Question

The efficiency of a heat engine can never be

• A. $10\%$
• B. $20\%$
• C. $40\%$
• D. $100\%$

Answer 1

The correct option is D

$100\%$

1. A heat engine is a device that converts heat into work.
2. The efficiency of a heat engine is a measure of how efficiently the engine can utilize its input energy and converts it into useful work done or the output energy.
3. For a heat engine to have $100\%$efficiency, it must convert all the heat into useful work or mechanical energy.
4. But no engine (or machine) can convert all of its input energy into work.
5. Some amount of work is evidently spent (or lost) in overcoming friction as soon as the different parts of the engine start moving.
6. In moving systems, it can be observed very clearly that some of the mechanical energy is converted into heat, sound, etc.
7. So the output of a steam engine is always less than the input.
8. Therefore, the efficiency can never be 100%.

Hence, option D) is correct.