4.5 describe a variety of everyday and scientific devices and situations, explaining the fate of the input energy in terms of the above relationship, including their representation by Sankey diagrams

Sankey diagrams show what happens to the input energy and in what proportion is transferred into various forms. This is a Sankey of a filament lamp

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PC: BBC BITESIZE 

Note: Downwards arrow is the wasted energy.

So if we take into account that energy efficiency = total useful output / total useful input,

  • Light is the useful energy (75 J)
  • Efficiency = ( 75 / 100 )  x 100
  • Efficiency =  75%
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