Thermodynamics

Collier's New Encyclopedia (1921)

the branch of theoretical physics which treats of heat as a mechanical agent, and is the basis on which the modern doctrine of energy is built. The second interpretation given by Newton of his third law of motion all but enumerates the principle of the conservation of energy. Ignorant, however, of the true nature of heat, he was unable to trace the mechanical loss caused by friction to its final issue. Not till more than a century after the publication of the "Principia" was any attempt made to fill the gap, and then the valuable experimental results of Rumford (1798) and Davy (1799), though conclusively disproving g the accepted caloric theory, were given to an unappreciative world, and failed to excite any real interest till 40 years later, when their discoveries were rediscovered. In 1812 Davy wrote, "The immediate cause of the phenomenon of heat then is motion, and the laws of its communications are precisely the same as the laws of the communication of motion." Here then the dynamical theory of heat was enunciated, but it was carried no further, and not till the experiments of Colding and Joule, executed independently about 1840, were published, can thermodynamics be regarded as established. About the same time Séguin and Mayer approached the same object, and deduced from experiment values of the mechanical equivalent of heat. They, however, went to work on hypotheses or rather different forms of the same hypothesis which are now known to be false; so that their claims as the founders of the doctrine of energy cannot be maintained against those of Colding and Joule, who went to work in a legitimate way. Mayer, however, deserves great merit for the manner in which he developed and applied the conservation principle. In the more restricted sphere of thermodynamics, Clausius, Rankine, and Thomson have been the great developers; and to the last-mentioned is due the modification of Carnot's forgotten cycle of operations to suit the true theory, and the deduction therefrom of the doctrine of the dissipation of energy. Thermodynamics is based on two laws. The first law enunciates heat to be a form of energy and subject to the conservation principle-an experimental truth rigorously established by Joule. Clerk Maxwell gives it in the form: When heat is transformed into work or work into heat, the quantity of work is mechanically equivalent to the quantity of heat. A given quantity of work can always be transformed into an equivalent quantity of heat; but in transforming heat into work a certain limitation exists which is expressed in the second law of thermopress dynamics. This law asserts that it is impossible, by physical processes, to transform any part of the heat of a body into mechanical work except by allowing heat to pass from that body to another at a lower temperature; or in Thomson's words, it is impossible by derive mechanical effect from any portion have of matter by cooling it below the temperature of the coldest of the surrounding objects. This is the law on which Carnot's principle is based-a principle which has led to results of the highest consequence. The principle is that the efficiency of a reversible engine is the greatest that can be obtained from a given range of temperature. Now a reversible engine in Carnot's sense is an altogether unrealizable heat engine, which can be made to go through a complete cycle of operations, either forward or backward. In other words, not merely is the engine able to do work while it transforms a given quantity of heat from the boiler to the condenser, but, by an expenditure of an equivalent quantity of work upon it, may be made to take back the same quantity of heat from the condenser to the boiler. In subjecting such an engine to a cycle of operations, the engine must be brought back to its original condition before any conclusion can be drawn regarding the relation between the heat which has disappeared and the work which has been done. Séguin, when he assumed that the work done by an expanding heated body was the equivalent of the heat which it loses, and Mayer, when he went to work on the hypothesis that the amount of heat produced in compressing a gas is equivalent to the work done in compression, violated this principle, so that their conclusions were logically untrustworthy. It is easily demonstrable on the conservation principle that Carnot's reversible engine is the most perfect possible engine, and that consequently all reversible engines working between the same temperatures have the same efficiency. For a small difference of temperature the efficiency is a function only of the temperature, and this efficiency, divided by the difference of temperatures, is called Carnot's function. Thomson, defining temperature as the reciprocal of Carnot's function, has constructed a scale of temperature absolutely independent of the nature of the thermometric substance. Hence if t is the temperature measured according to this absolute scale of the source of heat, and t' that of the refrigerator or condenser, the efficiency may be expressed by the t-t' fraction Now the efficiency is det fined as the ratio of the work done to the heat supplied expressed in dynamical measure. Hence, if W is the work done, H the heat supplied to the engine, and h the heat given out to the condenser, means of inanimate material agency to all expressed in dynamical measure we Wt-t' Ht W=H-h k t' whence H t or, in a reversible engine, the heat rejected is to the heat received as the absolute temperature of the refrigerator is to the absolute temperature of the boiler. From the first of these equations it is evident that the heat supplied cannot be wholly transformed into work unless the refrigerator is at absolute zero of temperature, a practical impossibility. Consequently, in a material system only a part of the intrinsic energy is available for work, and this available portion or entropy is continually diminishing because of the universal tendency of heat to diffuse itself, and reduce the system to a uniform temperature, when of course no work can be produced. This is Thomson's principle of the dissipation of energy.