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Physics: Energy Effiency
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This document has notes on how energy is transferred, energy efficiency formula and a few questions at the end. These are my own notes taken during class.

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  • September 18, 2021
  • 1
  • 2021/2022
  • Lecture notes
  • Harry
  • All classes

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Energy Efficiency


All the energy transfers (useful and wasted) that are
associated with a device can be represented by a Sankey
diagram.
A Sankey diagram uses arrows to represent all the output
energies.
The thickness of each arrow shows the amount of energy




The formula for energy efficiency is:




 Useful energy is measured in joules (J).
 Total energy is measured in joules (J).
 Energy efficiency does not have any units.
 It is a number between 0 and 1 which can be converted into a percentage by
multiplying by 100.
Efficiency of a lightbulb
This filament bulb is supplied with 100 J of electrical energy, which it converts to 45 J of light
energy.
1. How much energy is wasted?
2. In what form is the energy wasted?
3. In what form is the energy wasted?
Answers
1. Wasted energy= Total – Useful
= 100 J – 45 J
= 55 J
2. Heat
3. Efficiency = Useful/ Total = 45 J/ 100 J =0.45 or 45%

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