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**Unformatted text preview: **2. Statistical mechanics of particles reading: Shu Vol.II, Ch.1 2.1 The need for a statistical description of particles Astrophysical objects contain a large number of particles. The sun for example is made up of approximately 10 57 individual atomic nuclei, not to count electrons, photons, and neutrinos. The particles will interact via collisions and, if charged, via electromagnetic fields. A gas that contains so many free charged particles that their collective Lorentz force significantly influence the property of the medium is referred to as a plasma. Desirable though it may appear, it is impossible to follow the path of each individual particle on account of their sheer number. Each interaction process may have a large number of dif- ferent outcomes, each occuring with a certain probability, P i . Given a number of interactions (interacting particles), n , we expect for the number of particles that suffer outcome i or to fall into an interval dx of a physical parameter x N exp = n P i or dN exp dx = n dP dx (2 . 1) If the number of interacting particles is sufficiently large, then N exp will also be large and therefore will be very close to the measured number of particles with outcome i . The same will apply to dN exp dx even for infinitesimal intervals dx , so by using the limes dx 0 we can properly and meaningfully describe the ensemble of particles by a distribution that mathematically is in the form of a differential....

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