The Second Law of Thermodynamics

Abstract

As everyday experience shows, all natural phenomena are irreversible. The irreversibility establishes the arrow of time. Natural irreversibility has its own rules, which are stated in the second law of thermodynamics. We shall start giving the two historical statements, one due to Clausius, one due to Lord Kelvin, and then prove their equivalence. After having demonstrated the fundamental theorems by Carnot and by Clausius, we shall define the function of state called entropy and see how the second law is expressed, in the most precise form, as the law of increasing entropy. We shall also introduce the concept of thermodynamic temperature that allows extending the scale down to the absolute zero.