Explain how the second law of thermodynamics predicts that a heat engine can never be 100% efficient. - AQA - A-Level Physics - Question 5 - 2020 - Paper 5

The second law of thermodynamics states that in any energy transfer or transformation, some energy is always lost to a colder reservoir. Therefore, it implies that a heat engine cannot convert all the heat energy from the hot reservoir into work, as some of that energy must be expelled to a cold reservoir. Hence, for a heat engine to be 100% efficient, the temperature of the cold reservoir would have to be absolute zero (0 K), which is unattainable. This establishes that no real heat engine can achieve 100% efficiency.