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Calculating Efficiency20 April 2023
Objectives
Be able to calculate the efficiency of different systems.
HSW: AF4: Be able to identify significant variables and recognise which
are independent and which are dependent. AF5: Explain ways of modifying work to improve accuracy and reliability.
Used before in: AF5, lesson 3 conservation of energy
Will use again in:
PLTS: Team Workers: collaborate with others to work towards a common goal.
Used before in: Will use again in:
KeywordsEfficiency, variable, independent, dependent, categoric, continuous, accuracy,
precision, reliability
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Outcomes
• All students should be able to: • Describe what efficiency means in terms of
input energy, useful energy and wasted energy.
• Most students should be able to: • Use the equation to calculate efficiency.• Some students should be able to:• Apply the equation for efficiency to
experimental evidence and comment on the types of variable being tested as well as the accuracy and reliability.
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Efficiency is a measure of how good a device is at changing energy from one form to another.
All devices waste energy, so the efficiency of a device is never 100%.
An energy-efficient light bulb is more efficient than a filament bulb because it wastes less energy.
The more efficient a device is the less energy is wasted.
What is energy efficiency?
It converts a greater proportion of the energy that it is supplied with into light energy and not heat.
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The energy efficiency of a device can be calculated using this formula:
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.
Useful energy is measured in joules (J).
Total energy is measured in joules (J).
energy efficiency = useful output energy
total input energy
How is energy efficiency calculated?
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0.45 or 45%
a) How much energy is wasted?
b) In what form is the energy wasted?
c) What is the efficiency of the bulb?
Efficiency = Useful Total
= 45 J100 J
= 0.45 or 45%
This filament bulb is supplied with 100 J of electrical energy, which it converts to 45 J of light energy.
55 J
heat
Wasted energy = Total – Useful = 100 J – 45 J
= 55 J
Calculate the efficiency of a bulb
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a) How much energy is wasted?
b) In what form is the energy wasted?
c) What is the efficiency of the radio?
This radio is supplied with 300 J of electrical energy which it converts to 96 J of sound energy.
Efficiency = Useful Total
= 96 J 300J
= 0.32 or 32%
204 JWasted energy = Total – Useful = 300 J – 96 J
= 204 J
heat
0.32 or 32%
Calculate the efficiency of a radio
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a) What useful energy does a television produce?
b) How much useful energy is produced?
This television converts 2 000 J of electrical energy into useful energy at an efficiency of 65%.
Efficiency = Useful Total
Useful = Efficiency x Total
= 0.65 x 2 000 J = 1 300 J
light and sound
1 300 J
Calculate the efficiency of a television
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Energy efficiency calculations – activity
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Bouncing Balls – A Demo
• P250 core science.
