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3. Conservation of Energy(Law of Conservation of Energy Energy Efficiency (qualitative) Energy Losses and Useful Energy )
Year 8 • Physics
Conservation of Energy
This lesson explains the law of conservation of energy, energy efficiency, useful energy, and energy losses using real-life examples suitable for Year 8 UK (KS3).
Lesson Objectives
- State the law of conservation of energy.
- Explain what is meant by useful and wasted energy.
- Understand energy losses in everyday systems.
- Describe energy efficiency in a qualitative way.
1. Law of Conservation of Energy
The law of conservation of energy states that energy cannot be created or destroyed.
It can only be transferred from one energy store to another.
Key Points:
• Total energy before a change = total energy after the change
• Energy only changes form or store
• Energy is always conserved in a closed system
• Total energy before a change = total energy after the change
• Energy only changes form or store
• Energy is always conserved in a closed system
2. Useful Energy
Useful energy is the energy transferred in the way we want and helps the device do its job.
Examples:
• Light energy from a lamp
• Kinetic energy from a moving car
• Thermal energy heating water in a kettle
• Light energy from a lamp
• Kinetic energy from a moving car
• Thermal energy heating water in a kettle
3. Energy Losses
Not all energy transfers are useful. Some energy is transferred to the surroundings and is called wasted energy.
Common Energy Losses:
• Thermal energy (heat to surroundings)
• Sound energy (noise)
• Light energy escaping where it is not needed
• Thermal energy (heat to surroundings)
• Sound energy (noise)
• Light energy escaping where it is not needed
4. Energy Efficiency (Qualitative)
Energy efficiency describes how much of the input energy is converted into useful energy.
• A device is more efficient if it wastes less energy
• No device is 100% efficient
• Reducing energy losses increases efficiency
• No device is 100% efficient
• Reducing energy losses increases efficiency
5. Improving Energy Efficiency
• Lubrication reduces friction
• Insulation reduces heat loss
• Streamlined shapes reduce air resistance
• Turning off unused devices saves energy
• Insulation reduces heat loss
• Streamlined shapes reduce air resistance
• Turning off unused devices saves energy
Practice Questions
A. Fill in the Blanks
- Energy cannot be created or __________.
- Energy that helps a device work is called __________ energy.
- Energy lost to the surroundings is called __________ energy.
- A more efficient device wastes __________ energy.
- Heat loss can be reduced using __________.
B. True or False
- Energy can disappear in a system.
- No device is 100% efficient.
- Sound is always useful energy.
- Insulation reduces thermal energy loss.
- Efficient devices save energy.
C. Short Answer
- State the law of conservation of energy.
- What is meant by useful energy?
- Give two examples of wasted energy.
- Why is some energy always wasted?
✅ Show Answer Key
A. Fill in the Blanks
- destroyed
- useful
- wasted
- less
- insulation
B. True or False
- False
- True
- False
- True
- True
C. Short Answer
- Energy cannot be created or destroyed, only transferred or stored.
- Energy transferred in the way we want.
- Heat and sound (any two).
- Because energy spreads to the surroundings.
© Aviate Learning – Conservation of Energy (Year 8)