What is the Second Law of Thermodynamics?
There are many ways of stating the Second Law of Thermodynamics—there are many textbook definitions associated with different scientists. But its essential conclusion is that a heat engine (an engine that uses heat and operates in a cycle) cannot convert 100% of the heat produced within it (from, say, fuel combustion) to an equivalent amount of useful work. Nature has supposedly saddled us with an efficiency limitation that cannot be broken.
