# Entropy, thermaldynamics

1. Oct 5, 2011

### Liquidxlax

1. The problem statement, all variables and given/known data

A cylinder contains 1000mol o He gas at an initial temp o 2000k and initial pressure of 1MPa. The He gas is now cooled to a final temp o 500k in a reversible process in which the volume and pressure are constrained to vary as PV3 = constant. Assume that the He is a monatomic ideal gas. Denote the initial and final states of the gas by A and B, respectively.

a) Find the initial volume VA of the gas

b) use the 1st law o thermodynamics to show that

dQin = (3/2)nRdT + (PAVA3dV)/V3

c) eleminate P from the two process equations PV = nRT and PV3 = PAVA3

and hence that

blah blah blah

d) Use dS/dQinrev/T to find the entropy change ΔS = SB - SA

2. Relevant equations

all given in the question

3. The attempt at a solution

I solved a) b) c) with no trouble, but i'm just uncertain about d)

doing the integral i found that the change in entropy ΔS = -1.152x104 and i was wondering i this is reasonable, because i thought entropy is supposed to be ≥0.

I thought to myself that the ΔSuniverse = 0 such that ΔS + ΔSsurrounding = 0

would this be a good assumption?

2. Oct 5, 2011

### Mapes

Yes; that's what "reversible" implies.

3. Oct 5, 2011

### Liquidxlax

derp :facepalm: thanks for the help