Introduction:

The second law of thermodynamics is one of the four basic laws of thermodynamics, which expresses the irreversibility of thermodynamic process, that is, an isolated system spontaneously evolves in the direction of thermodynamic equilibrium-maximum entropy state. Similarly, the second perpetual motion machine will never be realized.

In an ideal situation, when the system is in thermodynamic equilibrium or undergoes a (hypothetical) reversible process, the total entropy of the system and its surrounding environment can remain unchanged. In all processes (including spontaneous processes), the total entropy of system and environment increases, which is irreversible in thermodynamic sense. The increase of entropy explains the irreversibility of natural processes and the asymmetry between the future and the past.

Although this law is an empirical law in the field of thermodynamics and cannot be explained, it has been explained with the development of statistical mechanics. The history of this law can be traced back to Nicholas Cano's research on the efficiency of heat engines and his Carnot theorem in 1824.