THERMODYNAMICS ( from Gk. eipaa, thernie, heat + .36papAs, dynavni,s, power). The application of the principles of mechanics to heat-phenomena. It is shown in the article HEAT that all heat-effects can he traced for their cause to work having been done against the molec ular forces of the body—e.g. friction, compression, etc.—or to the reception of energy by the minute portions of the body—e.g. radiation, conduction, etc. In short, it may he regarded as proved ex that heat-effects always accom pany changes in the intrinsic energy of a body; and the idea that the numerical value of the beat-effects depends on the energy added to the minute portions of the body and on that alone is now accepted by every one. If a small amount of energy AQ is added to a body, its intrinsic energy is changed (dU) and as a rule a certain amount of external work is done by the expan sion of the body (pdv, where p is the external pressure on the body and dv is the change in volume). By the conservation of energy, then, if no other work is done, oQ=dU+ydr.
This is sometimes called the 'first principle of thermodynamics.' In any form of heat engine—e.g. a steam en gine—the 'working substance,' water, starts at ordinary temperature; heat-energy is added to it by the boiler ; it reaches a high temperature. that of the boiler, and a high pressure; it ex pands, doing work in pushing out the piston; its temperature and its pressure therefore fall ; the 'cylinder' is now joined to the 'condenser,' and the steam passes out and is condensed to water, giving up heat-energy; the piston is drawn hack and the process is repeated. So far as the beat-energy and work are concerned, heat-en ergy at a high temperature has been given to the working substance, heat-energy at a low temperature is taken away from it, external work (W) is done. By the first principle of thermodynamics \V = if we assume that the water at the end of its cycle of changes has the same energy that it had at the beginning. The 'efficiency' of the process is defined as the W ratio Q, or ". In considering the efficiencies of various processes Ca root in 1S24 was led to imagine one which bears his name and which can be discussed theoretically. This process is one which consists of a working substance in closed in a cylinder with a movable piston pass ing around a cycle so as to return to its initial condition. There are supposed to be two large reservoirs of a liquid, at different temperatures. The cylinder containing the working substance at a high pressure and small volume is imagined placed in the high-temperature tank ; the sub stance is allowed to expand, so slowly that its temperature remains practically unchanged at that, of the surrounding liquid in the tank; in doing tills, heat-energy must flow into the work substance, otherwise its temperature would fall : after the volume has increased sufficiently, the cylinder is removed from the tank, and the inclosed substance is allowed to expand under such conditions that no heat-energy can enter, and its temperature falls; when it reaches that of the second tank the cylinder is placed in it, the piston is forced in by external force, thus reduc ing the V0111111e: if this process is done slowly the temperature will not rise, but heat-energy will flow out into the liquid of the tank; when the volume is sufficiently reduced, the cylinder is removed and the piston is pushed in under such conditions that no heat-energy can leave or enter; if the volume at. the beginning of this last
step is chosen correctly the workinz substance will after a sufficient compression be restored to its original condition. There have been four steps, two 'isothermal' and two 'adiabatic.' The net result is: No change in the working substance; external work (NV) done by the sub stance; heat-energy Q, withdrawn from the high temperature tank; heat-energy Q, given out to the low-temperature tank. Therefore, by the first principle of thermodynamics, W = Q, — This process of Carnot's is perfectly 'rever sible:' by doing an amount of work W on the working substance it may be made to pass around the cycle in the reverse way; the low temperature bath losing an amount of beat-en ergy Q, and the high-temperature bath gaining an amount Q,. The conditions for reversibility are that each 'point' of the whole cycle should be one of equilibrium; and it is apparent that for this to be satisfied the changes must all be made slowly and that when the substance re ceives or gives out heat-energy it must be in contact with a large tank of liquid at the same temperature, within an infinitesimal amount, as it itself is at that instant. (lf a gas expands out of a high-pressure reservoir into the open air—e.g. illuminating, gas rushing out of a burner—the process is irreversible; if in an engine the flame is at a higher temperature than the steam in the boiler—as it. always is—the process is irreversible.) It was stated by Car not that the efficiency of his -' was the same for all working substances and de pended alone on the temperatures of the two baths. His proof was, however, erroneous, and was corrected by Clausius in 1850, who showed that the statement. was correct if one assumed that heat-energy of itself always passes from high to low temperature. The statement that heat-energy of itself does always pass from high to low temperature is called the 'second prin ciple of thermodynamics.' Lord Kelvin has shown that this principle is identical in its con clusions with the assumption that it is impos sible by any material agency to derive mechanical effect from any portion of matter by cooling it below the temperature of the coldest of the sur rounding objects.