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### ΔS(universe)

Posted: Sat Feb 03, 2018 5:11 pm
Can ΔS(universe) decrease? If so, what does that mean?

### Re: ΔS(universe)

Posted: Sat Feb 03, 2018 5:14 pm
I think the entropy of the universe can only increase. I'm not sure how to explain it, but I know it can't decrease.

### Re: ΔS(universe)

Posted: Sat Feb 03, 2018 5:39 pm
Hi,

So delta S according to the Second Law of Thermodynamics will always be increasing in the universe. Delta S of the system can decrease, in other words, if you freeze liquid water to ice, you are reducing the possible number of arrangements (microstates) that the H20 molecule can be in. So delta S of that system is decreasing. However, the environment around us is so large that the decrease in entropy of that system would not outweigh the delta S of the surroundings, where usually delta S increases. So that's partly why overall, the delta S in the universe is always increasing.

I would think back to the analogy that Dr. Lavelle mentioned about lighting a match outside and/or if dropping few water droplets in the ocean will really change the volume of the ocean...I think those analogies help put the Second Law of Thermodynamics into perspective.

Hope that helps!

### Re: ΔS(universe)

Posted: Sat Feb 03, 2018 5:54 pm
This is a really helpful explanation! The analogy that Lavelle really helps to provide an example as to why the Second Law of Thermodynamics proves to be true. Compared to the universe, the entropy observed in a system will always be too small to have any notable effect on the universe's entropy, and in the universe, entropy is always increasing.

### Re: ΔS(universe)

Posted: Sat Feb 03, 2018 8:30 pm
The change in entropy of the universe is the same always.

### Re: ΔS(universe)

Posted: Sat Feb 03, 2018 9:11 pm
snehabhargava wrote:The change in entropy of the universe is the same always.

Wait how can this be true? I think my TA said that delta S can be zero for reactions at equilibrium or something like that?

### Re: ΔS(universe)

Posted: Sun Feb 04, 2018 12:11 am
Delta S can only be zero in an equilibrium, one way to think about that could be that disorder and order are equal. Likewise, entropy in the universe is always increasing.