Thermodynamics

252 | ThermodynamicsThus,andv 2 0.001 10.009211.004 0.0012 m 3 /kg 0.010 m 3 /kgm 2 V v 20.006 m3 0.6 kg0.01 m 3 /kgTherefore, after 30 min there is 0.6 kg water (liquid vapor) left in thepressure cooker.Discussion Note that almost half of the water in the pressure cooker hasevaporated during cooking.TOPIC OF SPECIAL INTEREST*General Energy EquationOne of the most fundamental laws in nature is the first law of thermodynamics,also known as the conservation of energy principle, which providesa sound basis for studying the relationships among the various formsof energy and energy interactions. It states that energy can be neither creatednor destroyed during a process; it can only change forms.The energy content of a fixed quantity of mass (a closed system) can bechanged by two mechanisms: heat transfer Q and work transfer W. Then theconservation of energy for a fixed quantity of mass can be expressed in rateform asQ # W # dE sysorQ # W # d redVdtdtsys(5–47)where Q . Q . net,in Q . in Q . out is the net rate of heat transfer to the system(negative, if from the system), W . W . net,out W . out W . in is the net poweroutput from the system in all forms (negative, if power input) and dE sys /dt isthe rate of change of the total energy content of the system. The overdotstands for time rate. For simple compressible systems, total energy consistsof internal, kinetic, and potential energies, and it is expressed on a unit-massbasis ase u ke pe u V 22 gz(5–48)Note that total energy is a property, and its value does not change unless thestate of the system changes.An energy interaction is heat if its driving force is a temperature difference,and it is work if it is associated with a force acting through a distance,as explained in Chap. 2. A system may involve numerous forms of work, andthe total work can be expressed asW total W shaft W pressure W viscous W other(5–49)where W shaft is the work transmitted by a rotating shaft, W pressure is the workdone by the pressure forces on the control surface, W viscous is the work done*This section can be skipped without a loss in continuity.