Which law of thermodynamics is commonly applied in heat engines?

The second law of thermodynamics is fundamental to the operation of heat engines. It states that not all energy can be transformed into useful work during a process, and there will always be some energy lost as waste heat to the environment. In the context of heat engines, this law helps us understand that no engine can be 100% efficient; some energy will always be converted to heat rather than useful work. The efficiency of a heat engine can be influenced by factors such as the temperature difference between the heat source and the heat sink, and this relationship is encapsulated by the Carnot efficiency, which is derived from the second law.

In contrast, while the first law of thermodynamics describes the conservation of energy, effectively stating that energy in a closed system is constant, it does not give insights into the direction of energy transformations which is crucial for heat engine functionality. The third law of thermodynamics addresses the behavior of systems as they reach absolute zero and does not apply to traditional heat engines. The law of thermodynamic equilibrium deals with the state when a system has uniform properties and does not directly pertain to the efficiency and functionality of engines. Thus, the second law is the most relevant to the principles governing heat engines.