Can we consider the air condition as reversing entropy? what are the other process like that?
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- 7$\begingroup$ what do you mean by "reversing entropy"? $\endgroup$hyportnex– hyportnex2025-10-24 13:28:09 +00:00Commented Oct 24 at 13:28
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2 Answers
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1 An air conditioner always increases entropy. The principle function of the AC, expressed in terms of entropy, is that when it is set to a temperature lower than the measured temperature, it moves entropy outside.
Doing this requires mechanical work, and there are no perfectly efficient machines, so running the AC unit increases total system entropy. However there is no requirement that the entropy increase from inefficiency either at the AC unit or at the power source must be larger than the entropy decrease associated with the drop in the interior temperature. That entropy decrease has already been paid for by the increase in entropy in the air blown past the heat rejection coils outside.
If the exterior temperature is colder than the interior temperature, we could in principle build an air conditioner unit that worked as a heat engine and would produce electrical power while still slowly reducing the difference between exterior and interior temperature and hence cooling the interior while decreasing the system's thermodynamic free energy. It would still increase entropy at the AC unit - part of the power would have to go to overcoming mechanical inefficiencies.
- 1$\begingroup$ It requires work, but not necessarily mechanical work. $\endgroup$kangermu– kangermu2025-10-24 19:24:16 +00:00Commented Oct 24 at 19:24
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2 If "reversing entropy" means "decreasing" entropy, then it is true for the room (system) but not for the universe. The energy required for running A/C is generated from coal, gas etc in an electric power plant, transported through a wire, converted to other forms of energy from electrical energy through the machines in A/C and the net increase in entropy in all those processes is greater than the decrease in case of room.
- 2$\begingroup$ This answer is somewhat misleading, as an air conditioner powered by a reversible engine (that produces no entropy) would still not decrease total entropy. The key aspect is that the entropy removed from the room is moved directly outside the room, and additional entropy is also generated. This all occurs at the location of the air conditioner, not the location of the power source. $\endgroup$Chemomechanics– Chemomechanics2025-10-24 17:07:36 +00:00Commented Oct 24 at 17:07
- $\begingroup$ @Chemomechanics I was considering real world scenario so did not consider reversible engine. However, I have edited the answer to include some more possible sources which increase entropy. $\endgroup$M. Saamin Rahman– M. Saamin Rahman2025-10-25 04:34:59 +00:00Commented Oct 25 at 4:34