What is the second law of thermodynamics?

Formally speaking, the definition required in an exam would be that the second law of thermodynamics states that the entropy of an isolated system never decreases. The second law was conceived with the concept of entropy in mind, which is a quantitative measure of the amount of disorder in a system. Therefore, the second law simply states that the amount of disorder in an isolated system (such as the universe), can never decrease with the passage of time. There are cases that it may seem that this is simply not true, there are for example many chemical reactions that would initially seem to disobey this. However, when you consider the total system including its surroundings you will find that this law is never violated.

Some important ramifications of this is that we can never have the spontaneous flow of thermal energy from a cold to a hot bodies. Recall that heat always flows from hot to cold. Similarly, the law also states that no system is perfect, for example a heat engine can never completely convert its input energy into useful work since there are always losses. This last fact has caused many individuals for the perpetual motion machine, without any luck.