Unit 3: Forces and energy
Conservation Of Energy — HIA International - Grade 8 Revision Notes & Topical Questions
5 min read 8 parts Lesson 11 of 16 2 quizzes
Lesson summary
Conservation of energy — HIA INTERNATIONAL notes: State the law of conservation of energy and use energy diagrams to find missing energy values.Meaning — the total quantity of energy stays the same when energy is stored, changed, transferred or dissipated.Not the same as saving —…
- Created and destroyed
- Missing values
- Sankey diagrams
- Efficiency
- Bottle racer
- Energy input
Section 1 of 8Energy is conserved
1Meaning
- Meaning the total quantity of energy stays the same when energy is stored, changed, transferred or dissipated.
2Not the same as saving
- Not the same as saving saying energy is conserved is different from using less energy.
3Law of conservation of energy
- Law of conservation of energy energy cannot be created or destroyed, only changed from one form to another.
4Created and destroyed
- Created and destroyed created means made or brought into existence, and destroyed means damaged or made to cease to exist.
Section 2 of 8Energy diagrams
Energy diagrams
Input
Input the energy supplied to a system, e.g. electrical energy to a lamp.
Output
Output the energy that comes out: useful and wasted energy together.
Total
- Total the total energy output equals the total energy input and can never be greater.
System
System something chosen to study, especially for energy changes.
Section 3 of 8Examples
| System | Energy input | Energy outputs |
|---|---|---|
| Electric lamp | 100 J electrical | 10 J light and 90 J thermal |
| Car engine | 100% chemical energy in fuel | 25% kinetic, 35% thermal and sound, 40% exhaust gas |
| Wood fire | Chemical energy in wood | 70% thermal, the rest light and waste chemicals |
Section 4 of 8Useful and wasted energy
Useful
- Useful light from a lamp.
Wasted
- Wasted thermal energy spreads out into the surroundings and becomes less useful.
Dissipated
- Dissipated wasted energy is dissipated but not destroyed.
Missing values
- Missing values subtract the known outputs from the input, e.g. 1000 J input and 500 J kinetic leaves 500 J thermal.
Section 5 of 8Energy stores and transfers
Stores
- Stores kinetic, gravitational potential, elastic, chemical, thermal and nuclear.
Transfers
- Transfers by heating, forces, electric current and waves such as light and sound.
Sankey diagrams
- Sankey diagrams arrow width is proportional to the amount of energy.
Efficiency
- Efficiency useful energy output ÷ total energy input × 100%.
Section 6 of 8Observing conservation
1
Fill two identical bottles with water and hang them 40–50 cm apart from a string.
2
Pull one bottle sideways and release it.
3
Watch the second bottle begin to swing as energy transfers to it.
4
The bottles eventually stop because energy is dissipated by friction and air resistance.
- Bottle racer
- twisting an elastic band stores elastic potential energy, which is transferred to kinetic energy.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Law of conservation of energy | Energy cannot be created or destroyed, only changed from one form to another. |
| Energy input | The energy supplied to a system. |
| Energy output | The useful and wasted energy that comes out of a system. |
| Dissipated | Spread out into the surroundings and less useful. |
| System | Something chosen to study, especially in terms of energy changes. |
| Efficiency | Useful energy output divided by total energy input. |
Section 8 of 8Exam technique
1
Conserving energy means using less energy — in physics it means the total quantity stays the same.
2
Output can be greater than input — total output equals total input.
3
Wasted energy is destroyed — it is dissipated into the surroundings.
4
Wasted energy has gone — it has spread out and cannot easily be recovered.
Marked practiceQuizzes for this lesson
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