I'm having an issue with a question I'm trying to solve. Here is the question:

A gas with an initial pressure of 800Kpa, 0.02m3 expands following a polytropic process. The final conditions are 350Kpa, 0.042m3. Find the following:

a) Index of expansion

b) Work done

c) Change in Internal Energy

d) Heat transfer

I'm stuck on c). If someone could guide me through it would be of help.

Answers so far:

a) n = 1.11 using P1/P2 = (V2/V1)^n

b) 11.82KJ using P1V1-P2V2/n-1

Why I'm stuck: During my studies I've found internal energy to be only tied to temperature. What am I missing?

Thanks gentlemen.