Physics / Unit 5: Work, Energy and Power
5.6 Efficiency
Notes
No real machine turns all its input energy into useful output; some becomes wasted heat or sound.
- Efficiency $\displaystyle = \frac{\text{useful energy out}}{\text{total energy in}} \times 100\%$ (or use power instead of energy).
- Efficiency is always less than 100% for real machines.
- Example: an LED bulb uses 10 J of electrical energy and gives out 4 J of light. Efficiency $\displaystyle = \frac{4}{10} = 40\%$.
Practice
A motor takes in 500 J of electrical energy and does 400 J of useful work. What is its efficiency?
$\displaystyle \frac{400}{500} = 0.80 = 80\%$.
A machine claims to be 110% efficient. What is wrong with this claim?
Output can never exceed input energy.
A 25% efficient engine must deliver 3,000 J of useful work. How much fuel energy must it use?
Input $\displaystyle = \frac{3000}{0.25} = 12000 \text{ J}$.
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