The second law

In this chapter we will discuss the basic concepts that lead to the statement of the second law of thermodynamics. We shall present a description based on simple probabilistic ideas and an intuitive postulate: in an isolated system (which may also be the entire Universe) every microscopic state (microstate) accessible to the system is equally probable. A microstate is a particular configuration of all the elements forming the system. For example, for a gas it would be a particular value of the position and the velocity of each particle. If the particles of the gas had internal degrees of freedom then we would also need to specify their values, for example their status of rotation and vibration, their magnetic moment, electronic state etc. In the next section we will start the discussion with a simple toy system, which will already provide a useful framework to discuss another important concept, that of equilibrium.