| Περίληψη: | The molecular description of thermodynamic systems is presented. It is observed that many microscopic states correspond to a macroscopic state and that averaging of molecular variables is required for the calculation of macroscopic quantities. Time averaging of evolving in time variables is found to be difficult for complex systems and is replaced by statistical ensemble averaging. The latter is based on the consideration of ensemble of macroscopically identical systems that can differ microscopically. Macroscopic quantities are associated with averages at a single moment, provided the probabilities that microstates occur are known. A general method is presented for the determination of the probabilities as well as the averages and the fluctuations of thermodynamic variables. In the case of a closed system, a special ensemble is developed with the macroscopic states defined through the energy, the volume and the number of particles.
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