Conservation of Energy
• Lesson Objectives:
• All must know that energy an be neither
created nor destroyed but it can be changed
from one form into another.
• All must know that some energy is wasted
(usually as heat) when energy is transferred.
• Most should be able to draw, label and use
simple SANKEY DIAGRAMS.
• Some may be able to work out efficiency for
some electrical devices.
• http://www.bbc.co.uk/learningzone/clips/ener
gy-transfer/263.html
What are the main energy transfers for a television?
light
electrical
sound
heat
What are the main energy transfers for a car engine?
kinetic
chemical
sound
heat
What are the main energy transfers for a radio?
sound
electrical
heat
In which two forms is energy usually
wasted?
A. Sound and light
B. Heat and sound

C. Heat and light
D. Light and chemical
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What is the intended energy transfer
for a Bunsen burner?
A. Sound  light
B. Heat  chemical
C. Chemical  light
D. Chemical  heat

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What is the energy change for a falling
rock?
A. Kinetic  gravitational
B. Gravitational  kinetic
C. Kinetic  speed

D. Mass  speed
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In what form is energy wasted for all
electrical devices?
A. Heat

B. Electrical
C. Sound
D. Light
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Sankey Diagrams
• These are ways of representing
USEFUL and WASTED energy.
A simple
filament light
bulb.
Which is the
useful energy?
The useful energy is shown by the HORIZONTAL arrow.
The wasted energy is the curved, downward pointing arrow.
It could also be shown as an upward pointing arrow.
More Sankey Diagrams
• Sankey diagram for energy saving light bulb.
Why are energy saving light bulbs better value for money?
Because they waste little energy as heat. Bills go down!
Drawing an accurate Sankey diagram
Energy Transfer diagrams
Energy is measured in joules.
Consider a light bulb. Let’s say that the bulb runs on 100 joules per second
and transfers 20 joules per second into light and the rest into heat. Draw
this as a diagram:
Total input energy must = total output energy.
“Input” energy
100 J/s
electrical
energy
“Output” energy
20 J/s
light energy
80 J/s heat
energy (given to
the surroundings)
Example questions
Consider a kettle:
2000 J/s
electrical
energy
Sound
energy
Wasted
heat
Heat to
water
1) Work out each energy value.
2) How much energy is wasted?
Consider a computer:
150 J/s
electrical
energy
10 J/s
wasted
sound
20 J/s
wasted
heat
Useful
light and
sound
1) How much energy is converted
into useful energy?
Energy Transfer Values
Electrical energy
100%
Kinetic energy
58%
(Useful spinning
of the drill bit)
Kinetic energy
22%
(Wasted vibration
of the drill)
Draw a Sankey Diagram for this drill.
Heat energy
20%
Sankey Diagram for an ipod.
How much energy is wasted energy?
Efficiency
Efficiency is a measure of how much USEFUL energy you
get out of an object from the energy you put INTO it.
Efficiency = Useful energy given out by the device
Energy put into it
x100%
e.g. if 2000 joules of electrical energy are put into a
kettle and 500 joules of heat energy are gained from it,
its efficiency is 500/2000 x 100% = 25%
Efficiency for a radio
If you have a radio and
it is supplied with 300J
of electrical energy
which it converts to 96J
of sound energy.
a) How much energy is
wasted? 204J
b) In what form is the
energy wasted? heat
c) What is the efficiency of
the radio?
Efficiency
= Useful
Total
= 96J/300J
= 0.32 or 32%
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Energy efficiency
Energy efficiency
Energy efficiency
Some examples of efficiency…
1) 5000J of electrical energy are put into a motor. The
motor converts this into 100J of movement energy.
How efficient is it?
2) A laptop can convert 400J of electrical energy into
240J of light and sound. What is its efficiency?
Where does the rest of the energy go?
3) A steam engine is 50% efficient. If it delivers
20,000J of movement energy how much chemical
energy was put into it?
Working out efficiency from Sankey Diagrams
Example questions
Consider a kettle:
2000 J/s
electrical
energy
Sound
energy
Wasted
heat
Heat to
water
What is the kettle’s efficiency?
Consider a computer:
150 J/s
electrical
energy
10 J/s
wasted
sound
What is the computer’s
efficiency?
20 J/s
wasted
heat
Useful
light and
sound
If a plant converts 200J of light energy into
140J of chemical energy by photosynthesis,
what is the efficiency of the process?
A. 30J
B. 30%
C. 70J
D. 70%

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• When you have finished the work set
please follow the following and do the
exercises.
• Subjects>science>revision>Lonsdale
Revision Guides>Physical Processes>70
Forms of Energy and 75 Transfer and
Conservation of Energy
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© Boardworks Ltd 2003