Virial Theorem
Virial Theorem
| Topics: |  Astrophysics |
Introduction
In mechanics, the virial theorem provides a general equation relating the average over time of the total kinetic energy,
\left\langle T \right\rangle
, of a stable system, bound by potential forces, with that of the total potential energy,
\left\langle V_\text{TOT} \right\rangle
, where angle brackets represent the average over time of the enclosed quantity. Mathematically, the theorem states
-
2 \left\langle T \right\rangle = -\sum_{k=1}^N \left\langle \mathbf{F}_k \cdot \mathbf{r}_k \right\rangle
where Fk represents the force on the kth particle, which is located at position rk. The word "virial" derives from vis, the Latin word for "force" or "energy", and was given its technical definition by Clausius in 1870.[1]Fritz Zwicky was the first to use the virial theorem to deduce the existence of unseen matter, what is now called dark matter.
The significance of the virial theorem is that it allows the average total kinetic energy to be calculated even for very complicated systems that defy an exact solution, such as those considered in statistical mechanics; this average total kinetic energy is related to the temperature of the system by the equipartition theorem. However, the virial theorem does not depend on the notion of temperature and holds even for systems that are not in thermal equilibrium. The virial theorem has been generalized in various ways, most notably to a tensor form.
If the force between any two particles of the system results from a potential energy V(r) = αr n that is proportional to some power n of the inter-particle distance r, the virial theorem adopts a simple form
-
2 \langle T \rangle = n \langle V_\text{TOT} \rangle.
Thus, twice the average total kinetic energy
\left\langle T \right\rangle
equals n times the average total potential energy
\left\langle V_\text{TOT} \right\rangle
. Whereas V(r) represents the potential energy between two particles, VTOT represents the total potential energy of the system, i.e., the sum of the potential energy V(r) over all pairs of particles in the system. A common example of such a system is a star held together by its own gravity, where n equals −1. As another example of its many applications, the virial theorem has been used to derive the Chandrasekhar limit for the stability of white dwarf stars.
Although the virial theorem depends on averaging the total kinetic and potential energies, the presentation here postpones the averaging to the last step.
External Links
- Virial Theorem - Cornell University, Department of Astronomy, Astronomy 201 course Material. (View this for an explanation of the Virial Theorem that is geared toward astronomy.)
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The Virial Theorem (Source: Cornell University.)
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Original content retrieved from ""http://en.wikipedia.org/wiki/Virial_Theorem""
Citation
Wikipedia. (Contributing Author); Bernard Haisch (Topic Editor). 2009. "Virial theorem." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published March 19, 2009].
<http://www.cosmosportal.org/articles/view/138226//a>>
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