Efficiency  (η)

η = Puseful/Pin     or   η = Euseful/Ein

  • η            efficiency  (no dimension)
  • Puseful     useful power out in W
  • Pin          total power in in W
  • Euseful      useful energy out in J
  • Ein              total energy in in J

Example

In example 2 the water is heated by a an electric heating element with a power of  1000 W.

The water was heated for  1.5 minutes.

Calculate the efficiency of the process

Ein  = P  t = (1000)( 1.5 x 60)  = 9.0 x 104  J

Euseful = Q = 6.39 x 104  J

η = (6.39 x 104)/(9.0 x 104) = 0.71  or  71 %  

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