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G7 4.1 2 energy conversions
1. 4.1 ENERGY
Grade 7 Science – Interactive Science Book 1B
Diocesan Boys' School
This material is for self-reference only, please don’t reproduce and share to the others
without permission. 1
2. Textbook p.3
ENERGY IS NEEDED TO CARRY OUT ALL KINDS OF ACTIVITIES
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3. Textbook p.3
ENERGY IS NEEDED TO CARRY OUT ALL KINDS OF ACTIVITIES
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4. Textbook p.3
We need energy to carry out all kind of activities.
Energy can do work, therefore, we need energy.
Can you give more examples of the uses of energy?
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5. Textbook p.4
COMMON FORMS OF ENERGY
How can you describe
the Sun?
In fact, the Sun is the
most original source
of energy of all the
living things (most
non-living as well) on
the Earth
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6. Textbook p.4
1. HEAT ENERGY (熱能)
Hot objects like the Sun gives out heat energy.
ALL the objects contains heat energy……even for
things cold like ice have heat energy, it’s only the
matters of more or less only.
Heat is defined as “the energy transferred from one
to another” as a result of temperature difference
between the bodies.
Notes: more appropriately, physicists use the term
“internal energy(內能)” instead of “heat energy”
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7. Textbook p.4
WHY DO I FEEL HOT WHEN I TOUCH
SOMETHING?
It is because that things I touch is “hotter than”
me…even for a very small temperature difference
When you catch a fever…
Why do we feel hot when we touch our forehead?
Why?
Can you explain in terms of heat energy?
It is because there is a temperature difference
between our hand (less heat energy) and forehead
(more heat energy).
Energy is gained from our forehead to our hand.
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8. Textbook p.4
WHY CAN YOU SEE THE SUN?
Warning: DON’T directly watch the sun!
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9. Textbook p.4
The Sun is an extremely hot object.
We can see the Sun because it emits light, but
why it emits light?
Light is emitted when one thing is “over-heat”.
熱極發光
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10. Textbook p.4
2. LIGHT ENERGY (光能)
Light energy always
comes together with heat
energy.
The energy of the Sun
transfers to the Earth
through radiation(幅射).
Light energy is a kind of
radiation.
The lighter the object, the
more the light energy.
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11. Textbook p.6
6. CHEMICAL ENERGY (化學能)
When the energy from the Sun
reach the Earth, plants on the Earth
can carry out photosynthesis (光合
作用).
Because of photosynthesis, human
beings or animals can get some
plants or crops for food.
Chemical energy is stored in food.
They are energy stored in
“chemicals”.
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12. Textbook p.6
Fuels and dry cells also store
chemical energy.
Question:
So how can we release the
chemical energy of a piece of
paper?
BURNING
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13. Textbook p.6
When you play your iPhones…what energy are
you using?
Chemical energy of the lithium cells (鋰電池).
Chemical energy in the cell change into
electricity…therefore, we have…
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14. Textbook p.6
5. ELECTRICAL ENERGY
Electrical energy is a useful form of energy in
our daily life.
All electrical appliances need electrical energy
to work.
At home, we get the supply
of energy by connecting the
appliances to the socket.
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15. Textbook p.6
How can you come back to DBS from your
home?
Walking?
Public Transportation?
Private Cars?
You can come back to school because you can
move, moving is a kind of…
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16. Textbook p.5
4. KINETIC ENERGY (動能)
When an object is moving, it has kinetic energy.
The faster the movement, the more the kinetic
energy (K.E.).
Note 1: The amount of kinetic energy
also depends on its mass, when two
objects move in the same speed, the
higher the mass, the more the KE.
Note 2: Stationary objects also have
kinetic energy microscopically, but we
DO NOT consider it at the moment.
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17. Textbook p.7
You walk from Prince
Edward Road to the
DBS campus, you need
to walk for more than
180 steps…of course,
you need kinetic
energy.
However, you climb
higher and higher…
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18. Textbook p.7
7. POTENTIAL ENERGY (勢能/位能)
And the potential energy (PE) of you is also
increasing…
When an object is lifted up (from the reference
point), it gains potential energy.
The higher the position of an object, the more
the potential energy.
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19. Textbook p.7
7. POTENTIAL ENERGY
I am an old boy of DBS…
I used to go back to DBS everyday through the
Steps…
Up and down, up and down, up and down, up
and down, up and down, up and down…
When I leave DBS today and go down to Prince
Edward Road. Will I gain any energy potential
energy finally, in the reference of Prince Edward
Road?
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20. Textbook p.7
7. POTENTIAL ENERGY
In fact, there are 2 kinds of potential energy…
When you stretch a rubber band, the potential
energy of it also increases…
More examples…
stretched string of guitar
compressed spring
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21. Textbook p.7
7. POTENTIAL ENERGY (MORE)
The PE stored in the rubber band of the fork is
called elastic PE (compressed or stretched
object).
The PE I gained when I
go up the steps is called
gravitational PE.
Note: Both KE and PE can be classified
as mechanical energy (ME) (機械能).
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22. Textbook p.5
Now I am here to talk to you…
Can you hear me???
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23. Textbook p.5
3. SOUND ENERGY
When an object vibrates, it gives out sound.
It is also a form energy, called sound energy.
The stronger the vibration, the more the sound
gives out.
My vocal cord is vibrating when I am talking to
you…
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24. Textbook p.7
OUR STORY
Chemical Energy
Heat Energy Light Energy
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25. Textbook p.7
OUR STORY
Electrical Energy
Sound Energy
Potential Energy
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26. Textbook p.5
SUMMARY
We have learnt 7 types of energy form:
Heat energy
Light energy
Chemical energy
Electrical energy
Kinetic energy
Potential energy
Sound energy
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29. Textbook p.8
CONVERTING ENERGY FROM ONE FORM TO
ANOTHER
(Remember: you need the same kind of things to
exchange another same kind of things. 等價交換!)
Energy can only change from one form to another form.
Energy can’t be created or destroyed.
The total energy before change and after change is thus
equal.
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30. Textbook p.8
EXAMPLE OF ENERGY CONVERSION
Chemical energy Kinetic energy
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31. Textbook p.8
EXAMPLE OF ENERGY CONVERSION
Find out more examples on textbook p.8
Elastic PE KE + Gravitational PE
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32. Textbook p.8
CONVERTING ENERGY FROM ONE FORM TO
ANOTHER
(Remember: 等價交換!)
Misconception:
Mass won’t change to energy, and energy won’t
change to mass. Mass won’t loss when release
energy.
For your interest only: although relativity (相對論) tells us some different views, just
forget it right now…please!!!
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33. Textbook
p.9-11
Experiment
Time
Textbook p.9-11
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34. Textbook
p.11
QUICK SUMMARY
Energy can be converted
from one form to another.
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36. Textbook
p.12
The 3 Idiots
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Machine is a…(examples)
What was the definitions of machine the characters
mentioned?
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37. Textbook
p.12
ENERGY CONVERTER
In fact, machines are energy converters.
They change energy from one form to
another
Form Another
of Energy Form of
energy converter energy
(Machine)
(Input) (Output)
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38. Textbook
p.12
ENERGY CONVERTER
Electrical Kinetic
Energy
Fan Energy
(Input) (Machine) (Output)
Light
Energy
+ Sound
Telephone Energy
Electrical
Energy
(Machine) (Output)
(Input)
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