Chen explained that heat engines illustrate only the second law of thermodynamics because they involve the flow of thermal energy from hot to cool areas. His friend Mia disagreed and explained that these engines illustrate only the first law of thermodynamics because they are sometimes 100 percent efficient if they conserve enough energy.

Mia is incorrect because a heat engine cannot convert all absorbed heat into useful work. By the second law of thermodynamics, some thermal energy must be released to a cooler surroundings or reservoir, so a heat engine cannot be 100 percent efficient. Heat engines also illustrate the first law through conservation of energy, so Chen is mistaken to say they illustrate only the second law.

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