Heat Engine Efficiency
A heat engine absorbs 1000 J of heat from a hot reservoir and rejects 600 J to the cold reservoir in each cycle, so what is the efficiency of this engine?
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About This Question
- Subject
- physics
- Chapter
- thermodynamics
- Topic
- heat engine efficiency
- Difficulty
- Easy
- Year
- 2025
Solution
Correct Answer:
40 percent
Building on NCERT Class 11, Chapter 12 (Thermodynamics), the efficiency of a heat engine is defined as the ratio of useful work output to the heat absorbed from the hot reservoir, η=QHW, where by energy conservation W=QH−QC. Here QH=1000 J and QC=600 J, so the work output per cycle is W=1000−600=400 J. The efficiency is therefore η=1000400=0.40, or 40 percent. The option 60 percent is wrong because it mistakenly uses the rejected heat fraction QC/QH rather than the work fraction. The option 100 percent is wrong because it would violate the Second Law, which forbids converting all absorbed heat into work in a cyclic engine. The option 25 percent is wrong because it divides work by the rejected heat instead of by the absorbed heat. A plausibility check: efficiency must lie between 0 and 1 for any real engine, and 0<0.40<1 is consistent; also the heat rejected (600 J) exceeds the work (400 J), which is typical for low-efficiency engines.
This easy difficulty physics question is from the chapter thermodynamics, covering the topic of heat engine efficiency. It appeared in the 2025 exam.
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