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Gas in a piston with regards to pressure

Posted: Thu Jan 18, 2018 10:11 am
In discussion this week, my TA did this problem with the class:

Gas in a piston does 171 KJ of work. 242 KJ of heat are added. Find deltaU. Is the final pressure of gas higher or lower than the initial?

I understand how to find deltaU with q+w and we ended up getting 71 KJ. I am confused, however, about the second thing that the problem asks us to do. How do we know if the pressure increases or decreases? He used PV=nRT to explain his answer (that it increases), but I still don't understand how he reached that conclusion.

Re: Gas in a piston with regards to pressure

Posted: Thu Jan 18, 2018 10:27 am
Gas in a piston does 171kJ of work means that the volume increases. Using PV=nRT, the amount of gas and T stays the same (and R is a constant). Since V increases, the pressure should decrease to satisfy the equation.

Re: Gas in a piston with regards to pressure

Posted: Thu Jan 18, 2018 10:55 am
My TA said that the pressure increases not decreases though. Also why do you think that T is constant? I thought that it might increase because heat was added.

Re: Gas in a piston with regards to pressure

Posted: Thu Jan 18, 2018 12:01 pm
I agree---if gas in a piston is doing work, it means its pushing the piston, therefore increasing the volume and decreasing the overall pressure. I don't know exactly why T is constant (besides looking at PV=nRT), but maybe it has to do with system vs surroundings? I also don't know for sure if adding heat automatically means raising temp...
sorry I couldn't be of more help---you raise some interesting points