2012 practice final#2A

Volume: $\Delta S = nR\ln \frac{V_{2}}{V_{1}}$
Temperature: $\Delta S = nC\ln \frac{T_{2}}{T_{1}}$

Lily Wei 1D
Posts: 10
Joined: Fri Sep 25, 2015 3:00 am

2012 practice final#2A

For this problem, why is the change in entropy calculated using only $\Delta S = nR ln(\frac{V2}{V1})$
and not $\Delta S = nR ln\frac{P1}{P2}$ ? The problem states that "The He in the metal case is initially held at a pressure of 1.0 atm", so doesn't that mean that the pressure is going to change and we should take that into account?

Irena Roy 4H
Posts: 40
Joined: Fri Sep 25, 2015 3:00 am

Re: 2012 practice final#2A

The question gives you no other information that could lead you to conclude that the pressure did actually change in the process of He leaking.

Alex Nguyen 3I
Posts: 100
Joined: Fri Sep 25, 2015 3:00 am

Re: 2012 practice final#2A

Using the equation with volume change already accounts for the pressure change because of PV=nRT. Therefore you can use one or the other when finding the entropy. Only when there's a temperature change with a volume/pressure change do you need 2 equations with a combined entropy.

Suraj Palaparty 2F
Posts: 27
Joined: Fri Sep 25, 2015 3:00 am

Re: 2012 practice final#2A

The case is initially at 1.00 atm, taking into account the change in pressure later in the problem, while the safe is isolated.