Change in Internal Energy

lilkrazyrae

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.

mezarashi

I can't see why it could be wrong. Why do you suspect so?

vaishakh

You don't know W????
Or is the doubt about signs.
Well is it that the book gives some differant answer than what you got?

lilkrazyrae

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.

lightgrav

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!

lilkrazyrae

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

lando45

Your conversions seem alright to me, I don't see how that could be incorrect...