Thermodynamics

62 | ThermodynamicsHotbodyCaloricContactsurfaceColdbodyFIGURE 2–16In the early nineteenth century, heatwas thought to be an invisible fluidcalled the caloric that flowed fromwarmer bodies to the cooler ones.W = 30 kJm = 2 kg∆t = 5 sW = 6 kWw = 15 kJ/kJ/kgINTERACTIVETUTORIALSEE TUTORIAL CH. 2, SEC. 4 ON THE DVD.30 kJworktemperature increased; and when caloric was removed from a body, its temperaturedecreased. When a body could not contain any more caloric, muchthe same way as when a glass of water could not dissolve any more salt orsugar, the body was said to be saturated with caloric. This interpretation gaverise to the terms saturated liquid and saturated vapor that are still in usetoday.The caloric theory came under attack soon after its introduction. It maintainedthat heat is a substance that could not be created or destroyed. Yet itwas known that heat can be generated indefinitely by rubbing one’s handstogether or rubbing two pieces of wood together. In 1798, the AmericanBenjamin Thompson (Count Rumford) (1754–1814) showed in his papersthat heat can be generated continuously through friction. The validity of thecaloric theory was also challenged by several others. But it was the carefulexperiments of the Englishman James P. Joule (1818–1889) published in1843 that finally convinced the skeptics that heat was not a substance afterall, and thus put the caloric theory to rest. Although the caloric theory wastotally abandoned in the middle of the nineteenth century, it contributedgreatly to the development of thermodynamics and heat transfer.Heat is transferred by three mechanisms: conduction, convection, andradiation. Conduction is the transfer of energy from the more energetic particlesof a substance to the adjacent less energetic ones as a result of interactionbetween particles. Convection is the transfer of energy between a solidsurface and the adjacent fluid that is in motion, and it involves the combinedeffects of conduction and fluid motion. Radiation is the transfer of energydue to the emission of electromagnetic waves (or photons). An overview ofthe three mechanisms of heat transfer is given at the end of this chapter as aTopic of Special Interest.2–4 ■ ENERGY TRANSFER BY WORKWork, like heat, is an energy interaction between a system and its surroundings.As mentioned earlier, energy can cross the boundary of a closed systemin the form of heat or work. Therefore, if the energy crossing theboundary of a closed system is not heat, it must be work. Heat is easy torecognize: Its driving force is a temperature difference between the systemand its surroundings. Then we can simply say that an energy interaction thatis not caused by a temperature difference between a system and its surroundingsis work. More specifically, work is the energy transfer associatedwith a force acting through a distance. A rising piston, a rotating shaft, andan electric wire crossing the system boundaries are all associated with workinteractions.Work is also a form of energy transferred like heat and, therefore, hasenergy units such as kJ. The work done during a process between states 1and 2 is denoted by W 12 , or simply W. The work done per unit mass of asystem is denoted by w and is expressed asw W m 1kJ>kg2(2–17)FIGURE 2–17The relationships among w, W, and W # .The work done per unit time is called power and is denoted W . (Fig. 2–17).The unit of power is kJ/s, or kW.