Overview
This lecture explains the concept of energy efficiency, how to calculate it using energy or power, and discusses why devices are never 100% efficient.
What is Efficiency?
- Energy can be converted from one form to another, but not all energy becomes useful output.
- Efficiency is the proportion of input energy that is transferred into useful output energy.
- Devices often lose energy as waste, commonly as heat.
Calculating Efficiency
- Use the formula: Efficiency = Useful Energy Output / Total Energy Input.
- For power: Efficiency = Useful Power Output / Total Power Input.
- Efficiency can be expressed as a decimal or percentage (multiply decimal by 100).
Efficiency Example: Lamps
- Incandescent bulb: 45 J light output from 300 J input; efficiency = 0.15 or 15%.
- LED bulb: 225 J light output from 300 J input; efficiency = 0.75 or 75%.
- LED lamp is 5 times more efficient than the incandescent (0.75 / 0.15 = 5 or 75% / 15% = 5).
Common Mistakes & Conservation of Energy
- Do not invert the efficiency equation; this would give a value >1 or >100%, which is impossible.
- Efficiency cannot exceed 100% due to the conservation of energy principle.
Efficiency with Power Example
- Microwave is 70% efficient with 800 W input.
- Convert percentage to decimal: 70% = 0.7.
- Useful power output = 0.7 × 800 W = 560 W.
Special Case: Heaters
- Devices designed to produce heat (e.g., electric heaters) treat thermal energy as useful output, not waste.
Key Terms & Definitions
- Efficiency — The ratio of useful output to total input energy or power.
- Useful Energy Output — The energy transferred in the desired form (e.g., light from a lamp).
- Waste Energy — Energy output that is not useful, often as heat.
- Conservation of Energy — Principle stating energy cannot be created or destroyed, only transferred.
- Power — The rate at which energy is transferred or converted, measured in watts (W).
Action Items / Next Steps
- Practice calculating efficiency for different devices using both energy and power.
- Review the principle of conservation of energy for better understanding.