Questions: Efficiency: Useful Energy Out vs. Total Energy In

Question 1

A motor uses 500 J of electrical energy and produces 350 J of useful mechanical work. What is its efficiency?

• A. 50%
• B. 70%
• C. 85%
• D. 35%

Answer: (B)

Efficiency = (useful output / total input) × 100% = (350/500) × 100% = 70%.

Question 2

It is possible to build a machine that is 100% efficient.

• T. True
• F. False

Answer: False

In any real machine, some energy is inevitably converted to heat through friction and other processes. No machine can convert 100% of input energy into useful output.

Question 3

An incandescent bulb converts only 5% of electrical energy into light. Where does the other 95% go?

Model answer: The remaining 95% is converted to heat (thermal energy). The energy is not lost — it is just not in a useful form for lighting.

Conservation of energy means all 100% must go somewhere. In an incandescent bulb, the filament gets extremely hot, radiating mostly infrared (heat) rather than visible light.