems are more difficult that i didn't know if there was something else to it.
 
Last edited:
yes, Q = 3.35 kcal , positive because it is Energy absorbed by this "system".
The Work done by the system takes Energy out of the system, so it's subtracted. You can't subtract Joules from kcal ... convert one of them!
 
Ok so what I've got is 3.35kcal=3350cal=3350cal*1J/.289cal=14022.604J
Then U=14022.604J-90J=13092.604J and this is my change in internal energy.
Is this correct
 
Your conversions seem alright to me, I don't see how that could be incorrect...
 

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