A student that just received their driver’s license decides to buy a new car. He remembers from physics class that heat engines have a thermal efficiency that can be calculated. He asks the car dealer for a car that has a high thermal efficiency rating. The car dealer says he has a car that is 100% efficient. Which law of thermodynamics best describes why the car cannot be 100% efficient?

The law that  best describes why the car cannot be 100% efficient is the second law of thermodynamics.

Thermodynamics is the study of heat. Heat is a form of energy that exists as a result of temperature difference between bodies.

According to the second law of thermodynamics, the conversion of energy  from one form to another is not 100% efficient. As such, the thermal efficiency of the engine can be calculated.

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Second law of thermodynamics.

Explanation:

The second law of thermodynamics states that: "in all energy exchanges, if there is no energy entering or leaving the system, then it means the potential energy of that particular state will definitely be less than the potential energy it had at it's initial state."

Now, for a car to be 100% efficient, it means that there must be some form of heat transfer to the environment. However, this is a violation of the second law of thermodynamics and as such it can't be 100% efficient.