What is the essence of the second law of thermodynamics?

The essence of the second law of thermodynamics is that macroscopic physical processes related to thermal phenomena in nature are directional.

The second law of thermodynamics is expressed as follows: It is impossible to transfer heat from a low-temperature object to a high-temperature object without producing other effects, or it is impossible to take heat from a single heat source and completely convert it into useful work without producing Other effects, or small increases in entropy in irreversible thermodynamic processes are always greater than zero.

The discoverer of the second law of thermodynamics

In 1824, French engineer Sadi Carnot proposed Carnot's theorem. The German Rudolph Clausius and the Englishman Lord Kelvin re-examined Carnot's theorem after the establishment of the first law of thermodynamics and realized that Carnot's theorem must be based on a new theorem, the second law of thermodynamics.