Concepts

Wi will use the term physical quantity to denote any quantity which can be used in the context of physical, chemical, or physico-chemical calculations. These calculations follow the rules of quantity calculus. Thus, in particular situations, quantities must be commensurable in order to make the calculation meaningful. Moreover, the units attached to the respective quantities must be taken into account.

In many cases, a physical quantity can be regarded as a variable, which might attain different values in the course of the calculation. In some cases, the value must be fixed at first use, and not be changable thereafter. As discussed in the quantities concepts section, such a physical quantity will be denoted a constant. In still other cases, the value of a constant is universally fixed and can never be changed. A particular case of such unique constants are fundamental physical constants [Cohen/Cvitas/etal_2007] (p.111). We will refer to these also as natural constants. Recommended values for fundamental physical constants are compiled and accessible online [CODATA]. Their values may be experimentally determined (e.g., the elementary charge), calculated from other such constants' values (e.g., the Faraday constant), or defined (e.g., the zero of the Celsius scale) [Cohen/Cvitas/etal_2007] (p.111). The numerical values may be subject to changes with experimental progress.

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References

[Cohen/Cvitas/etal_2007] E.R. Cohen, T. Cvitas, J.G. Frey, B. Holmström, K. Kuchitsu, R. Marquardt, I. Mills, F. Pavese, M. Quack, J. Stohner, H.L. Strauss, M. Takami, and A.J. Thor, Quantities, Units and Symbols in Physical Chemistry, 3rd ed., Blackwell Science, Oxford, 2007.

[CODATA] http://physics.nist.gov/constants

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