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Virial Theorem - Wikipedia, the Free Encyclopedia Page 1 of 9 Virial theorem - Wikipedia, the free encyclopedia Page 1 of 9 Virial theorem From Wikipedia, the free encyclopedia In mechanics, the virial theorem provides a general equation relating the average over time of the total kinetic energy, , of a stable system consisting of N particles, bound by potential forces, with that of the total potential energy, , where angle brackets represent the average over time of the enclosed quantity. Mathematically, the theorem states 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] 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 Thus, twice the average total kinetic energy equals n times the average total potential energy . 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. Although the virial theorem depends on averaging the total kinetic and potential energies, the presentation here postpones the averaging to the last step. Contents 1 History of the virial theorem 2 Statement and derivation 2.1 Definitions of the virial and its time derivative 2.2 Connection with the potential energy between particles 2.3 Special case of power-law forces 2.4 Time averaging and the virial theorem 3 The virial theorem and special relativity 4 Generalizations of the virial theorem 5 Inclusion of electromagnetic fields 6 Virial radius http://en.wikipedia.org/wiki/Virial_theorem 8/17/2011 Virial theorem - Wikipedia, the free encyclopedia Page 2 of 9 7 See also 8 References 9 Further reading 10 External links History of the virial theorem In 1870, Rudolf Clausius delivered the lecture "On a Mechanical Theorem Applicable to Heat" to the Association for Natural and Medical Sciences of the Lower Rhine, following a 20 year study of thermodynamics. The lecture stated that the mean vis viva of the system is equal to its virial, or that the average kinetic energy is equal to 1/2 the average potential energy. The virial theorem can be obtained directly from Lagrange's Identity as applied in classical gravitational dynamics, the original form of which was included in his "Essay on the Problem of Three Bodies" published in 1772. Karl Jacobi's generalization of the identity to n bodies and to the present form of Laplace's identity closely resembles the classical virial theorem. However, the interpretations leading to the development of the equations were very different, since at the time of development, statistical dynamics had not yet unified the separate studies of thermodynamics and classical dynamics. [2] The theorem was later utilized, popularized, generalized and further developed by persons such as James Clerk Maxwell, Lord Rayleigh, Henri Poincaré, Subrahmanyan Chandrasekhar, Enrico Fermi, Paul Ledoux and Eugene Parker. Fritz Zwicky was the first to use the virial theorem to deduce the existence of unseen matter, which is now called dark matter. 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. Statement and derivation Definitions of the virial and its time derivative For a collection of N point particles, the scalar moment of inertia I about the origin is defined by the equation where mk and rk represent the mass and position of the kth particle. rk=| rk| is the position vector magnitude. The scalar virial G is defined by the equation where pk is the momentum vector of the kth particle. Assuming that the masses are constant, the virial G is one-half the time derivative of this moment of inertia http://en.wikipedia.org/wiki/Virial_theorem 8/17/2011 Virial theorem - Wikipedia, the free encyclopedia Page 3 of 9 In turn, the time derivative of the virial G can be written where mk is the mass of the k-th particle, is the net force on that particle, and T is the total kinetic energy of the system Connection with the potential energy between particles The total force on particle k is the sum of all the forces from the other particles j in the system where is the force applied by particle j on particle k. Hence, the force term of the virial time derivative can be written Since no particle acts on itself (i.e., whenever j = k), we have [3] where we have assumed that Newton's third law of motion holds, i.e., (equal and opposite reaction). It often happens that the forces can be derived from a potential energy V that is a function only of the distance rjk between the point particles j and k. Since the force is the negative gradient of the potential http://en.wikipedia.org/wiki/Virial_theorem 8/17/2011 Virial theorem - Wikipedia, the free encyclopedia Page 4 of 9 energy, we have in this case which is clearly equal and opposite to , the force applied by particle k on particle j, as may be confirmed by explicit calculation. Hence, the force term of the virial time derivative is Thus, we have Special case of power-law forces In a common special case, the potential energy V between two particles is proportional to a power n of their distance r where the coefficient α and the exponent n are constants. In such cases, the force term of the virial time derivative is given by the equation where VTOT is the total potential energy of the system Thus, we have For gravitating systems and also for electrostatic systems, the exponent n equals −1, giving Lagrange's identity http://en.wikipedia.org/wiki/Virial_theorem 8/17/2011 Virial theorem - Wikipedia, the free encyclopedia Page 5 of 9 which was derived by Lagrange and extended by Jacobi. Time averaging and the virial theorem The average of this derivative over a time τ is defined as from which we obtain the exact equation The virial theorem states that, if , then There are many reasons why the average of the time derivative might vanish, i.e., . One often-cited reason applies to stable bound systems , i.e., systems that hang together forever and whose parameters are finite. In that case, velocities and coordinates of the particles of the system have upper and lower limits so that the virial Gbound is bounded between two extremes, G and G , and the average goes to zero in the limit of very long times τ min max Even if the average of the time derivative is only approximately zero, the virial theorem holds to the same degree of approximation. For power-law forces with an exponent n, the general equation holds For gravitational attraction , n equals −1 and the average kinetic energy equals half of the average negative http://en.wikipedia.org/wiki/Virial_theorem 8/17/2011 Virial theorem - Wikipedia, the free encyclopedia Page 6 of 9 potential energy This general result is useful for complex gravitating systems such as solar systems or galaxies. A simple application of the virial theorem concerns galaxy clusters. If a region of space is unusually full of galaxies, it is safe to assume that they have been together for a long time, and the virial theorem can be applied. Doppler measurements give lower bounds for their relative velocities, and the virial theorem gives a lower bound for the total mass of the cluster, including any dark matter. The averaging need not be taken over time; an ensemble average can also be taken, with equivalent results. Although derived for classical mechanics, the virial theorem also holds for quantum mechanics, which was proved by Fock [4] (the quantum equivalent of the l.h.s. vanishes for energy eigenstates). The virial theorem and special relativity For a single particle in special relativity, it is not the case that . Instead, it is true that and The last expression can be simplified to either or . Thus, under the conditions described in earlier sections (including Newton's third law of motion, , despite relativity), the time average for N particles with a power law potential is In particular, the ratio of kinetic energy to potential energy is no longer fixed, but necessarily falls into an interval: where the more relativistic systems exhibit the larger ratios. Generalizations of the virial theorem http://en.wikipedia.org/wiki/Virial_theorem 8/17/2011 Virial theorem - Wikipedia, the free encyclopedia Page 7 of 9 Lord Rayleigh published a generalization of the virial theorem in 1903. [5] Henri Poincaré applied a form of the virial theorem in 1911 to the problem of determining cosmological stability. [6] A variational form of the virial theorem was developed in 1945 by Ledoux. [7] A tensor form of the virial theorem was developed by Parker, [8] Chandrasekhar [9] and Fermi.
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