thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed b ...

, the Massieu function (sometimes called Massieu–Gibbs function, Massieu potential, or Gibbs function, or characteristic (state) function in its original terminology), symbol ( Psi), is defined by the following relation: :

\Psi = \Psi \big( X_1, \dots, X_i, Y_, \dots Y_r \big) \,

where for every

system A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its open system (systems theory), environment, is described by its boundaries, str ...

with degree of freedom one may choose variables, e.g.

\big( X_1, \dots, X_i, Y_, \dots Y_r \big) ,

to define a

coordinate system In geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine and standardize the position of the points or other geometric elements on a manifold such as Euclidean space. The coordinates are ...

, where and are extensive and intensive variables, respectively, and where at least one extensive variable must be within this set in order to define the size of the system. The -th variable, , is then called the Massieu function.Inden, Gerhard. (2008). �
Introduction to Thermodynamics
��, ''Materials Issues for Generation IV Systems'', pgs. 73–112. Springer The Massieu function was introduced in the 1869 paper "On the Characteristic Functions of Various Fluids" by French engineer François Massieu (1832-1896). The name "Gibbs function" is the eponym of American physicist

Willard Gibbs Josiah Willard Gibbs (; February 11, 1839 – April 28, 1903) was an American mechanical engineer and scientist who made fundamental theoretical contributions to physics, chemistry, and mathematics. His work on the applications of thermodynami ...

(1839-1903), who cited Massieu in his 1876 '' On the Equilibrium of Heterogeneous Substances''. Massieu, as discussed in the first footnote to the abstract of Gibbs' ''Equilibrium'', “appears to have been the first to solve the problem of representing all the properties of a body of invariable composition which are concerned in reversible processes by means of a single function.” Massieu's 1869 paper seems to be the source for the generalized mathematical conception of the

energy Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...

of a system being equal to summations of the products of pairs of

conjugate variables Conjugate variables are pairs of variables mathematically defined in such a way that they become Fourier transform duals, or more generally are related through Pontryagin duality. The duality relations lead naturally to an uncertainty relation— ...

References

Further reading