Statistical physics

The substance Statistical Physics microstructure and microscopic particles understanding the interaction with the statistical probability of the method , making interpretation of microscopic physical properties of the macroscopic object consisting of a large number of particles and the orderliness of theoretical physics branch. Also known as statistical mechanics . The so-called large amount is based on the number of molecules contained in a mole of material 

Research object

The research object changes from a small number of individuals to a group composed of a large number of individuals, resulting in fundamental changes in the nature of the law and research methods. The statistical laws followed by a large number of particle systems cannot be reduced to mechanical laws. Statistical Physics by microscopic to macroscopic bridge, which provide the basis for a variety of macroscopic theory, has become a gas, liquid, solid, and plasma basic theory body, and play a role in the study of biology and chemistry. Gas kinetic theory (formerly known as gas molecule movement theory ) is an early statistical theory. It reveals the microscopic nature of macroscopic quantities such as gas pressure , temperature, and internal energy , and gives the relationship between them and the corresponding average of the microscopic quantities . The derivation of the average free path formula, the establishment of the gas molecule velocity or velocity distribution law, the energy sharing theorem, and the relevant data are obtained, so that people can balance the thermal motion , collision, and energy distribution of the ideal gas molecule in equilibrium . With a clear physical image and quantitative understanding, it also shows the special importance of probability and statistical distribution to statistical theory.

Theoretical basis

The establishment of the non- equilibrium distribution function and its evolution equations not only become the basis of the micro-statistic theory of the transportation process, but also the H function defined by it and the H theorem that it follows to understand, the irreversibility of the macro process and the process tending to equilibrium Played an important role. Elucidation of the statistical significance of entropy and microscopic statistical interpretation of the principle of entropy increase indicate that statistical theory has evolved from equilibrium to non-equilibrium, and has evolved from microscopic statistical interpretation of certain macro concepts and macro laws to the second law of thermodynamics Such general laws make microscopic statistical interpretations. However, the theory of gas kinetics uses molecules as statistical entities, and it is necessary to make unfounded guesses or assumptions about the structure of molecules and the interaction between molecules. This is the fundamental difficulty and limitation for its further development.

 Research method

JW Gibbs took the entire system as a statistical entity, and proposed to study the distribution of ensemble composed of a large number of systems in the phase space , which overcomes the difficulty of gas kinetic theory and establishes statistical physics. In the statistical theory of equilibrium state , the micro-regular ensemble is used for the isolated system with fixed energy and particle number ; for the system that can exchange energy with the large heat source but the particle number is fixed, the regular ensemble is used ; for the system that can exchange energy with the large heat source and The particle system uses giant regular ensemble . These are three commonly used systems, and the distribution density function of each ensemble in the phase space has been obtained. Quantum statistics and classic statistics have the same research objects and research methods , and the concept of ensemble is still applicable in quantum statistics. Quantum Statistical difference is that microscopic particle motion follows the mechanics of quantum rule rather than mechanical classic law of microscopic motion state having a discontinuity , required quantum state rather than to buildings described.

Non- equilibrium statistical physics has a wide range of contents, and it is a subject that is still developing rapidly and is far from mature. The system is near equilibrium, which tends to study equilibrium relaxation time and temperature dependencies; do not too far away from equilibrium, maintaining the temperature difference, concentration difference, the potential difference and the like various transport process system undergo , To study its various linear transport coefficients, in addition, to study fluctuations . Relaxation , transportation, and fluctuation are the main non-equilibrium processes near the equilibrium state.

Historical origin

Since the 1960s, to far from equilibrium of physical phenomena have been extensively studied, the most important thing is far from equilibrium mutation, ordered structures arise, the establishment of a dissipative structure theory , but has not yet formed a complete theoretical system