Applied Thermodynamics Second Law. Entropy

Common parlance refers to thermal, mechanical, renewable, fossil, nuclear and other forms of "energy". It even goes so far as to mention energy losses. Yet the first principle of thermodynamics stipulates the conservation of energy. Energy loss is therefore scientific nonsense. Correct language should refer to different forms of energy or different types of energy. In this case, for example, "mechanical energy" is lost, while "thermal energy" is gained. This is a conversion from one type of energy to another. The first principle thus expresses an equivalence between different types of energy: one type of energy is quantitatively worth another.

However, intuitive common sense does not place all forms of energy on the same level, hence the notion of "loss". For example, mechanical energy is spontaneously converted into thermal energy by friction, and electrical energy is just as easily transformed into thermal energy by the Joule effect. Better still, everyone knows, for example, that heat at high temperature is more interesting than heat at medium temperature. Similarly, it's better to have a gas at high pressure than at atmospheric pressure, as the former can perform a certain amount of work. In this way, the notion of energy quality is combined with that of quantity.

Quantity and quality are inseparable from energy concepts, and form the basis of the first and second principles of thermodynamics. If the first principle appears to be that of quantity, the second principle is that of quality, or the difference between forms of energy and the interest of intensive variables. In fact, to support this last point, we note that if it's more interesting to have thermal energy at high temperature, it's quite simply because heat flows naturally from hot to cold, i.e. from a high-temperature system to a lower-temperature one. Similarly, in hydraulics, water flows naturally from a higher altitude to a lower one, never the other way round, except by using a pump to do so. Temperature and altitude are intensive parameters (or variables), like pressure, electrical voltage, force, etc., and it is these parameters that regulate the direction of energy exchanges. What's more, the speed of exchanges (power) between two elements is linked to the intensity gradient between them.

The second principle is also concerned with the notions of entropy, irreversibility and entropy creation. It is the opposition between quasi-static, reversible exchanges (i.e. with zero power and entropy conservation) and efficient, irreversible exchanges (with finite power and entropy creation).

These concepts are the subject of this article.

In the following article