Conservation of Energy
Lipi Shorts
CBSE · Class 9 · Science
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Conservation of Energy
Central idea
Energy is money in a sealed wallet: you can change coins for notes, but the total never goes up or down on its own.
Central idea
The total energy of a closed system stays exactly the same before and after any process.
Energy is never created or destroyed: it only changes form.
The law
PE lost equals KE gained, so the total stays the same. At the bottom all the PE has become KE.
As something falls, its store of height-energy (PE) turns into movement-energy (KE).
Falling object
This energy is not destroyed: it just becomes too spread out to do useful work.
Friction and air resistance turn some energy into heat that spreads out and is wasted.
Wasted energy
No real machine is 100% efficient because some energy always escapes as heat or sound.
Efficiency is the fraction of input energy that does the useful job.
Efficiency
See it in a diagram
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Myth vs truth
✕ Myth
Energy gets used up and disappears.
✓ Truth
It is only transferred or changed in form; the total stays the same.
Myths vs truth
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Word meanings
conservation of energy
the principle that the total energy of a closed system stays constant
closed system
a system that does not exchange energy with its surroundings
kinetic energy
the energy a body has because of its motion
potential energy
stored energy due to position, such as height in a gravitational field
Word meanings
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Themes & message
•
Total energy in a closed system is always conserved.
•
Energy transfers (such as PE to KE) keep the total fixed.
•
Dissipation and efficiency explain why some energy is wasted as heat.
Themes & message
Key facts at a glance
The law
Energy cannot be created or destroyed, only transferred or changed in form
Closed system
Total energy stays constant
Energy unit
Joule (J)
Key facts at a glance
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