Overview
This lecture covers how to calculate the efficiency of energy transfers and ways to increase efficiency, focusing on practical examples with light bulbs and heating water.
Calculating Efficiency
- Efficiency shows what fraction of input energy is usefully transferred by a device.
- Use the formula: efficiency = useful output energy transfer ÷ total input energy transfer.
- Alternatively, efficiency = useful power output ÷ total power input.
- These formulas are not provided in exams; memorization is required.
- Efficiency can be expressed as a decimal or as a percentage (multiply decimal by 100).
- A result above 1 (or 100%) is not possible and means the calculation was done incorrectly.
Worked Examples
- Example 1: Electric hob transfers 12,000 J of thermal energy to water from 15,000 J input; efficiency = 0.8 or 80%.
- Example 2: Light bulb has 3 W useful output from 60 W input; efficiency = 0.05 or 5%.
Improving Efficiency of Energy Transfers
- Energy is often lost as unwanted thermal energy (e.g., heat to the air around a pan).
- Use a pan with a wider base and a lid to reduce heat loss.
- Place the heating element directly in the water (like in an electric kettle) to minimize energy lost to heating the container.
- Materials with low thermal conductivity (like plastic) reduce unwanted heat transfer compared to metal.
Key Terms & Definitions
- Efficiency — The ratio of useful energy output to total energy input.
- Useful energy output — The form of energy that is intended and beneficial for the task.
- Thermal conductivity — A measure of how well a material conducts heat.
Action Items / Next Steps
- Memorize both efficiency formulas.
- Practice efficiency calculations with example questions.
- (Higher tier) Review methods to increase efficiency in energy transfer systems.
- Read ahead on thermal conductivity for future lessons.