The first law of thermodynamics deals with relationships of quantitative values such as thermal energy and work, and it represents the energy conservation law, which denotes that the summation of all energy is constant. Considering only the first law, it seems easy to convert heat to work or work to heat, but it is known empirically that it is not true. Focusing on the relation between heat and work, for example, it is effortless to change kinetic energy to thermal energy. But in order for the opposite change to occur, another way must be devised. If it is desired to convert heat into work, the heat must be transferred from a high-grade heat source to a low-grade heat source, and a thermal engine is utilized for this. A thermal engine is a device that sends heat acquired from a high-grade heat source to a low-grade heat source and provides work to the outside.