Conservation of
Energy
Energy cannot be created or destroyed. It can be
stored and can be transferred from one type to
another.
Only part of the energy will be transferred usefully (to the
type you want); the rest is ‘wasted’.
Energy in = energy out
Energy is measured in joules (J). If 100J goes into an
appliance, then 100J will come out. But not all of the 100J
coming out will be the type of energy you want.
Appliance
Input
Useful
Waste
energy (J) energy (J) Energy (J)
Tungsten filament light bulb 100
20
80
Energy efficient light bulb
25
20
5
Electric kettle
200
180
20
Electric drill
500
300
200
Old washing machine
500
200
300
New washing machine
300
200
100
New energy efficient
washing machine
250
200
50
Time Available: 5 mins
Gr C: 3 Completed
Gr B: 5 Completed
Gr A: All 7 Completed
Efficiency
Efficiency tells us how much useful energy we get out
compared to how much wasted energy we get out.
The greater the amount of energy converted to the type of
energy you want the more efficient it is.
We can calculate this using the following equation;
Efficiency
We can calculate this using the following equation;
Example
A kettle has 1000 joules of input electrical energy and produces 200
joules of useful heat energy. The rest is wasted as sound. Work out
the efficiency.
Energy Input = 1000 J
useful Energy OUTPUT = 200 J
(200 / 1000) X 100
0.2 X 100 = 20% efficiency
Why Are They Called
Sankey Diagrams?
Captain Matthew Henry Phineas Riall
Sankey was an engineer.
In 1898 he was the first person to publish
a diagram that used the width of an arrow
to show the size of the energy flow.
The first ever Sankey diagram shows the
energy flow in a real steam engine
compared with a ‘perfect’ engine.
Captain Henry Sankey
(1853 – 1925)
Sankey diagrams are so useful that you
can buy computer programs to draw them!
Measuring Efficiency
Efficiency is the amount of the total input energy that
is converted into a useful form. We can show this
using a Sankey diagram.
The thickness of each arrow
is drawn to scale to show the
amount of energy
The total amount of energy
before is equal to the total
amount of energy after
(conservation of energy)
Although the total energy out
is the same, not all of it is
useful.
Sankey Diagram for a
filament bulb
Sub-Title 2
9J
wasted
energy
out
10 J
energy
input
1 J
useful
energy
out
Sankey Diagram for a
filament bulb
9J
wasted
energy
out
10 J
energy
input
1 J
useful
Sankey Diagram for
an energy efficient
bulb
6J
energy
output
wasted
energy
out
10 J
energy
input
4J
useful
energy
output
Create simple sankey diagrams for the below
appliances and calculate the efficiency of each.
Appliance
Input
energy
(J)
100
Tungsten filament light
bulb
Energy efficient light bulb 25
Useful
energy
(J)
20
Waste
Energy
(J)
80
% efficiency
20
5
80%
20%
Electric kettle
200
180
20
90%
Electric drill
500
300
200
60%
Old washing machine
500
200
New washing machine
300
200
300
40%
Success Criteria
100 Gr C: 66%
New energy efficient
washing machine
250
200
50 Gr A: 80%
Time Available:
Gr B:
Efficiency
Developing: To understand not all energy is transferred from one
form directly to another
Secure: To be able to calculate efficiency from given data.
Exceeding: To be able to create a Sankey diagram from given data
and calculate efficiency from it.
Entrance Activity
In the U.K. the government will give you a
grant to have your house insulated. This means
it is more difficult for heat to leave your house.
Do you think this should be done for all people
in the UK or just certain groups like the
elderly? Explain why
Key Words – Tool Box
Thermal, Insulation, Heat loss,
Conduction, Dissipated, Efficiency,
Sankey Diagram, Joules, Transfer
iGCSE Exam Q