The price of work
There is no such thing as a free lunch, says an old physics adage, but perhaps the opposite also holds: there is no free work. The second law, in its operational formulation (Clausius, Kelvin), tells us that to extract work from a hot reservoir we must always discard part of it into a cold reservoir. The usable fraction is never , and is bounded by the Carnot efficiency .
This thermodynamic “tax” is everywhere, once one learns to see it. It is why the refrigerator warms the kitchen instead of cooling it overall; it is why thermoelectric power plants always have a “heat graveyard” — the cooling towers — where they dissipate the energy they failed to convert; it is why perpetual motion cannot be built, however ingenious the mechanism.
Thermodynamics, more than any other physical discipline, teaches us to live with the constraints imposed by nature: it tells us not only what we can do, but above all what we will never be able to do.
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Topics: Macchine termiche Concepts: Secondo principio della termodinamica · Rendimento
Related exercises: Problema — Una macchina troppo efficiente · Problema — Vero o falso su macchine e frigoriferi · Problema — Carnot rispetto agli altri cicli