Energy is conserved; usefulness is not. Engines, entropy and the limits nature sets.
The first law ΔQ = ΔU + ΔW and internal energy as a state function, work done by a gas in isothermal, isobaric and isochoric processes, heat engines and efficiency (steam, petrol and diesel engines), the second law (Kelvin–Planck and Clausius), reversible and quasi-static processes, entropy, the Carnot engine and theorem, and refrigerators.