What principle does the concept of entropy illustrate in a thermodynamic system?

The concept of entropy is fundamentally tied to the direction of spontaneous processes in thermodynamics. Entropy is a measure of the disorder or randomness in a system, and the second law of thermodynamics states that the total entropy of an isolated system can never decrease over time. Instead, in any spontaneous process, the entropy of the system and its surroundings will increase.

This means that systems tend to move toward a state of greater disorder, and processes that result in an increase of entropy are naturally favored. For example, when a hot object comes into contact with a cold one, heat flows from the hot object to the cold one until thermal equilibrium is reached, and the overall entropy of the combined system increases. Thus, the principle illustrated by entropy is that natural processes tend to proceed in a direction that increases the overall entropy of the universe.