Dear RG community members, this pedagogical thread is related to the most difficult subject among the different fields that physics uses to describe nature, i.e. the physical kinetics (PK). Physical Kinetics as a subject is defined as a “method to study physical systems involving a huge number of particles out of equilibrium”.

The key role is given by two physical quantities:

  • The distribution function f (r, p, t), where r is a vector position, p is a linear momentum and t is the time for the function f which describes a particle in an ensemble.
  • The collision or scattering term W (p, p¨) gives the probability of a particle changing its linear momentum from the value p to the value p¨ during the collision.

If the following identity is satisfied for the distribution function df (r, p, t) / d t = 0, then we can directly link PK to the Liouville equation in the case that the distribution function does not depend on time directly. Physics students are tested on that, at the end of an advanced course in classical mechanics, when reading about the Poisson brackets.

However, is important to notice that not all phys. syst. are stationary and not always the identity df /d t = 0 follows, i.e., the distribution function - f is not always time-independent, i.e., f (r, p) is just true for some cases in classical and non-relativistic quantum mechanics, and the time dependence “t” is crucial for the majority of cases in our universe, since is out of equilibrium.

In addition, physical kinetics as a “method to study many-particle systems” involves the knowledge of 4 physics subjects: classical mechanics, electrodynamics, non-relativistic quantum mechanics & statistical mechanics.

The most important fact is that it studies the scattering/collision of particles without linear momentum conservation p, where: the time dependence & the presence of external fields are crucial to study any particular physical phenomena. That means that PK is the natural method to study out of equilibrium processes where the volume of the scattering phase space is not conserved & particles interact/collide with each other.

If the phase scattering space vol is not conserved, then we have the so-called out of equilibrium distribution function which follows the general equation:

df (r, p, t) / d t = W (p,p¨), (1)

where: d/dt = ∂/∂t + r´. ∂/∂r + p´. ∂/∂p, with units of t -1or ω/(2π).

The father of physical kinetics is Prof. Ludwig Eduard Boltzmann (1844 – 1906) [1]. He was able to establish the H theorem which is the basis for the PK subject and also he wrote the main equation (1), i.e., the Boltzmann equation to describe the out of equilibrium dynamics of an ideal gas. r´ & p´ in d/dt are derivatives, p¨ in W is another momentum position

Another physicist who established the first deep understanding and condensed the subject into a book was Prof. Lev Emmanuilovich Gurevich (1904 - 1990). He was the first to point out that the kinetic effects in solids, i.e., metals and semiconductors are determined by the "phonon wind", i.e., the phonon system is in an unbalanced state [2]

Physical kinetics has 3 main approaches:

  • The qualitative approach involves the evaluation of several physical magnitudes taking into account the order of magnitude for each of them.
  • The second approach is the theoretical approach which involves complicated theoretical solutions of the kinetic equation using different approximations for the scattering integral such as the t approximation. For graduate courses, I follow [8], an excellent textbook by Prof. Frederick Reif. For undergraduate teaching, I followed the brief introduction at the end of Vol V of Berkeley Phys C.
  • The numerical approach since most problems involving PK requires extensive numerical and complicated self-consistent calculations.

The fields where PK is useful are many:

  • The physics of normal metals and semiconductors out of equilibrium.
  • The hydrodynamics of reacting gases & liquids, quantum liquids, and quantum gases at very low temperatures.
  • The physics of superconductors, phase transitions, and plasma physics among others.

There is a quantum analog to the classical Boltzmann equation, we ought to mention three cases: the density matrix equation for random fields, the density matrix equation for quantum particles, and the Wigner distribution function. Main graph 1 is adapted from [4] to the English language, LB picture from [7], and LG picture from [3].

Any contributions to this thread are welcome, thank you all.

References:

1. https://www.wikiwand.com/en/Ludwig_Boltzmann

2. Fundamentals of physical kinetics by L. Gurevich. State publishing house of technical and theoretical literature, 1940. pp 242

3. Lev Emmanuilovich Gurevich. Memories of friends, colleagues, and students. Selected Works, by Moisey I. Kaganov et. at (1997) pp 318. ISBN:5-86763-117-6. Publishing house Petersburg Institute of Nuclear Physics. RAS

4. Белиничер В.В. Физическая кинетика. Изд-во НГУ.Новосибирск.1996.

5. Lifshitz E., Pitaevskii L. 1981. Physical Kinetics. Vol. 10, (Pergamon Press).

6. Thorne, K. S. & Blandford, R. D., Modern Classical Physics: Optics, Fluids, Plasmas, Elasticity, Relativity, & Statistical Physics (2017) (Princeton University Press).

7. https://es.wikipedia.org/wiki/Ludwig_Boltzmann

8. Fundamentals of Statistical and Thermal Physics: F. Reif Mc Graw-Hill, 1965

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