To understand the first law of thermodynamics and its origin. By relating heat, thermal energy, and work, the first law lays the groundwork for thermodynamics. The first law of thermodynamics generalizes the concept of energy conservation to include heat energy. You are probably already aware that loss of total mechanical energy (e.g., from nonconservative forces such as friction) does not destroy energy, but rather converts mechanical energy into thermal energy. This process, as well as the reverse process (conversion of thermal energy into mechanical energy), can be described quantitatively by the first law. Like the law of mechanical energy conservation that it generalizes, the first law relates the changes in energy that occur from the beginning to the end of some process. The first law involves the following physical quantities Using your knowledge of energy conservation, express AEth in terms of Q and W. • W: work done on the system by the outside world, Q: heat added to the system by the outside world, and AEth: thermal energy change of the system.