# Homework Help: Piston/cylinder expands polytropically

1. Feb 17, 2010

### OsDaJu

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

A piston/cylinder contains helium at 500 K and 1200 kPa. It expands polytropically with n=1.667 to a pressure of 100 kPa. Find the final temperature, the specific work and specific heat transfer in the process

2. Relevant equations
U2-U1=1Q2-1W2
1W2=P2V2-P1V1/(1-n)
P1*V1^n=P2*V2^n
PV=mRT

3. The attempt at a solution

(P1V1/T1)=mR=(P2V2/T2)

I'm trying to plug that into this equation (P1*V1^n=P2*V2^n) to find P or T but I can't since I don't have enough data to use it.

I only have intensive information to work with. Can I assume that this is an ideal gas or should I look for the stage it is in? What am I doing wrong?

2. Feb 17, 2010

### rock.freak667

$$\frac{P_1 V_1}{T_1}=\frac{P_2 V_2}{T_2}$$

make V1 the subject and then sub that into P1V1n=P2V2n

You should then get an equation involving P1,P2,T1,T2 and n