What is the change in work and heat in this thermodynamic process?

[casebj11]

A piston / cylinder arrangement with an initial volume of 1 ft^3 contains water at P = 20 psia, T = 320 F. The piston expands against a linear spring until the final temperature and pressure are P = 10 psia, T = 400 F.

D)calculate the work and heat transfer in the process, and report in BTU


I am confused with this problem since it does not state that heat is being added but the temperature increases. Is this implied?

or does the temperature increase due to the decrease in pressure and increase in volume?

 

[casebj11]

Can anyone help me?

 

[kjohnson]

Yes it is implied when it says calculate the heat transfer. So set up your energy equation Qin-Wout=deltaU. Find your internal energy U at state 1 and 2. Then calculate the work done by the system to move the piston from 1 to 2. Don't forget to add the initial PV term in addition to the work done by the spring. Then that will allow you to solve the heat transfer Qin.

 

[casebj11]

I just received some clarification with my prof. Apparently there is no initial heat transfer. Instead the linear spring has a negative spring constant. Can someone please explain how this would work.

 

[kjohnson]

That doesn't really make much sense, unless he is trying to convey that the spring is initially in tension. Even in such a case there would be heat transfer unless your change in internal energy was exactly equal to the work done by/on the system.