Lev P. Pitaevskii

The general theory of van der Waals forces

Publisher Summary The van der Waals forces refer to the attractive forces acting between any two neutral atoms or molecules that are separated by distance large compared to their own dimensions. These forces are of a long-range nature, which decrease with distance according to a power law. The basic idea of the theory is that the interaction between the bodies is considered to take place through a fluctuating electromagnetic field. This field is always present in the interior of a material medium and it also extends beyond its boundaries because of the thermodynamic fluctuations. Any change in the electrical proper ties of the medium in a certain region will, by Maxwells equations, lead to a change in the fluctuation field that extends beyond that region. Therefore, the part of the free energy that is related to electromagnetic fluctuations is not determined by the properties of the substance solely at the point considered.

Van der Waals forces in liquid films

Publisher Summary This chapter examines van der Waals forces in liquid helium films. The Maxwell stresses were carried out using the usual formulas for the field in vacuo. In the case of a vacuum gap amomg bodies, the forces of the interaction between them can completely be reduced to the forces described by the fluctuating electromagnetic fields. When, however, the gap is filled with a liquid medium, there are forces of yet another origin that are connected with the energies of the acoustic sound vibrations in that medium. Moreover, fluctuating surface vibrations at the interface of different media can also lead to a definite contribution.

SCATTERING OF ELECTROMAGNETIC WAVES BY INHOMOGENEITIES EXCITED IN A PLASMA BY A RAPIDLY MOVING BODY

Results are given of calculations of the scattering function F^a) and the effective scattering cross section a for a body moving rapidly in a plasma in a magnetic field; Strela and BESM-2 computers were used. It is found that a is represented by a function having many narrow lobes, the main lobe being that corresponding to reflection at a plane having the direction of flo, the magnetic field. The relation of a to height and to the wavelength X is discussed. It is found that the differential cross section can be 1000 m or more for motion at small angles to the direction of the magnetic field.