Thermodynamics

and when the process is reversed, the system passes through exactly thesame equilibrium states while returning to its initial state. That is, the pathsof the forward and reverse processes coincide for an internally reversibleprocess. The quasi-equilibrium process is an example of an internallyreversible process.A process is called externally reversible if no irreversibilities occur outsidethe system boundaries during the process. Heat transfer between areservoir and a system is an externally reversible process if the outer surfaceof the system is at the temperature of the reservoir.A process is called totally reversible, or simply reversible, if it involvesno irreversibilities within the system or its surroundings (Fig. 6–35). Atotally reversible process involves no heat transfer through a finite temperaturedifference, no nonquasi-equilibrium changes, and no friction or otherdissipative effects.As an example, consider the transfer of heat to two identical systems thatare undergoing a constant-pressure (thus constant-temperature) phasechangeprocess, as shown in Fig. 6–36. Both processes are internallyreversible, since both take place isothermally and both pass through exactlythe same equilibrium states. The first process shown is externally reversiblealso, since heat transfer for this process takes place through an infinitesimaltemperature difference dT. The second process, however, is externally irreversible,since it involves heat transfer through a finite temperature differenceT.6–7 ■ THE CARNOT CYCLEWe mentioned earlier that heat engines are cyclic devices and that the workingfluid of a heat engine returns to its initial state at the end of each cycle.Work is done by the working fluid during one part of the cycle and on theworking fluid during another part. The difference between these two is thenet work delivered by the heat engine. The efficiency of a heat-engine cyclegreatly depends on how the individual processes that make up the cycle areexecuted. The net work, thus the cycle efficiency, can be maximized byusing processes that require the least amount of work and deliver the most,Noirreversibilitiesoutsidethe systemChapter 6 | 299Noirreversibilitiesinsidethe systemFIGURE 6–35A reversible process involves nointernal and external irreversibilities.INTERACTIVETUTORIALSEE TUTORIAL CH. 6, SEC. 7 ON THE DVD.20°C20°CBoundaryat 20°CHeatThermal energyreservoir at 20.000 ...1°C(a) Totally reversibleHeatThermal energyreservoir at 30°C(b) Internally reversibleFIGURE 6–36Totally and interally reversible heattransfer processes.