Why do we change the use of the heat formula in the following problem from using Cp to then using Cv:
Problem: A gas is initially at 38 ̊C and having Cp= 0.92 KJ/kg.K and Cv=0.71KJ/kg.K is
placed within a cylinder. If 8.5 KJ of heat is added to 5 kg of the gas in a non-
work flow, constant pressure process. Determine:
(a) The final gas temperature
(b) Work done on or by the gas
Solution:
We use Q = mCp (T2 - T1) to find T2 because it is a constant pressure process; but then use
Q = mCv (T2 - T1) to find work.
Shouldn't we still use
Q = mCp (T2 - T1) to find Work ?
Thanks
The work they have calculated is the one that is consumed against atmosphere. Isn't it? They have assumed the process to be constant volume and found out the amount of heat that would have been required to raise the temp by 1.84 degrees Celcius. And then subtracted this heat transfer from previous one. This is not the right approach to calculate thermodynamic work done by the system. The work done by the system should have been difference between initial and final volumes multiplied by pressure. As we do not know either pressure or volume difference of piston we can not calculate work done by the system. Whatever they have calculated does not represent anything related to Thermodynamic work done by piston that can be useful.