The Energy whether possessed by an electrical system or mechanical system or thermal system has the same thing in common i.e., it can do some work. Therefore, mechanical,electrical and thermal energies must have the same unit. This is amply established by the fact that there exists a definite relationship among the units assigned to these energies. It will be seen that these units are related to each other by some constant.

1. Electrical To mechanical

                       1kWh = 1 kW x 1 hr 
                           = 1000 watts x 3600 seconds

                           = 36 x 10⁵  watt-sec. or Joules

                      so, 1 kWh = 36 x 10⁵  Joules 

       It is clear that electrical energy can be expressed in Joules instead of kWh.

2. Heat and Mechanical

   

    (a)    1 calorie = 4.18 Joules        (by experiment)

   

    (b)    1 C.H.U. = 1 lb x 1⁰C 

                  = 453.6 gm x 1⁰C 

                  = 453.6 calories 

                  = 453.6 x 4.18 joules

                  = 1896 Joules

         so, 1 C.H.U = 1896 Joules

    (c)    1 B. Th.U = 1 lb x 1⁰F 

                   = 453.6 gm x 5/9⁰C

                   = 252 calories

                   = 252 x 4.18 Joules

                   = 1053 Joules

         so,  1 B.Th.U = 1053 Joules

  It may be seen that heat energy can be expresses in joules instead of thermal units viz,calorie, B.Th.U and C.H.U.

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