The laws of thermodynamics were formulated in the nineteenth century after careful experimentation. Although the first law of thermodynamics states that energy is conserved in every process, it is the second law of thermodynamics that governs the ordering of events that occur during any process. The second law of thermodynamics can be stated in many ways but probably the most common way is to say that the entropy of an isolated system can never decrease. Entropy can be understood as a measure of a system's closeness to equilibrium or as a measure of a system's disorder. The second law of thermodynamics can be used to show that heat will not pass from a colder region to a hotter one unless work is done.

The history of the second law of thermodynamics can be traced back to Nicolas-Leonard-Sadi Carnot in the early 1800s...