What principle describes that energy will not spontaneously flow from a colder body to a hotter body?

The Second Law of Thermodynamics is fundamental in describing the direction of heat transfer. It states that energy will not spontaneously flow from a colder object to a hotter one. This principle establishes the concept of entropy, which describes the degree of disorder or randomness in a system. In any given process, the total entropy of an isolated system can never decrease over time; it either increases or remains constant.

This law explains why heating a cold room with cold air can’t spontaneously warm up hot objects within it without external work being done. To transfer heat from a cold body to a hot body, some external energy must be applied, as in refrigeration systems that utilize compressors to move heat against its natural flow, thereby creating a conditioned space that is cooler than its surroundings.

Understanding this law helps refrigeration operators anticipate how systems will function and ensure they are set up properly to manage heat transfer in an efficient manner.