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18     CHAPTER 1  Introduction




                            In Eq. (1.34) Q and W is the net energy transfer by heat and work across the sys-
                         tem boundary, respectively. The unit for Q and W is Joule (J). ∆E is the net change
                         of total energy within the system and its unit is Joule (J). Total energy, E, of a system
                         consists of internal energy (U), kinetic energy (KE), and potential energy (PE). The
                         change in total energy of a system is expressed by:
 ∆E=∆U+∆KE+∆PE                                    ∆= ∆+ ∆KE   + ∆PE                     (1.35)
                                                   E
                                                       U
                            Where:

                                      ∆U  = (U  −U  )∆KE =  1  m (V  2  − V  2  ) ∆PE  = mg (z  − z  )  (1.36)
 ∆U=U −U ∆KE=12mV22−V12∆PE=                  2  1      2    2  1          2  1
 2  1
 mgz −z
 2  1
                            Internal energy is represented by the symbol U, and U  − U  is the change in
                                                                         2
                                                                              1
                                                                                  1
 12mV 2                  internal energy of a process. V is the magnitude of system velocity,  mV  is the
                                                                                      2
                                                                                  2
                         kinetic energy, KE, of the body and ∆KE is the change in kinetic energy of the sys-
                         tem. Z is the magnitude of the system elevation relative to surface earth, mgz is the
                         gravitational potential energy, PE, and the change in gravitational potential energy
                         is ∆PE.
                            By inserting Eqs. (1.35), (1.36) into Eq. (1.34), the first law of thermodynamics
                         becomes:
                                                         (
                                                   U
                                           Q  − W  = ∆+  1  mV  2  − V  2 ) + mgz  − z  )  (1.37)
 Q−W=∆U+12m(V22−V12)+mg(z −z )                        2    2  1     ( 2  1
 2  1
                            The instantaneous time rate form of the first law of thermodynamics is:
                                                      dE  = QW

                                                            −
 dEdt=Q˙−W˙                                            dt                               (1.38)

                                   W
 W ˙                     where Q and     are the rate of heat and work transfer across the boundary, respectively.
 Q˙
                            The first law of thermodynamics for open system (control volume) is expressed as:
                          Time rate of change of the energy    Net rate of energy transfer   Net rate of energy transfer 
                           within the control volume   =  as heat and work to/from   +  by mass entering the  
                                                 
                                                                        control volume
 Time rate of change of the energywithin the control volume=Net rate of     control volume at time  t      
 energy transferas heat and work to/fromcontrol volume at time t+Net rate
 of energy transferby mass entering thecontrol volume  or
                                                                      
                                                     
                                    dE cv  =  Q − W +  mh +  V i 2  +  gz  i    − ∑  mh +  V e 2  +  gz e    (1.39)
                                              cv ∑




                                                                     e 
                                                    i 
 dEcvdt=Q˙cv−W˙cv+∑im˙ihi+Vi22       dt   cv      i    i  2      e    e  2    
 +gzi−∑em˙ehe+Ve22+gze
                         where subscripts i and e denote the inlet to and exit from the system, respectively, h
                         is the enthalpy, and  m    is the mass flow rate.
 m˙
                         1.8.3.3  The second law of thermodynamics
                         Second law of thermodynamics talks about the usefulness of energy as well as energy
                         transfer direction. In another word second law of thermodynamics put limit on the
                         first law of thermodynamics. Second law states that the total system work is always
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