Peter Debye

In existographies, Peter Debye (1884-1966) (CR:3) was a Dutch-born American physicist and chemical physicist, noted for []

Unit | Dipole moment
The unit debye, symbol D, Is a non-SI unit for electric dipole moment (Ѻ), defined as:

D = 3.36×10−30C·m

used in physical chemistry; water, e.g. has a dipole moment of 1.85 debye.

Quotes | By
The following are quotes by Debye:

“Forty years ago, discussions started about the interpretation of van der Waals' universal molecular attraction as a result of the Coulombic interactions of the electrical components of the molecule. The average electrical field around a molecule was analyzed, which led to its characterization by dipole-, quadrupole- and higher moments. The mutual orientation effect of such molecules was recognized as a reason for attraction. However, the higher the temperature, the smaller the effect of such an orientation has to become. So, the conclusion to be drawn was that at high enough temperatures molecular attraction should vanish. This obviously was in contradiction to the experimental facts, and so the picture of molecules as rigid electrical structures was abandoned. Instead, their structure was recognized as deformable under the influence of an electric field: polarizability was introduced. The result was that, under the combined influence of the electrical field carried by one molecule and the polarization induced by it in its partner an additional potential energy emerges which is proportional to the square of the field and drives the molecules to each other. This still was not enough, since it made the mutual attraction of single atoms like neon or argon, around which the average field is zero, not understandable. The last difficulty was overcome by London when he recognized that the field important for the interaction should be taken as the instantaneous new field. This introduction, unavoidable, quantum theory as a new and essential background.

It was at this juncture that colloid chemistry entered into the field with the recognition that the attraction between colloid particles was the result of essentially the same molecular forces as those which determine, for instance, the heat of vaporization of a liquid. Calculations appeared concerning the van der Waals attraction between spheres and plates, giving rise to a series of brilliant experiments, which directly measured van der Waals' forces between plates at distances of the order of the wavelength of visible light, and which also demonstrated that indeed van der Waals' attraction is a result of the electromagnetic interaction of the molecular stray fields.

However, the theory of molecular interaction had first to be refined. The molecular field introduced by London was an electrostatic field. Casimir (Ѻ) recognized that its finite velocity of propagation had to be taken account of. One of the most characteristic results of this refinement is the calculation of van der Waals' attraction between two perfect mirrors. This attraction depends solely on two fundamental constants, Planck's quantum of action h and the light velocity c, and illustrates emphatically how intimate the relation is between van der Waals' universal attraction and the quantum fluctuations of the electromagnetic field. "In recent times observations on the critical opalescence have also entered the field as appropriate for the measurement of molecular forces. In the vicinity of the critical point [for phase transition], interactions are observed which are apparently of long-range character extending over distances of the order of the wavelength of visible light. ... this can be understood as a result of long-range correlation, based on short-range interactions.”
— Peter Debye (1967), Molecule Forces [1]

References
1. (a) Debye, Peter. (1967). Molecular Forces: Baker Lectures (Introduction, pgs. #). Interscience.
(b) Ho, Mae-Won. (1993). The Rainbow and the Worm: the Physics of Organism (pgs. 107-08). World Scientific, 1998.

External links
Peter Debye – Wikipedia.

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