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Unit 3: Forces and energy

Where does energy go? — HIA International - Grade 8 Revision Notes & Topical Questions

5 min read 8 parts Lesson 8 of 16 2 quizzes

📚 HIA International - Grade 8 Hub / Topical Revision
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Lesson summary

Where does energy go? — HIA INTERNATIONAL notes: Explain useful and wasted energy, dissipation and efficiency.Every use — when energy is used to make something happen, some is useful and some is wasted.Motorcycle — about 25% of the chemical energy in the fuel becomes kinetic ener…

  • Dissipated energy
  • Thermal energy
  • Not collectable
  • Even when you want heat
  • Efficient lamp
  • Where the energy goes

Part 1 of 6Useful and wasted energy

1Every use
  • Every use when energy is used to make something happen, some is useful and some is wasted.
2Motorcycle
  • Motorcycle about 25% of the chemical energy in the fuel becomes kinetic energy, and the other 75% is wasted as thermal and sound energy.
3Wasted energy
  • Wasted energy it is dissipated and cannot be recovered.
4Dissipated energy
Dissipated energy energy that spreads out where there is no use for it.

Part 2 of 6Dissipation

Thermal energy
  • Thermal energy it is always wasted in some amount when energy is changed or transferred.
Not collectable
  • Not collectable you cannot gather thermal energy and sound back into one place to be stored, changed or transferred.
Even when you want heat
  • Even when you want heat when heating water over a fire, thermal energy also heats the rocks, container and air, and escapes in steam.
Always
  • Always for everything that changes or transfers energy, some is always dissipated.

Part 3 of 6Comparing two lamps

LampEnergy changed to lightEnergy wasted as thermal
Lamp AAbout 15%About 85%
Lamp BAbout 50%About 50%
  • Efficient lamp
    • Lamp B wastes less energy for the same brightness.

Part 4 of 6Energy stores and dissipation

DirectionDescription
Becomes less usefulEnergy that spreads out
Becomes more usefulEnergy that can be used later
Not useful but storableEnergy that can be stored

Part 5 of 6Cooling of hot tea

1
Measure the temperature of the hot tea every minute and stir before each reading.
2
Use a spoon to stir, not the thermometer.
3
Plot time on the horizontal axis and temperature on the vertical axis.
4
Describe the pattern: the temperature falls quickly at first and then more slowly.
Where the energy goes
  • Where the energy goes into the cup, the air and the surroundings by conduction, convection, radiation and evaporation.
End point
  • End point the tea stops cooling when it reaches room temperature.

Part 6 of 6Other ideas

Ripple tank wave
  • Ripple tank wave the energy in a wave dissipates as it travels.
Distance
  • Distance a hot object feels less hot as you move away and the temperature falls with distance.
Joule’s machine
  • James Prescott Joule used a falling mass to turn a paddle in water and showed the water temperature rose.

Key terms

TermMeaning
Useful energyEnergy changed into the form we want.
Wasted energyEnergy that is not changed into the useful form.
Dissipated energyEnergy that spreads out where there is no use for it.
RecoveredCollected back for reuse.
JouleThe unit of energy.
Thermal energyHeat energy that is often wasted.

Avoid these mistakes

1
Wasted energy disappears — it is dissipated into the surroundings and cannot easily be used again.
2
Only machines waste energy — every energy change wastes some energy.
3
Efficient devices waste none — no device is 100% efficient.
4
Wasted energy is destroyed — energy is conserved but spreads out.

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