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Post by Progenitor A on Mar 20, 2011 19:15:26 GMT 1
The definition of a 'closed' thermodynamic system is , effectively , a system in which no energy can escape and no energy can enter
Consider a sphere containing a heat source. The sphere is a perfect 'insulator' and no energy can enter the sphere from outside the sphere and no energy can escape from the sphere.
So the total energy within the sphere remains constant
The sphere is 'closed' thermodynamically
Now, if the energy within the sphere does not remain constant then by dfeinition , we do not have a 'closed' thermodynamic system
If a thermodynamic system is not closed, then , again by definition, it is open
Now consider the Universe
Does the total energy of the Universe remain constant?
If the answer is 'yes' then the Universe is a closed thermodynamic system
If the total energy of the Universe does not remain constant, then by definition the Universe is an open thermodynamic system
Is the Universe 'open' or closed thermodynamically/
Do we actually know?
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Post by eamonnshute on Mar 20, 2011 20:16:47 GMT 1
Does the total energy of the Universe remain constant? Yes, it is always zero because of the Law of Conservation of Energy. |
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Post by mak2 on Mar 20, 2011 20:19:22 GMT 1
It is an interesting question because the Second Law of Thermodynamics ( increasing entropy ) states that it only applies to closed systems.
It is a big jump from relatively small isolated systems, where the law undoubtedly applies, to the whole universe. If the universe is finite it probably can be regarded as a closed system. On the other hand, if the universe is infinite, it is not closed.
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Post by Progenitor A on Mar 21, 2011 8:15:58 GMT 1
Does the total energy of the Universe remain constant? Yes, it is always zero because of the Law of Conservation of Energy. That would seem good sense, but the conservation of energy implies that energy cannot be detroyed and cannot be created. I do not know if there are examples of energy being destroyed, but there is evidence (if Cosmologists are to be believed)of energy being created. We, the Universe, would not be here had not the energy of the Universe been created out of 'nothing', they say. And also physicists tell us that new energy is created all the time in the Universe so that the energy per unit Cubic metre of space remains constant as space expands. Now, if new energy is created in the universe, then by the definition of a closed thermodynamic system, the Univers is not closed, but open. The total energy of the Universe is not constant (according to this theory of continuous energy creation) but is increasing. |
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Post by Progenitor A on Mar 21, 2011 8:21:14 GMT 1
It is an interesting question because the Second Law of Thermodynamics ( increasing entropy ) states that it only applies to closed systems. But it is patently obvious, surely, that thermodynamic entropy applies equally in closed and open thermodynamic systems? It is nonsensical to suggest that no energy is 'wasted' in open thermodynamic systems; it is simply the case that new energy added to the system masks the effects of entropy, not that entropy does not aplly! |
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Post by mak2 on Mar 21, 2011 11:29:46 GMT 1
What I am saying, is that the second law of thermodynamics applies only to closed systems.
Clearly, open systems have entropy but, in some circumstances, it can decrease. That is, the system can become more ordered.
Applying the law of increasing disorder to the whole universe has always seemed a bit dubious to me, because it is not clear that the universe can be treated as a closed system.
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Post by speakertoanimals on Mar 21, 2011 13:27:39 GMT 1
Just to rain on this parade -- the law of conservation of energy becomes a bit dodgy in some sense when considering the entire universe! Why? Because the conservation of energy (or momentum, or angular momentum) apply only because we have certain symmetries of the laws of physics. In detail, energy because laws of physics don't depend on time, momentum because they don't depend on position in space, and angular because they don't depend on orientation. Hence strictly speaking, these conservation laws are only local, under conditions where the above can be applied. Which all boils down to the fact that in relativity, defining the energy of spacetime itself can be a bit problematic! math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html |
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Post by Progenitor A on Mar 21, 2011 13:54:39 GMT 1
What I am saying, is that the second law of thermodynamics applies only to closed systems. But I am saying that is not the case. In both systems, energy is 'wasted' and disperses as useless. In a closed system order can increase locally, as long as the total entropy is not complete. For if we do not have complete entropy in a closed system, we can perform work, and performing work is the essence of disorder to order Take, for example a closed system that has lots of available energy, and we use that energy to convert a pile of sand to a sandcastle. We have created order oiut of disorder I know that parallel to that activity we have increased thermal disorder, but that is the puzzle of the 2nd Law, In a closed system thermal disorder always increases but disorder as far as the organisation of molecules as such such can move fom the disordered to the ordered. Applying the law of increasing disorder to the whole universe has always seemed a bit dubious to me, because it is not clear that the universe can be treated as a closed system. Quite. We see order arisng out of disorder all around us! |
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Post by speakertoanimals on Mar 22, 2011 3:51:21 GMT 1
We see order arisng out of disorder all around us! .............
Only if you neglect the other controbutions to entropy from the heat generated by gravitational collapse, which gives a net increase of energy, even if matter in chunks looks tidier than strewn all over the place. WHY do you keep only doing half the job, and keep trying to maintain that there is more than one sort of entropy, or that the second law isn't quite true..............
Matter collapsing under gravity, rather than being more ordered as you keep implying, is actually, at the end, the greatest generator of random energy (heat ) that we have. Matter collpases under gravity,and converts useful gravitational potential energy to heat. Stars then burn hydrogen to helium and heavier elements, converting more useful nuclear energy to heat. And finally, black holes convert the whole shebang into random Hawking radiation. where's your order then?
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