# Energy of gas as the system

1. Oct 27, 2005

### huskydc

A cylinder (cross section is 0.2m2) with a free moving piston is filled with gas. The piston is attached to a heavy weight W = 10000N. Outside the cylinder, the air is at 300K and 1atm. Initially the gas is at 300K, then it is heated to 400K. The heat capacity of the gas under the constant pressure is 500J/K.
If the length of the gas in the cylinder l increases by 20cm during the heating, find the change in the internal energy of the gas in Joule J.

i figured....heat is being transferred to the gas, and work is done by the gas, so both Q and W should be postive...

delta U = Q - W

i'm not really sure how to start this....but i tried the following:

C = Q/ delta T

500 = Q / (400-300)

Q = 50000 J

W = P delta V (i'm not sure if the P is of the system or the surrounding)

but with the problem, I've been given a clue that says this:

P system = (P surrounding - W force)/cross area...

and I came up with 456500 J...

Q- W

50000 - 456500 = delta U...apparently the answer didn't work, any clues?

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2. Oct 28, 2005

### huskydc

can i even use the equation: delta U = 1.5 n R delta T
but that would include finding out n....which i don't have enough info to calculate....any one help??
i'm lost... =(

Last edited: Oct 28, 2005