# Homework Help: Entropy with non constant specific heats

1. Mar 12, 2012

### Goatsenator

1. The problem statement, all variables and given/known data
Consider air inside a piston-cylinder device that is compressed adiabatically from p1 = 100kPa, T1 = 300 K, and V1 = 0.5 m^3 to V2 = 0.03 m^3 and then heated at constant pressure until T3 = 2000 K. The heat is exchanged with a heat reservoir at TH = 2000 K. Determine P2, T2, P3, V3, W1-2, W2-3, Q2-3, and ∆1−2, without assuming constant specific heats, instead using data given in Table A7.1 in the text.

The table just gives standard entropy.

2. Relevant equations

I really just need to find T2 and can get everything else from there. The equation the professor gave was

S0(T2) - S0(T1) = R*ln(V2/V1) + R*ln(T2/T1)

where S0 is the standard entropy. I derived this myself as well and understand where it comes from.

3. The attempt at a solution

I tried to substitute S0(T1) + R*ln(P2/P1) for S0(T2) but then it ends up cancelling T2 in the equation so I have no idea what to do.