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may  be  interpreted  graphically  as  the  area  under  the  curve  between  the
            limits V  and V .
                      1
                                2
                According to the convention stated above, the work is positive when a
            system expands. Thus, if a system  expands from 1 to 2 in Fig.2.3.2, the

            area is regarded as positive. A compression from 2 to 1 would give rise to a
            negative area; when a system is compressed, its volume decreases and it
            does negative work on its surroundings.

                If  the  pressure  remains  constant  while  the  volume  changes,  then  the
            work is
                                                           A   p V  V                                          (2.3.4)
                                                            2    1



                                                2.4 Heat Capacity


                  The heat capacity  C of the system is determined as the amount of heat
            (expressed  in    joules)  needed  to  raise  the  system  temperature  by  one
            degree  (expressed in  Kelvin).

                                                        Q
                                                          C                                                     (2.4.1)
                                                        T

               It is expressed in units of thermal energy per degree of temperature. The
            SI unit of heat capacity is joule per kelvin,


                      The amount of heat   Q  needed to increase the temperature of one
            mole       1 mole   of a substance by one degree    T              1 K  is the molar

            heat capacity.


                                                            Q
                                                          C M                                               (2.4.2)
                                                             T


                                                                                                    J     
            It is expressed in joule per mole per degree   Kelvin     C                 M              
                                                                                      
                                                                                                           
                                                                                                mole   K  
            The amount of heat needed to increase the temperature of one kilogram of
            a substance by one degree is the specific heat capacity.


                                                         Q
                                                          c                                                (2.4.3)
                                                      m   T


            From (2.4.2) and (2.4.3) we obtain relation between specific an molar heat
            capacity.



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