Internal Energy: gas inside piston

[cokeaddict]

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.


Im thinking:

C = Q / delta T

C (delta T) = Q

500 x (400-300) = 50000 J = Q

so...delta U = Q - W

W = P delta V

i guess P is the pressure inside the piston...I've told something like this:

Inside pressure = outside pressure - weight_force/cross_sectional_area

I tried that, i came up with 456500 = ( 101300 - 10000) / .2

? i don't know...any help would be appreciated...

 

[cokeaddict]

oops, i actually got it...thanks though...