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َ هُمام. د.أ غص ٍييب Prof. Dr. Humam B. Ghassib STATISTICAL MECHANICS –1 (0342756) Preamble تمهيد ْ 1. Statistical Mechanics is that branch of theoretical physics which seeks to draw a connection between the macroscopic viewpoint of matter and its microscopic constitution. The behavior of bulk macroscopic matter is described in such thermodynamic terms as volume, density, pressure and temperature – none of which is meaningful when applied to single atoms or molecules. Yet, bulk laboratory matter does consist entirely of atoms and molecules. The instantaneous states of these elementary constituents are described classically by their positions and velocities (or momenta); and the motions are governed by the laws of mechanics. The task of Statistical Mechanics is, then, to infer the behavior of bulk matter (i.e., the macroscopic system) from a knowledge of the structure of the particles composing it, using the laws of mechanics. The use of quantum, rather than classical, mechanics does not seriously affect this fundamental task. It merely alters the ‘mechanical’ description of the constituent particles – precise assessments of position and velocity being replaced with probability distributions or wave functions. -1- 2. The scope of Statistical Mechanics is almost as wide as the very range of natural phenomena. In fact, it has been applied, with considerable success, to the study of matter in the solid, liquid and gaseous states; matter in the ‘fourth state’, i.e., plasma; matter composed of several phases and/or several components; matter under extreme conditions of density and temperature [sometimes called the ‘fifth state’] (in astrophysics, for example); matter in equilibrium with radiation; living matter, i.e., matter in the form of a biological specimen; soft condensed matter, e.g., suspensions, emulsions and foams, liquid crystals, polymers, and membranes; and so forth. Moreover, Statistical Mechanics is suitable for investigating equilibrium, as well as nonequilibrium, states of matter. 3. However, in this course, we shall concentrate almost exclusively on the equilibrium state (in which the system undergoes no macroscopically-observable changes in a given interval of time). Our aim is twofold: first, to study thoroughly the theoretical principles involved; and, second, to consider some applications. Prof. Dr. Humam Ghassib -2- َ ُهمام.د.أ غ ِصيب -3-