Entropy | Definition & Equation

Entropy is a fundamental concept in physics that measures the thermal energy per unit temperature that is unavailable for doing useful work. It is also a measure of the molecular disorder or randomness of a system. Introduced by physicist Rudolf Clausius in 1850, the concept of entropy provides deep insight into the direction of spontaneous change in various phenomena.

The idea of entropy allows us to mathematically encode the notion of which processes are impossible, even though they do not violate the conservation of energy. For example, the melting of a block of ice on a hot stove is an irreversible process, while the thawing or freezing of a block of ice in an ice-water bath is a reversible process. Entropy is a measure of the molecular disorder and randomness associated with these processes.

Clausius defined entropy as a way to express the second law of thermodynamics, stating that spontaneous change in an isolated system always proceeds in the direction of increasing entropy. This means that the total entropy of a system and its surroundings always increases for irreversible processes.

The concept of entropy is closely related to temperature. According to Clausius, if heat flows into a heat reservoir at a temperature above absolute zero, the entropy increase is equal to the amount of heat divided by the temperature. This equation provides an alternate definition of temperature that aligns with the usual definition.

Entropy also plays a crucial role in determining the efficiency of heat engines. The maximum possible efficiency of a heat engine is determined by the condition that the net entropy change is greater than or equal to zero. This condition limits the amount of work that can be done by the engine.

Entropy is an extensive property that depends on the amount of material in the system. In statistical interpretation, entropy is proportional to the natural logarithm of a quantity representing the maximum number of microscopic ways in which the macroscopic state can be realized.

Overall, entropy is a fundamental concept in physics that helps us understand the direction of spontaneous change and the limits of thermodynamic processes. It provides insights into the behavior of systems and the efficiency of heat engines.

The original article: https://www.britannica.com/science/entropy-physics