The second law of thermodynamics is one of the basic laws of thermodynamics, which means that heat can only be transferred from heat to cold (in natural state) forever. It is an experience summary about the irreversibility of all physical and chemical processes related to thermal motion in a limited space and time.

Under natural conditions, heat can only be transferred from high-temperature objects to low-temperature objects, but not from low-temperature objects to high-temperature objects automatically. That is to say, under natural conditions, this transformation process is irreversible. Only by consuming work can the direction of heat transfer be reversed.

Any form of energy in nature is easy to turn into heat, but conversely, heat can't completely turn into other forms of energy without other influences, which shows that this transformation is irreversible under natural conditions. Thermal energy can continuously change heat into mechanical work, which is bound to be accompanied by heat loss. The second law is different from the first law. The first law denies the possibility of creating energy and eliminating energy. The second law clarifies the directionality of the process and denies the possibility of using energy in a special way.