2. 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.

3. light electrical sound heat
What are the main energy transfers for a television?
light
electrical
sound
heat

4. kinetic chemical sound heat
What are the main energy transfers for a car engine?
kinetic
chemical
sound
heat

5. What are the main energy transfers for a radio?
sound
electrical
heat

6. In which two forms is energy usually wasted?
Sound and light
Heat and sound
Heat and light
Light and chemical 

7. What is the intended energy transfer for a Bunsen burner?
Sound  light
Heat  chemical
Chemical  light
Chemical  heat 

8. What is the energy change for a falling rock?
Kinetic  gravitational
Gravitational  kinetic
Kinetic  speed
Mass  speed 

9. In what form is energy wasted for all electrical devices?
Heat
Electrical
Sound
Light 

10. These are ways of representing USEFUL and WASTED energy.
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.

11. Sankey diagram for energy saving light bulb.
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!

12. Drawing an accurate Sankey diagram

13. 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
“Output” energy
20 J/s light energy
100 J/s electrical energy
80 J/s heat energy (given to the surroundings)

14. Example questions Consider a computer: Consider a kettle:
150 J/s electrical energy
10 J/s wasted sound
20 J/s wasted heat
Useful light and sound
How much energy is converted into useful energy?
Consider a kettle:
Work out each energy value.
How much energy is wasted?
Sound energy
Wasted heat
Heat to water
2000 J/s electrical energy

15. Energy Transfer Values
Kinetic energy
58%
(Useful spinning of the drill bit)
Energy Transfer Values
Kinetic energy
22%
(Wasted vibration of the drill)
Electrical energy
100%
Draw a Sankey Diagram for this drill.
Heat energy
20%

16. Sankey Diagram for an ipod.
How much energy is wasted energy?

17. 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
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%
x100%

18. 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.
How much energy is wasted?
In what form is the energy wasted?
What is the efficiency of the radio?
204J
Efficiency = Useful
Total
= 96J/300J
= 0.32 or 32%
heat

19. Energy efficiency
Energy efficiency

20. Energy efficiency
Energy efficiency

21. Energy efficiency
Energy efficency

22. Some examples of efficiency…
5000J of electrical energy are put into a motor. The motor converts this into 100J of movement energy. How efficient is it?
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?
A steam engine is 50% efficient. If it delivers 20,000J of movement energy how much chemical energy was put into it?

23. Working out efficiency from Sankey Diagrams

24. Example questions Consider a computer: Consider a kettle:
150 J/s electrical energy
10 J/s wasted sound
20 J/s wasted heat
Useful light and sound
What is the computer’s efficiency?
Consider a kettle:
What is the kettle’s efficiency?
Sound energy
Wasted heat
Heat to water
2000 J/s electrical energy

25. If a plant converts 200J of light energy into 140J of chemical energy by photosynthesis, what is the efficiency of the process?
30J
30%
70J
70% 

26. 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