Thermodynamics -- PV Diagram of an Ideal Gas 1. The problem statement, all variables and given/known data Right, I've got a PV diagram attached to make this easier to explain. The substance is an ideal gas. I need to calculate the heat absorbed along the different paths ACB, ADB and AB. The first two are easy. The red lines in the diagram are isotherms. 2. Relevant equations PV = NKT for an ideal gas. QAB = ΔU + ΔW = ΔU + PΔV, U is internal energy, U = U(T) = CVT For a monatomic ideal gas, CV = 3/2 3. The attempt at a solution The change in internal energy is obviously just 3/2NK(4T1 - T1) = 9/2NKT1 The work done is P1(2V1 - V1) = NKT1 Thus the heat absorbed on the path AB is 11/2NKT1. My problem is the solution manual says it's 6NKT1. It also says that ΔU + ΔW = 3/2(NK)(2T1) + 3/2P1V1 = 6NKT1 which obviously isn't true so I'm questioning it's integrity. It uses the fact that QAB = 6NKT1 in the proceeding problems as well so it's a little confusing. Thanks for any help.