Second law of thermodynamics

The second law of thermodynamics is an expression of the universal law of increasing entropy. In simple terms, it is an expression of the fact that over time, differences in temperature, pressure, and density tend to even out in a physical system that is isolated from the outside world. Entropy is a measure of how far along this evening-out process has progressed. The most common enunciation of second law of thermodynamics is essentially due to Rudolf Clausius:

There are many versions of the second law, but they all have the same effect, which is to explain the phenomenon of irreversibility in nature.