I have a question about the deduction of First Thermodynamic Law. The book that I have is written by Paul A. Tipler and Gene Mosca and it is called Physics: For Scientists and Engineers. The way to deduct it is given here: @constant volume, Qv=Cv(T2-T1) Because W=0, E2-E1=Cv(T2-T1) After all, Cv=d(E2-E1)/d(T2-T1) @constant pressure, Given that E2-E1=Qp-P(V2-V1) and Qp=Cp(T2-T1) We have Cp(T2-T1)=(E2-E1)+P(V2-V1) Now, the author replaces E2-E1 with Cv(T2-T1), I cannot understand this because he incites something under constant volume into a formula under constant pressure. My question is what determines the internal heat in a system is defined by CvP, which can even be used in a situation in which a different condition is given. Please help me if you know! Thank you in advance!