Thermodynamics, entropy and waterwheels

In this paper the analogy between a thermal engine and a waterwheel is developed in details, showing that the analogous of the flow of water in an hydraulic engine is the flow of entropy in a thermal one. This analogy mat serve to analyse in details the aspects of a thermal engine that are quite distant from pupils' intuition. The discussion is complemented by an illustration of the working of thermal and hydraulic engines at the microscopic level, thus furnishing a view of what heat and work are at this level, and introducing the statistical and information aspects of entropy. Both hydraulic and thermal non-ideal engines are also discussed.