Un po’ di aria modulata per Neutrino.
I numerosi commenti di FermiGas mi hanno indotto a dedicare una altro post alla termodinamica, perché mi pare che la possibilità dei sistemi aperti e chiusi di diminuire la loro entropia porti a considerazioni scorrette. E’ vero che sistemi chiusi e aperti possono aumentare o diminuire la loro entropia, però essi devono essere descritti, ciò che non è necessario per un sistema isolato. La combinazione di idrogeno e ossigeno avviene con una diminuzione di entropia del sistema, ma con un aumento di entropia di sistema + ambiente. La sintesi dell’acqua porta a un notevole aumento dell’entropia dell’universo perché la reazione è fortemente esotermica.
Il collegamento della termodinamica al destino di una montagna, che ha dato inizio al nostro confronto, è descritto in modo elegante da
Christoph Schiller – The Adventure of Physics.
Ancora una volta Clausius è il riferimento d’obbligo al SPT.
The non-conservation of entropy has far-reaching consequences. When a piece of rock is detached from a mountain, it falls, tumbles into the valley, heating up a bit, and eventually stops. The opposite process, whereby a rock cools and tumbles upwards, is never observed. Why? We could argue that the opposite motion does not contradict any rule or pattern about motion that we have deduced so far.
Rocks never fall upwards because mountains, valleys and rocks are made of many particles. Motions of many-particle systems, especially in the domain of thermodynamics, are called processes. Central to thermodynamics is the distinction between reversible processes, such as the flight of a thrown stone, and irreversible processes, such as the afore-mentioned tumbling rock. Irreversible processes are all those processes in which friction and its generalizations play a role. Irreversible processes are those processes that increase the sharing or mixing of energy. They are important: if there were no friction, shirt buttons and shoelaces would not stay fastened, we could not walk or run, coffee machines would not make coffee, and maybe most importantly of all, we would have no memory.
Irreversible processes, in the sense in which the term is used in thermodynamics, transform macroscopic motion into the disorganized motion of all the small microscopic components involved: they increase the sharing and mixing of energy. Irreversible processes are therefore not strictly irreversible – but their reversal is extremely improbable.
We can say that entropy measures the ‘amount of irreversibility’: it measures the degree of mixing or decay that a collective motion has undergone.
Entropy is not conserved. Indeed, entropy – ‘heat’ – can appear out of nowhere, spontaneously, because energy sharing or mixing can happen by itself. For example, when two different liquids of the same temperature are mixed – such as water and sulphuric acid – the final temperature of the mix can differ. Similarly, when electrical current flows through material at room temperature, the system can heat up or cool down, depending on the material.
The second principle of thermodynamics states:
The entropy in a isolated system tends towards its maximum. In summary, the concept of entropy, corresponding to what is called ‘heat’ in everyday life – but not to what is called ‘heat’ in physics! – describes the randomness of the internal motion in matter. Entropy is not conserved: in a isolated system, entropy never decreases, but it can increase until it reaches a maximum value. The non-conservation of entropy is due to the many components inside everyday systems. The large number of components lead to the non-conservation of entropy and therefore explain, among many other things, that many processes in nature never occur backwards, even though they could do so in principle.
La sintesi dell’acqua e la sua trasformazione in ghiaccio, ricordate da FermiGas in Neutrino 3, aiutano a capire che le trasformazioni irreversibili comportano sempre e comunque un aumento di entropia dell’Universo.