Hey. Here is a question i was given:
Consider a process that takes steam from an initial state (P = 1 bar and T = 400°C) to a final state (P = 0.5 bar and T = 200°C). Evaluate the change in internal energy for the steam using (a) the steam tables and (b) ideal gas heat capacity.
I managed to get the correct answer, but i want to just confirm that i used the correct reasoning (for part b).
Since internal energy is a state property, the change in internal energy will not depend on how the system goes from state 1 to state 2, so i created a hypothetical situation where the gas remains at constant volume while lowering in temperature and pressure. Since the volume doesnt change, i can calculate the change in internal energy by using cv.
Is my reasoning correct, or did i just get lucky when solving this?