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The following excerpt comes from the physics textbook Understanding Physics, by David Cassidy, Gerald Holton, and James Rutherford:

A ball lying on the floor will not somehow gather energy from its surroundings and suddenly leap up. An egg will not unscramble itself. An ocean liner cannot be powered by an engine that takes heat from the ocean water and ejects ice cubes.


I'm currently reading about entropy and that no closed system has a negative entropy, and in reality, nor does it have an entropy of 0.


I understand the first 2 examples, but the 3rd peeves me.

Why can't ocean liner be powered by an engine that takes heat from the ocean water and eject ice cubes? The water is has more heat (energy) than ice cubes, so it makes some sense to me that the differential of heat can be of some use. Of course, this can't be true, because I've never heard of such an ocean liner. How come?


If you could make the explanation as if you were Feynman explaining it to a layman, :)

that would be great, as I'm a high school freshman, and Understanding Physics is not a college textbook.

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  • $\begingroup$ These are very sloppy examples of the second law of thermodynamics in the Clausius formulation, which basically says that heat only flows from hot to cold, unless something else happens. The latter is simply an observation. Nobody has ever observed heat flow against a temperature gradient unless there was a second physical process that caused it (which is what the electric motor/compressor in a fridge does all day). That an ocean liner can't run on heat from seawater by making ice cubes is simply based on the observation that nobody has ever seen such a machine (or any of its equivalents). $\endgroup$ – CuriousOne Apr 18 '16 at 19:55
  • $\begingroup$ @CuriousOne -- Oh. So, is it possible to make such an ocean liner? $\endgroup$ – Fine Man Apr 18 '16 at 19:57
  • $\begingroup$ :-) Nothing stops you from trying (except for common sense, that is). The point is that the second law of thermodynamics reflects the experience that nobody has been able to find physical processes that violate it, which reflects the scientific method: science observes and then derives descriptions. We don't have postulates around here. Laws are simply short hand notations that summarize large classes of unchallenged evidence. Find one counterexample and the law is gone. In this case you would, in my honest opinion, be wasting your time, though. $\endgroup$ – CuriousOne Apr 18 '16 at 20:01
  • $\begingroup$ @CuriousOne -- Hmm... I'm now a bit more confused than when I ask the question (darn entropy :-]). If a fridge can do a similar process, why can't an ocean liner? Also, what makes such an ocean liner different than one that uses heated water that ejects steam? $\endgroup$ – Fine Man Apr 18 '16 at 20:06
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    $\begingroup$ Nothing stops you from putting a nuclear reactor on the ship to propel if forward and that reactor will have to use sea water for its cooling system. Why you would also want to make large amounts of ice cubes along the way would remain a mystery, of course. $\endgroup$ – CuriousOne Apr 18 '16 at 20:14
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Simple-Every physical or chemical process always aims at decreasing the energy and increasing the entropy(randomness of things).Lets take some examples When an object falls on the ground then it would break and result in a large number of pieces completely scattered(randomness) thereby increasing entropy.The pieces would never come together to make the object because this would result in decreasing entropy which is against the law.In your example if the water is converted into ice there would be decrease in entropy because ice is more structured than water.So it never happens.Hope you get this.

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  • $\begingroup$ Careful there. Poincare is about to come down with a vengeance and a recurrence theorem on you. :-) $\endgroup$ – CuriousOne Apr 18 '16 at 20:07
  • $\begingroup$ But I need one tiny clarification: What makes cool water more "entropic" (<-- tell me if this is incorrect terminology) than even cooler ice (aside from the crystalline structure of such solid)? I assume the lack of such clarification is why @CuriousOne has corrected you. :-] $\endgroup$ – Fine Man Apr 18 '16 at 20:10
  • $\begingroup$ @SirJony: It's an inside joke. Every physicist past the second semester or so has heard about the Poincare recurrence theorem and knows why it doesn't matter. $\endgroup$ – CuriousOne Apr 18 '16 at 20:12
  • $\begingroup$ @CuriousOne -- It also shows my lack of knowledge of physics, as of today. :-] Oops. So, is WhiteKnight correct in his explanation? $\endgroup$ – Fine Man Apr 18 '16 at 20:14
  • $\begingroup$ @SirJony: And so is Poincare. Closed systems will, eventually, violate the second law by fluctuations. The second law simply doesn't talk about fluctuations, so it's an incomplete description of nature. However, and this is a big however, Poincare only violates it substantially on extremely long time scales and only for perfectly isolated systems, both of which belong into the realm of the fantastic. They do not represent actual physical experience for macroscopic systems. $\endgroup$ – CuriousOne Apr 18 '16 at 20:17

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