Helmholtz theorem (classical mechanics)

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For other uses, see Helmholtz theorem (disambiguation).

The Helmholtz theorem of classical mechanics reads as follows:



be the Hamiltonian of a one-dimensional system, where


is the kinetic energy and


is a "U-shaped" potential energy profile which depends on a parameter V. Let \left\langle \cdot \right\rangle _{t} denote the time average. Let

E = K + \varphi,
T = 2\left\langle K\right\rangle _{t},
P = \left\langle -\frac{\partial \varphi }{\partial V}\right\rangle _{t},
S(E,V)=\log \oint \sqrt{2m\left( E-\varphi \left( x,V\right) \right) }\,dx.


dS = \frac{dE+PdV}{T}.


The thesis of this theorem of classical mechanics reads exactly as the heat theorem of thermodynamics. This fact shows that thermodynamic-like relations exist between certain mechanical quantities. This in turn allows to define the "thermodynamic state" of a one-dimensional mechanical system. In particular the temperature T is given by time average of the kinetic energy, and the entropy S by the logarithm of the action (i.e.\oint
dx\sqrt{2m\left( E-\varphi \left( x,V\right) \right) }).
The importance of this theorem has been recognized by Ludwig Boltzmann who saw how to apply it to macroscopic systems (i.e. multidimensional systems), in order to provide a mechanical foundation of equilibrium thermodynamics. This research activity was strictly related to his formulation of the ergodic hypothesis. A multidimensional version of the Helmholtz theorem, based on the ergodic theorem of George David Birkhoff is known as generalized Helmholtz theorem.


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  • Helmholtz, H., von (1884b). Studien zur Statik monocyklischer Systeme. Sitzungsberichte der Kö niglich Preussischen Akademie der Wissenschaften zu Berlin, I, 159–177 (also in Wiedemann G. (Ed.) (1895) Wissenschafltliche Abhandlungen. Vol. 3 (pp. 163–178). Leipzig: Johann Ambrosious Barth).
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