Heat Pump And Energy Transfer
A heat pump uses 200 J of work to deliver a total of 800 J of heat into a warm room during each operating cycle, so how much heat does it extract from the cold outside air?
Select the correct option:
Solution
600 J
As presented in NCERT Class 11, Chapter 12 (Thermodynamics), a heat pump is a device that transfers heat from a colder region to a warmer region using work input, and it obeys energy conservation in the form QH=QC+W. Here the heat delivered to the warm room is QH=800 J and the work input is W=200 J, so the heat extracted from the cold outside air is QC=QH−W=800−200=600 J. The option 800 J is wrong because that is the total heat delivered, which already includes the work converted to heat. The option 1000 J is wrong because it adds the work to the delivered heat instead of subtracting, violating the energy balance. The option 400 J is wrong because it subtracts twice the work or otherwise mishandles the conservation equation. A plausibility check: the heat delivered to the warm side must always exceed the heat drawn from the cold side by exactly the work input, and indeed 800=600+200 balances perfectly, confirming both the sign convention and the magnitude.
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About This Question
- Subject
- physics
- Chapter
- thermodynamics
- Topic
- heat pump and energy transfer
- Difficulty
- Easy
- Year
- 2025
Solution
Correct Answer:
600 J
As presented in NCERT Class 11, Chapter 12 (Thermodynamics), a heat pump is a device that transfers heat from a colder region to a warmer region using work input, and it obeys energy conservation in the form QH=QC+W. Here the heat delivered to the warm room is QH=800 J and the work input is W=200 J, so the heat extracted from the cold outside air is QC=QH−W=800−200=600 J. The option 800 J is wrong because that is the total heat delivered, which already includes the work converted to heat. The option 1000 J is wrong because it adds the work to the delivered heat instead of subtracting, violating the energy balance. The option 400 J is wrong because it subtracts twice the work or otherwise mishandles the conservation equation. A plausibility check: the heat delivered to the warm side must always exceed the heat drawn from the cold side by exactly the work input, and indeed 800=600+200 balances perfectly, confirming both the sign convention and the magnitude.
This easy difficulty physics question is from the chapter thermodynamics, covering the topic of heat pump and energy transfer. It appeared in the 2025 exam.
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