Reflections on the Motive Power of Fire (el. knyga) (skaityta knyga) | knygos.lt

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Sadi Carnot's 1824 Reflections asked how much work heat can yield, and answered with the reasoning now called the Carnot cycle and Carnot's theorem - a ceiling on the efficiency of every engine, fixed by temperature alone. Reasoned from the mistaken caloric theory, his result outlasted it to become the seed of the second law. This edition reproduces R. H. Thurston's 1897 English translation with Lord Kelvin's account of the theory, the founding document of thermodynamics restored to reach. Carnot's Reflections appeared at a moment when the steam engine had already remade the world but no one understood why it worked or how far it could be improved. For three-quarters of a century, since Thomas Newcomen and then James Watt, engineers had built engines of steadily greater power and economy by trial, intuition, and mechanical ingenuity. Britain had been transformed by them; France, in Carnot's view, lagged dangerously behind. Yet there existed no theory of the engine as such - no account of what determined the motive power obtainable from a given quantity of fuel, and no way to say whether an engine was near the limit of possible performance or far from it. Carnot set out to supply exactly that: a general theory of heat engines, independent of working substance and mechanical design, that would fix the limits within which all such engines must operate. The intellectual setting was the caloric theory of heat. In Carnot's day heat was understood as a weightless, indestructible fluid - caloric - that flowed from hot bodies to cold and was conserved in all its transformations. On this view, an engine produced work as caloric fell from a high temperature to a low one, much as a water-wheel produces work as water falls from a high level to a low one. Carnot took the analogy seriously and reasoned with it. The motive power of an engine, he argued, depends not on the substance employed - steam, air, alcohol vapor, anything - but solely on the temperatures between which the caloric passes, just as the work of a water-wheel depends on the height of the fall and not on the kind of water. This was the decisive abstraction, and it was correct, even though the conserved-caloric premise from which Carnot drew it was wrong. From it Carnot constructed the reasoning now called the Carnot cycle and the Carnot theorem. He imagined an idealized engine carried through a closed sequence of operations - two isothermal stages, in which the working substance exchanges heat with a hot or a cold reservoir at constant temperature, and two adiabatic stages, in which it changes temperature without exchanging heat - and required that every stage be conducted reversibly, slowly enough that the substance is always in equilibrium and the whole cycle can be run backward without loss. He then proved, by a now-famous argument from the impossibility of perpetual motion, that no engine working between two given temperatures can be more efficient than a reversible one, and that all reversible engines working between the same two temperatures have exactly the same efficiency, whatever their working substance. The efficiency of an ideal engine is therefore a property of the two temperatures alone. This is Carnot's theorem, and it sets an absolute ceiling, the Carnot efficiency, on the work obtainable from heat - a ceiling no real engine can reach and none can exceed. The result was too far ahead of its time to be received. The book sold poorly and was scarcely cited; Carnot died in 1832 with his work almost unknown.