Entropy Decrease: Sample Phenomena Explained

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Entropy can decrease locally if compensated by an increase elsewhere, maintaining a positive total change in an isolated system. An example is an air conditioner, which lowers the entropy in a room while increasing it in the external environment. Cooling a subsystem, like water, can decrease its entropy, but this requires work that raises entropy in another part of the system. Some argue that spontaneous decreases in entropy can occur, particularly when a hot object cools, but this does not violate the Second Law of Thermodynamics, which states that the total entropy of the universe cannot decrease spontaneously. Overall, the discussion emphasizes the balance of entropy changes within thermodynamic systems.
Domenicaccio
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Sorry but I suck at thermodynamics...

Can entropy decrease LOCALLY, provided that the decrease is compensated by an increase in the rest of the universe (or whatever isolated system we are in), so that the total change of entropy is positive?

Can you provide a sample phenomenon where this happens?
 
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Yes, entropy can decrease locally at the expense of increased entropy elsewhere. An everyday example is an air conditioner.
 
temporarily yes...
take a room filled with gas.
the density in any small given sample volume of the room will have varying number of molecules, thus varying randomess, or entropy.
However the overall entropy of the isolated system (here, room) will be constant.
 
Yes- nearly all of biology/biochemistry works using this principle.
 
D H said:
Yes, entropy can decrease locally at the expense of increased entropy elsewhere. An everyday example is an air conditioner.

How easily can you guarantee that the entropy of a certain subsytem (e.g. an amount of water in a container) will be decreased by COOLING it?

(Obviously, the refrigerator would cause an entropy increase somewhere else)
 
to decrease entropy you must apply work, this work will coase an increase of entropy somewhere else. But I do not think there is an example of entropy decreasing spontaneously, unles you go back to the beginning of life (for which we cannot say exactly what happened)
 
bilha nissenson said:
to decrease entropy you must apply work, this work will coase an increase of entropy somewhere else. But I do not think there is an example of entropy decreasing spontaneously, unles you go back to the beginning of life (for which we cannot say exactly what happened)

This is completely wrong. Whenever a hot object cools, its entropy decreases. No work is necessary. There is no problem with the entropy of an object or a system decreasing spontaneously. What the Second Law forbids is the tendency for the total entropy in the universe to decrease spontaneously.
 

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