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About the learning unit
- Prof. Dr. Guenter Gauglitz
- Prof. Guenter Gauglitz
- Dr. Manuela Reichert
- Copyright © 1999-2016 Wiley Information Services GmbH. All rights reserved.
One physical constant or Natural constant (occasionally also Elementary constant ) is a physical quantity, the value of which cannot be influenced and which does not change spatially or temporally.
as fundamental natural constant are the constants that refer to general properties of space, time and physical processes that apply equally to every kind of particle and interaction. These are the speed of light, Planck's quantum of action and the gravitational constant (see also natural units).
Further elementary (or fundamental) natural constants relate to the individual types of particles and interactions, e.g. B. their masses and charges. Derived natural constants can be calculated from the fundamental and elementary constants. For example, the Bohr radius, a constant that is decisive for atomic physics, can be calculated from Planck's quantum of action, the speed of light, the elementary charge and the mass of the electron.
In some cases, parameters or coefficients that are only constant in a certain arrangement or constellation are called constant such as the Kepler constant, the decay constant or the spring constant etc. Strictly speaking, however, they are not constants, but parameters of the arrangement being examined.
Some natural sciences combine important constants into groups of Fundamental constants together, e.g. B. in astronomy and geodesy these are the exact reference values of the earth and solar mass, the earth radius, the astronomical unit or the gravitational constant.
Reference values commonly used in practice, such as the duration of a year, the pressure of the standard atmosphere or the acceleration due to gravity, are not natural constants. They are useful to humans in their earthly environment, but as a rule they do not have a fundamental meaning beyond that and do not prove to be really constant with increasing measurement accuracy. However, they were used for the initial definition of units of measurement (also e.g. for seconds, meters, kilograms). Modern efforts have been aimed at defining the units of measurement as far as possible through direct reference to (fundamental or elementary) natural constants. The natural constants selected for this receive a firmly defined, unchangeable numerical value. From the 26th General Conference on Weights and Measures, all units of the International System of Units were replaced by three fundamental natural constants (c, H, e), a special atomic transition (νCs) and three arbitrary constants (kB., NA., KCD) Are defined.
Continuous fractionation and solution properties of PVC, 4 †. Pressure dependence of the solubility in a mixed solvent ‡
Using a self-constructed light scattering apparatus, the pressure dependence of the demixing temperature of solutions of PVC 20,000, PVC 37,000 and PVC 70,000 in THE / water was determined up to 1,000 bar for different compositions of the mixed solvent. (The numbers in the codes of the PVC specimens are their approximate molecular weights.) In contrast to the thetasolvents O-xylene and phenetole, the solubility decreases with increasing pressure for all molecular weights and compositions under investigation typically by about 1 K / 100 bar. The evaluation of the experimental findings demonstrates that the volume fraction of the nonsolvent in the mixed solvent, φ, is the variable that governs phase separation: For the present region of p other T, a given polymer solution demixes at a characteristic constant value of φ, no matter how it is reached. This fact allows the prediction of pressure influences from known pVT data of the pure components and measurements at atmospheric pressure. A qualitative theoretical understanding of the observed influences of p other T can be reached on the basis of the solubility parameter theory.