Engineering Thermodynamics Work And Heat Transfer

They are not properties of a system. You cannot look inside a pressure cooker and say, "This contains 50 joules of heat." You can only say, "Heat transferred into the cooker." Once the energy crosses the boundary, it becomes part of the system’s internal energy.

A gas expands adiabatically ((Q=0)) against a piston. Then (-\Delta U = W)—the work done comes entirely from a decrease in internal energy (temperature drops). engineering thermodynamics work and heat transfer