Thermodynamics

72 | Thermodynamics(Adiabatic)∆E = 8 kJW sh, in = 8 kJbetween the total energy entering and the total energy leaving the systemduring that process. That is,orTotal energyaentering the system b a Total energy Change in the totalb aleaving the system energy of the system bE in E out ¢E systemFIGURE 2–41The work (shaft) done on an adiabaticsystem is equal to the increase in theenergy of the system.W b,in = 10 kJ∆E = 10 kJ(Adiabatic)FIGURE 2–42The work (boundary) done on anadiabatic system is equal to theincrease in the energy of the system.∆E = (15 – 3) + 6= 18 kJQ in = 15 kJQ out = 3 kJW sh, in = 6 kJFIGURE 2–43The energy change of a system duringa process is equal to the net work andheat transfer between the system andits surroundings.This relation is often referred to as the energy balance and is applicable toany kind of system undergoing any kind of process. The successful use ofthis relation to solve engineering problems depends on understanding thevarious forms of energy and recognizing the forms of energy transfer.Energy Change of a System, E systemThe determination of the energy change of a system during a processinvolves the evaluation of the energy of the system at the beginning and atthe end of the process, and taking their difference. That is,or(2–32)Note that energy is a property, and the value of a property does not changeunless the state of the system changes. Therefore, the energy change of asystem is zero if the state of the system does not change during the process.Also, energy can exist in numerous forms such as internal (sensible, latent,chemical, and nuclear), kinetic, potential, electric, and magnetic, and theirsum constitutes the total energy E of a system. In the absence of electric,magnetic, and surface tension effects (i.e., for simple compressible systems),the change in the total energy of a system during a process is the sumof the changes in its internal, kinetic, and potential energies and can beexpressed aswhereEnergy change Energy at final state Energy at initial state¢E system E final E initial E 2 E 1¢E ¢U ¢KE ¢PE¢U m 1u 2 u 1 2¢KE 1 2 m 1V 2 2 V 2 1 2¢PE mg 1z 2 z 1 2(2–33)When the initial and final states are specified, the values of the specificinternal energies u 1 and u 2 can be determined directly from the propertytables or thermodynamic property relations.Most systems encountered in practice are stationary, that is, they do notinvolve any changes in their velocity or elevation during a process (Fig.2–44). Thus, for stationary systems, the changes in kinetic and potentialenergies are zero (that is, KE PE 0), and the total energy changerelation in Eq. 2–33 reduces to E U for such systems. Also, the energy