# Homework Help: Thermodynamics- pV^n = constant.

1. Sep 8, 2009

### NBAJam100

1. The problem statement, all variables and given/known data
A closed system consisting of 2 lb of a gas undergoes a process in which pV^n=constant. For: p1=20 lb/in^2 , V1=10 ft^3 and p2=100lb/in^2 V2=2.9 ft^3.

Find n.

2. Relevant equations

pV^n=constant
3. The attempt at a solution

Im just having trouble realizing how to find n. This seems very basic and maybe im just rusty from having the summer off, this is a review question.

(p1)(V1)^n = (p2)(V2)^n

Now Im having trouble realizing how to re-arrange that to find n... Ive tried using the relation p1V1^n=5P1(.29V1)^n but everything keeps canceling out. Can someone push me in the right direction?

Ive also tried solving for n in terms of C and plugging that back in and that just gives me a jumbled mess.

2. Sep 8, 2009

### Mapes

The logarithm function turns exponential factors into multiplicative factors...

3. Sep 8, 2009

### NBAJam100

Yah I understand that. But Even if I do that wont I be left with:

n*ln(P1V1)=n*ln(P2V2)

So... All I can do is cancel things out. I cant really isolate n... right? unless I want #n=0.

Feel free to call me a moron if im missing something obvious :tongue:

4. Sep 8, 2009

### Mapes

You're not taking the logarithm correctly.

5. Sep 8, 2009

### NBAJam100

Oh God I suck....

So does this sound good to you?

ln(P1(V1)^n)=ln(P2(V2)^n)

ln(P1) + n*ln(V1) = ln(P2) + n*ln(V2)

6. Sep 8, 2009

Better!

7. Sep 8, 2009

### NBAJam100

Awesome, thanks Mapes! How did I not see that

I gotta say, I appreciate your help. I havent been on the boards in quite a while but i remember you helping me in the past as well. Thanks again Mapes!