I was studying the second law of thermodynamics and there was written "It implies that the universe will end in a 'heat death' in which everything is at the same temperature. This is the ultimate level of disorder; if everything is at same temperature no work can be done, and all the energy will end up as the random motion of atoms and molecules". Can anyone explain this more clearly?
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$\begingroup$ What is the use of such a question that will never happen ? You could also ask what will happen in the Universe if the Sun and the stars do not exist ? $\endgroup$– MauriceCommented Jan 28 at 17:53
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3$\begingroup$ To run a heat engine you need a temperature difference. A hot and a cold side. No temperature difference means no work can be done. $\endgroup$– Jon CusterCommented Jan 28 at 18:06
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$\begingroup$ @Maurice en.wikipedia.org/wiki/Einstein%27s_thought_experiments are of no use, for example? I would argue the application of scientific methodology to hypothetical situations is fundamental to growing one understanding of the subject. $\endgroup$– Ian BushCommented Jan 28 at 18:18
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$\begingroup$ @JonCuster That is a very good observation, power generation from thermodynamic cycles would be impossible. However, I don't think that work will disappear (as the OP's book suggests). Water can fall by nature like in dams, and we can use it to generate electric energy. Other ideas are possible. Maybe not everything is lost in this future... $\endgroup$– Metal StormCommented Jan 28 at 20:23
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$\begingroup$ Additionally, if matter flows naturally across the earth, friction will always play a part and there will be temperature differences, even though they are very small. E.g., some potential energy at the top of the dam will be converted as heat once it hits the bottom, and molecules will suffer a temperature increase once they absorb the heat. $\endgroup$– Metal StormCommented Jan 28 at 20:36
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2 Answers
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Implications get everyone in trouble. The Second Law states that a system at equilibrium has a constant Free Energy and can do no work. The Laws state what happens under certain constraints or conditions, but they are oblivious to the mechanism of maintaining those conditions. The Third Law might be an exception it purports to be absolute starting at absolute zero, while sidestepping zero point energy. The Earth resides in an almost steady state of gravity and radiation. Thankfully it is nowhere close to equilibrium altho we are making a rash attempt with Global Warming. There is difficulty in predicting what happens in a simple situation here on Earth [What happens if a dead battery ostensibly at thermodynamic equilibrium, E = 0 is left too long in a device] so what hope is there to predict the fate of the Universe. It is nice to study the Universe and ponder the fate of the Universe; it is wise to consider our own fight against equilibrium and to use our own steady state wisely.
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No work can be done by a system that is in a thermodynamic equilibrium.
It consists from several partial equilibrii:
- mechanical equilibrium
- chemical equilibrium
- radiative equilibrium
- thermal equilibrium.
The above textbook quote limits itself just to thermal equilibrium, that is required but not sufficient condition for thermodynamic equilibrium. At the thermal equilibrium, work cannot be done by heat engines using thermal balancing of reservoirs of different temperature. The typical example is the closed medium Stirling engine, doing exactly that - providing work using temperature difference.
But, if some of other three conditions are not satisfied, there is still possible of a work done. Note that universe thermal equilibrium is reachable in mathematically finite, but practically infinite time.
