Second law of thermodynamics

A principle stating that the total entropy of an isolated system never decreases. It explains why natural processes have preferred directions and why some transformations cannot be completely reversed.

Entropy: A thermodynamic state function that measures energy dispersal and, statistically, the number of microscopic arrangements compatible with a system's state. Its change provides the quantity constrained by the second law.
Thermodynamic system: A chosen quantity of matter or region of space studied through macroscopic properties and exchanges with its surroundings. The law applies to the total entropy of a system isolated from its surroundings.
Carnot's theorem: A theorem stating that no engine operating between two heat reservoirs can be more efficient than a reversible engine. It turns the second law into a precise upper bound on engine efficiency.
Sadi Carnot: A French physicist whose 1824 analysis of heat engines established fundamental limits on their efficiency. His engine analysis helped motivate the principle later formalized as the second law.
First law of thermodynamics: The principle that energy is conserved, changing form or moving between systems but not being created or destroyed. Energy conservation alone permits transformations that the second law rules out.
Statistical mechanics: A framework that explains macroscopic physical properties through the statistics of microscopic states. It derives the law's statistical direction from the overwhelming prevalence of higher-entropy states.
Isolated system: A system that exchanges neither matter nor energy with its surroundings. Isolation is the condition under which total entropy cannot decrease.
Carnot cycle: An ideal reversible cycle of two isothermal and two adiabatic processes between heat reservoirs. It represents the limiting cycle used to derive maximum possible efficiency.
Rudolf Clausius: A German physicist who formulated the second law and introduced the modern concept of entropy. He stated that heat does not spontaneously flow from cold to hot and defined entropy.
Zeroth law of thermodynamics: The principle that thermal equilibrium is transitive, providing the basis for defining temperature. It defines thermal equilibrium, while the second law constrains how systems approach it.

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