1. The problem statement, all variables and given/known data 2. Relevant equations q = w+Δh 3. The attempt at a solution I fail to understand how they obtained the 3 equations at the bottom. For the first one I understand that q = Δh since w = 0 but why is it h1-h4 rather than the other way around? Also, why does this h1-h4 equal h14 rather than h41? Is there some sort of convention? If yes, why isn't it followed with the next line which is h3-h2 = -h23 ? Similarly, why is Win = h1-h2 rather than the opposite way and why does this equal -h12? Later on in the lecture it says: so now qh = h2-h3 ? So is it h3-h2 or the other way round?