Editing Adiabatic process (section)

==Conceptual significance in thermodynamic theory==

The adiabatic process has been important for thermodynamics since its early days. It was important in the work of [[James Prescott Joule|Joule]] because it provided a way of nearly directly relating quantities of heat and work.

Energy can enter or leave a thermodynamic system enclosed by walls that prevent [[mass transfer]] only as heat or work. Therefore, a quantity of work in such a system can be related almost directly to an equivalent quantity of heat in a cycle of two limbs. The first limb is an isochoric adiabatic work process increasing the system's [[internal energy]]; the second, an isochoric and workless heat transfer returning the system to its original state. Accordingly, Rankine measured quantity of heat in units of work, rather than as a calorimetric quantity.<ref>{{cite journal |author=Rankine |first=W. J. MacQ. |date=1854 |title=On the geometrical representation of the expansive action of heat, and theory of thermodynamic engines |journal=Proceedings of the Royal Society |volume=144 |pages=115–175}} [https://archive.org/stream/miscellaneoussci00rank#page/340/mode/1up Miscellaneous Scientific Papers p. 339]</ref> In 1854, Rankine used a quantity that he called "the thermodynamic function" that later was called entropy, and at that time he wrote also of the "curve of no transmission of heat",<ref>{{cite journal |author=Rankine |first=W. J. MacQ. |date=1854 |title=On the geometrical representation of the expansive action of heat, and theory of thermodynamic engines |journal=Proceedings of the Royal Society |volume=144 |pages=115–175}} [https://archive.org/stream/miscellaneoussci00rank#page/341/mode/1up/search/transmission Miscellaneous Scientific Papers p. 341].</ref> which he later called an adiabatic curve.<ref name="Rankine 1866"/> Besides its two isothermal limbs, Carnot's cycle has two adiabatic limbs.

For the foundations of thermodynamics, the conceptual importance of this was emphasized by Bryan,<ref>{{cite book|author-link=George H. Bryan|last=Bryan |first=G. H. |date=1907 |url=https://archive.org/details/ost-physics-thermodynamicsin00bryauoft |title=Thermodynamics. An Introductory Treatise dealing mainly with First Principles and their Direct Applications |publisher=B. G. Teubner |location=Leipzig}}</ref> by Carathéodory,<ref name="Carathéodory"/> and by Born.<ref>{{cite book|author-link=Max Born|last=Born |first=M. |date=1949 |url=https://archive.org/details/naturalphilosoph032159mbp |title=Natural Philosophy of Cause and Chance |publisher=Oxford University Press |location=London}}</ref> The reason is that calorimetry presupposes a type of temperature as already defined before the statement of the first law of thermodynamics, such as one based on empirical scales. Such a presupposition involves making the distinction between empirical temperature and absolute temperature. Rather, the definition of absolute thermodynamic temperature is best left till the second law is available as a conceptual basis.<ref name="Bailyn Ch 3">{{cite book |last=Bailyn |first=M. |title=A Survey of Thermodynamics |date=1994 |publisher=American Institute of Physics |isbn=0-88318-797-3 |location=New York, New York |language=en-us |chapter=Chapter 3}}</ref>

In the eighteenth century, the law of conservation of energy was not yet fully formulated or established, and the nature of heat was debated. One approach to these problems was to regard heat, measured by calorimetry, as a primary substance that is conserved in quantity. By the middle of the nineteenth century, it was recognized as a form of energy, and the law of conservation of energy was thereby also recognized. The view that eventually established itself, and is currently regarded as right, is that the law of conservation of energy is a primary axiom, and that heat is to be analyzed as consequential. In this light, heat cannot be a component of the total energy of a single body because it is not a [[State function|state variable]] but, rather, a variable that describes a transfer between two bodies. The adiabatic process is important because it is a logical ingredient of this current view.<ref name="Bailyn Ch 3"/>