A device consumes 120,000 J of energy and delivers 90,000 J of useful work. What is the efficiency?

• A. 60%
• B. 75%
• C. 50%
• D. 30%

Answer: (A)

Efficiency is the ratio of useful work produced to the energy put in, expressed as a percentage. It shows how well the device converts input energy into the desired work.

Here, the device delivers 90,000 J of useful work from 120,000 J of energy input. So the efficiency is 90,000 J ÷ 120,000 J = 0.75, which is 75%. This means about a quarter of the input energy is lost to other forms (like heat, friction, or sound) rather than being used as useful work.