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Thermodynamics, field of physics that describes and correlates the physical properties of macroscopic systems of matter and energy. The principles of thermodynamics are of fundamental importance to all branches of science and engineering.

A central concept of thermodynamics is that of the macroscopic system, defined as a geometrically isolable piece of matter in coexistence with an infinite, unperturbable enviroment. The state of a macroscopic system in equilibrium can be described in terms fo such measurable properties as temperature, pressure, and volume, which are known as thermodynamic variables. Many other variables (such as density, specific heat, comperssibility, an the coefficent of thermal expansion) can be identified and correlated, to produce a more complete description of an object and its relationship to its enviroment.

When a macroscopic system moves from one state of equliibrum to another, a thermodynamic process is said to take place. Some processes are reversible and others are irreversible. The laws of thermodynamics, discovered in the ninteenth century through painstaking experimentation, govern the nature of all thermodynamic processes and place limits on them.

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