Final answer:
A heat engine converts thermal energy to mechanical energy, but cannot be 100% efficient due to the second law of thermodynamics, which ensures some energy is always lost as heat.
Step-by-step explanation:
What is a Heat Engine?
A heat engine is a system that converts thermal energy into mechanical energy. Examples of heat engines include car engines, steam turbines, and the old-style steam engines on trains. These systems operate on the principles of thermodynamics, particularly the first and second laws.
Efficiency of Heat Engines
A heat engine's efficiency is defined as the ratio of the work output to the heat input. This efficiency can never be 100% due to the second law of thermodynamics, which states that some energy will always be lost as heat during the conversion process.
Second Law of Thermodynamics
According to the second law of thermodynamics, it is impossible for a heat engine to achieve 100% efficiency because a certain amount of energy is unavoidably lost to the surroundings as heat, which cannot be used to do work. This inherent energy loss is why no heat engine can convert all its input energy into mechanical work.