Thermodynamic

Thermodynamics is a branch of physics that studies the relationships between heat, work, temperature, and energy in systems. It focuses on macroscopic properties and uses state variables to describe the state of a system, regardless of its microscopic composition.

A system exchanges energy with its surroundings through heat and work. Processes are characterized by conditions like isothermal, isobaric, isochoric, and adiabatic. Equilibrium and the concept of reversibility distinguish idealized processes.

The four laws form the foundation. The Zeroth Law establishes temperature and thermal equilibrium. The First Law, the law of energy conservation, states that energy cannot be created or destroyed; for a closed system, dU = δQ − δW. The Second Law introduces entropy and states that the total entropy of an isolated system never decreases; it also explains the direction of natural processes and the concept of irreversibility. The Third Law asserts that as temperature approaches absolute zero, the entropy of a perfect crystal approaches a minimum value.

Thermodynamic potentials such as internal energy U, enthalpy H, Helmholtz free energy F, and Gibbs free energy G express energy content under different constraints. Useful relations include H = U + pV and G = H − TS. The Second Law leads to limits like the Carnot efficiency, η = 1 − Tc/Th for heat engines.

Applications span engines, refrigerators, chemical reactions, phase changes, and material science. Thermodynamics also connects to statistical mechanics, which explains macroscopic laws from microscopic behavior.

Historical notes: foundations laid by Carnot, Clausius, and Kelvin, with later formalizations of entropy and statistical links.