Statistical Mechanics in a Nutshell
2011
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arXiv (Cornell University), 2017
Thermodynamics makes definite predictions about the thermal behavior of macroscopic systems in and out of equilibrium. Statistical mechanics aims to derive this behavior from the dynamics and statistics of the atoms and molecules making up these systems. A key element in this derivation is the large number of microscopic degrees of freedom of macroscopic systems. Therefore, the extension of thermodynamic concepts, such as entropy, to small (nano) systems raises many questions. Here we shall reexamine various definitions of entropy for nonequilibrium systems, large and small. These include thermodynamic (hydrodynamic), Boltzmann, and Gibbs-Shannon entropies. We shall argue that, despite its common use, the last is not an appropriate physical entropy for such systems, either isolated or in contact with thermal reservoirs: physical entropies should depend on the microstate of the system, not on a subjective probability distribution. To square this point of view with experimental results of Bechhoefer we shall argue that the Gibbs-Shannon entropy of a nano particle in a thermal fluid should be interpreted as the Boltzmann entropy of a dilute gas of Brownian particles in the fluid.
A short review of basic concepts and formulae of statistical mechanics is presented, paving the way to liquid state theory and computational methods. The Legendre- Laplace transformation of statistical ensembles is illustrated. These notes support the lectures on Liquid State and Phase Transitions of the TCMM Intensive Course. It is presupposed that the participants are familiar with the fundamentals of thermodynamics and statistical mechanics at the level of an undergraduate course on physical chemistry.
Physics and Chemistry of Liquids, 2015
Some relations are derived using statistical mechanics to describe the effects of surroundings on the properties of systems for sizes below the thermodynamic limit. A general expression for the free energy of closed, small systems is derived and then used to obtain the dependence of the thermal properties on density and temperature, including general expressions for equations of state and internal energies. Comparisons between predictions of the current theory and the results of molecular dynamics (MD) simulations are made for 3D hard-sphere and Lennard-Jones fluids for which the surroundings are modelled as reflecting hard walls that confine the system along one direction. The analytical predictions are in excellent agreement with the MD results.
Europhysics Letters (EPL), 2004
We use an off-lattice bead-spring model of a self-avoiding polymer chain immersed in a 3-dimensional quenched random medium to study chain dynamics by means of a Monte-Carlo (MC) simulation. The chain center of mass mean-squared displacement as a function of time reveals two crossovers which depend both on chain length N and on the degree of Gaussian disorder ∆. The first one from normal to anomalous diffusion regime is found at short time τ1 and observed to vanish rapidly as τ1 ∝ ∆ −11 with growing disorder. The second crossover back to normal diffusion, τ2, scales as τ2 ∝ N 2ν+1 f (N 2−3ν ∆) with f being some scaling function. The diffusion coefficient DN depends strongly on disorder and drops dramatically at a critical dispersion ∆c ∝ N −2+3ν of the disorder potential so that for ∆ > ∆c the chain center of mass is practically frozen. The time-dependent Rouse modes correlation function Cp(t) reveals a characteristic plateau at ∆ > ∆c which is the hallmark of a non-ergodic regime. These findings agree well with our recent theoretical predictions.
2004
Competing styles in Statistical Mechanics have been introduced to investigate physico-chemical systems displaying complex structures, when one faces difficulties to handle the standard formalism in the well established Boltzmann-Gibbs statistics. After a brief description of the question, we consider the particular case of Renyi statistics whose use is illustrated in a study of the question of the ''anomalous'' (non-Fickian) diffusion that it is involved in experiments of cyclic voltammetry in electro-physical chemistry. In them one is dealing with the fractal-like structure of the thin film morphology present in eletrodes in microbatteries which leads to fractional-power laws for describing voltammetry measurements and in the determination of the interphase width derived using atomic force microscopy. The fractional-powers associated to these quantities are related to each other and to the statistical fractal dimension, and can be expressed in terms of a power index, on which depends Renyi's statistical mechanics. It is clarified the important fact that this index, which is limited to a given interval, provides a measure of the microroughness of the electrode surface, and is related to the dynamics involved, the non-equilibrium thermodynamic state of the system, and to the experimental protocol. \\