Introduction

One of the main reasons why many persons experience conceptual difficulties when studying classical thermodynamics is probably that the concept of temperature and its main features are more or less taken for granted. Entropy is simpler and has a more direct connection to basic properties of matter than temperature. This connection is, however, outside the realm of classical thermodynamics; it can be established first in statistical thermodynamics. A basic postulate in statistical mechanics is that all available microscopic states for an isolated system occur equally likely at equilibrium. Due to the quantization in quantum mechanics, the number of microstates is well defined in that case, but in classical mechanics there is no such way to define a microstate and to calculate the number of such states.