The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time. However, if the drop of water falls into water, splashes decrease and disappear over time, which looks like a decrease in entropy. How does it comply with the second law?

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    $\begingroup$ A drop of water is not an isolated system once it falls into a larger pool. Also, how are you certain you know what a decrease in entropy "looks like"? $\endgroup$ – probably_someone Jun 3 '18 at 17:13
  • $\begingroup$ Can you reconstruct the waterdrop? By "reconstruct" I mean down to the exact atoms that made it by collecting them from the body of water. Let us suppose that in a hypothetical future we have the technology to do so, it would be significantly harder to do in comparison to the amount of effort required to let the drip fall in the water. This is what entopy is: effective irreversibility of a process. Another example would be trying to obtain a whole egg from an omlette $\endgroup$ – Jepsilon Jun 3 '18 at 19:51

It seems to me that the potential energy of the original drop is ultimately converted to internal energy of the pond water (including the original drop) and the surrounding air (and everything else surrounding). So the entropy of everything afterwards is a little higher than everything before, and roughly equal to the potential energy of the original drop divided by the absolute temperature of the pond, air, and greater surroundings. So, in this essentially large-scale isolated system, as expected, when a spontaneous process takes place, the entropy of the system increases.


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