In this section a bunch of articles are given in loose order, that elaborate further some of the concepts developed in the two basic articles. As opposed to the those articles, math is not avoided, though not forced anyway.
It is often stressed that entropy be related to information. However, entropy is an objective quantity, only depending on the system and not on an observers information about it. How can this quantity be measured? A procedure is presented to get the entropy of, say, a glass of water. Like for potential energy, only entropy differences have physical meaning, and a reference state is needed to give absolute entropies. Based on this reference state, absolute entropies of a few substances are given, and a practical example shows how such values can be obtained.
The meaning of "temperature" is discussed from a classical thermodynamical point of view. It is explained how Gay-Lussac discovered the absolute temperature scale back in the first decade of the nineteenth century, fifty years before anybody knew if temperature is to mean anything at all. The meaning of the term became clear only when Lord Kelvin succeeded in defining temperature completely independently of any thermometric substance. For this purpose, Kelvin relied on the efficiency of the heat engine; thus, he traced temperature back to fundamental terms like energy and entropy. An expression showing how temperature depends on these two terms is derived from the First and Second Law of thermodynamics.
While one always can take readings of a whatevermeter (e.g. a thermometer), these do not always make sense. The conditions are briefly examined, for which measurements of temperature can be reliable. Finally, some personal remarks (a somewhat more elaborated article on reality and objectivity is available German only) are presented as to why I think we can trustfully believe that certain terms relate to "real" things or "real" properties of things, while other terms do not.
Thermodynamics is older than the atomistic view of matter, hence thermodynamic quantities such as temperature are not defined atomistically. The definition of temperature allows negative absolute temperatures. But do such temperature have any empirical meaning? While this might be questionable, it is certainly interesting to discuss it because it helps understand the concept of temperature. As follows, a simplified statistical concept of temperature is discussed and used to explain the state of matter in lasers as well as the technique to attain very low temperature (adiabatic demagnetization)
Last update Jun. 26, 1998 gVa