conservation of energy lesson objectives: all must know that energy an be neither created nor...
TRANSCRIPT
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/energy-transfer/263.html
electrical
light
sound
heat
What are the main energy transfers for a television?
chemical
kinetic
sound
heat
What are the main energy transfers for a car engine?
electrical
sound
heat
What are the main energy transfers for a radio?
© Boardworks Ltd 2003
In which two forms is energy usually wasted?
A. Sound and light
B. Heat and sound
C. Heat and light
D. Light and chemical
© Boardworks Ltd 2003
What is the intended energy transfer for a Bunsen burner?
A. Sound light
B. Heat chemical
C. Chemical light
D. Chemical heat
© Boardworks Ltd 2003
What is the energy change for a falling rock?
A. Kinetic gravitational
B. Gravitational kinetic
C. Kinetic speed
D. Mass speed
© Boardworks Ltd 2003
In what form is energy wasted for all electrical devices?
A. Heat
B. Electrical
C. Sound
D. Light
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 diagramsEnergy 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.
100 J/s electrical energy
“Input” energy “Output” energy
80 J/s heat energy (given to
the surroundings)
20 J/s light energy
Example questionsExample questionsConsider a kettle:
1) Work out each energy value.
2) How much energy is wasted?
Sound energy
Wasted heat
Heat to water
2000 J/s electrical energy
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?
Electrical energy100%
Heat energy
20%
Kinetic energy
22%(Wasted
vibration of the drill)
Kinetic energy
58%(Useful
spinning of the drill bit)
Energy Transfer Values
Draw a Sankey Diagram for this drill.
Sankey Diagram for an ipod.
How much energy is wasted energy?
EfficiencyEfficiency
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%
© Boardworks Ltd 2003
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?
b) In what form is the energy wasted?
c) What is the efficiency of the radio?
Efficiency = Useful
Total
= 96J/300J
= 0.32 or 32%
204J
heat
Energy efficiencyEnergy efficiencyEnergy efficiencyEnergy efficiency
Energy efficiency
Energy efficiencyEnergy efficiencyEnergy efficiencyEnergy efficiency
Energy efficiency
Energy efficiencyEnergy efficiencyEnergy efficiencyEnergy efficiency
Energy efficency
Some examples of Some examples of efficiency…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 questionsExample questionsConsider a kettle:
What is the kettle’s efficiency?
Sound energy
Wasted heat
Heat to water
2000 J/s electrical energy
Consider a computer:
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?
© Boardworks Ltd 2003
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%
© Boardworks Ltd 2003
• 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
© Boardworks Ltd 2003