Abstract: | It is a common question of an engineer: the used model is a description of the underlying mechanisms or just a useful tool for design predictions with no further physical meaning? For the engineer, a good model is the one that closely represents the reality and can be treated without too many mathematical or other computational and experimental complexities. For that reason, instead of using a complicated theoretical model, which includes a number of difficult to be measured parameters, engineers prefer to use several simplified empirical models, which are based, in most of the cases, on a number of âreasonableâ assumptions and approximations. However, advocating the use of simple theories and models makes it necessary to point out the pitfalls, which are encountered in their application. A theory is completely useless unless it reflects the most important properties of the system to which it is applied. Any theory must first be checked as to whether it meets this criterion [1]: its âphysical significanceâ is always a critical issue. |