Talk:Second law of thermodynamics/Archive 3

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Archive 1 2005—March 2006 Creationism discussion (Nov 2005—Feb 2006) Archive 2 March 2006—May 2008

No such thing as equilibrium under real conditions.

A few years ago I spent considerable time analyzing natural ambient temperature fluctuations on a macro scale by means of two thermistors and an appropriate amplifier. Each thermistors was ~ 1mm in length. Regardless of how well the system was insulated there were always thermal fluctuations. On a nanoscopic scale it is a violent world where particles are in random rapid motion. Regardless of sample duration, temperature in any closed system will vary from measurement to measurement. One interesting universal effect is 1/f noise (also referred to as Pink_noise, Occurrences) where the noise (in this case temperature fluctuations) is relative to the reciprocal of frequency. The 1/f noise alone prevents consistent measurements regardless of sample duration. Another interesting study in terms of 2LoT is the Universe where one region of space is cold while another is hot such as our Sun. One may suggest the Universe is not yet at equilibrium, but then such a question becomes meaningless when asked at what point in time would the Universe be at 100.0...0% equilibrium.--PaulLowrance (talk) 16:52, 12 July 2008 (UTC)

Please reserve this space for discussion of potential improvements to the article. General discussion of the topic itself is not appropriate. - Eldereft (cont.) 21:30, 12 July 2008 (UTC)

The obvious point is the article makes various references to equilibrium when such a state is impossible. The article should make that clear.--PaulLowrance (talk) 23:25, 12 July 2008 (UTC)

The article already says "the second law applies only to macroscopic systems with well-defined temperatures." That means that if you only view the "nanoscopic" as "reality", then thermodynamic equilibrium is not "real." It's just an oversimplified bulk generalization. However (fortunately, I think) human beings are larger than a millimeter shortly after conception, and we don't have the sensitivity of an amplified thermistor, so equilibrium makes a lot of sense to us out here in our larger world. It's a useful concept to us big people who read Wikipedia. Maybe you will be able to understand us someday, millimeter-man. Flying Jazz (talk) 13:36, 4 August 2008 (UTC)

I agree thermodynamic equilibrium is an oversimplified bulk generalization. On a scale of say the size of an apple objects appear to be stable as far as the human eye's concerned, but take for example Brownian motion on particles that the unaided human eye can barely detect such as a grain of pollen. Anyone with a good magnifying glass can see such particles jittering around on water at room temperatures, even inside the best isolated chambers. In fact it was such pollen that helped confirm the existence of the atom-- reference: Einstein's 1905 paper. Such Brownian motion that occurs at macro scales always exists due to the natural ambient thermal energy. My point is that thermodynamic equilibrium does not exist at any scale. The Universe is in constant change. Even the Earth's spin that causes daily temperature fluctuations between night and day will affect the best insulated systems to some degree, as it would require infinite insulation. It would nice if the wiki article included a section on how the mathematical thermodynamic equilibrium is an impossible state. --PaulLowrance (talk) 18:08, 8 August 2008 (UTC)

I agree with PaulLowrance. I know next to nothing about thermodynamics, but already reading his comments has confirmed some thoughts I had been formulating after reading the article. --203.55.211.33 (talk) 04:37, 7 January 2009 (UTC)

The Sun

The section concerning the sun is largely irrelevant to this article.67.163.246.108 (talk) 05:40, 15 July 2008 (UTC)

The example is cited fairly regularly in second law contexts (I think my thermodynamics course put it between solar flux received by Earth and before degenerate white dwarves). I believe that its purpose here is as a material demonstration that figuring out how the second law holds can be fairly subtle. - Eldereft (cont.) 10:28, 15 July 2008 (UTC)

In any case, the section does not describe how the second law holds. As it is written, it seems fairly irrelevant. I think this section should be deleted. Jacob2718 (talk) 14:20, 11 September 2008 (UTC)

Dubious

The figure of 1kW/m² of at the sun's surface is surely wrong. This is the approximate value of the sun's energy at the surface of the earth, not the sun. Jdpipe (talk) 05:12, 19 July 2008 (UTC)

Section on the Sun

I deleted the section discussing heat transport in the sun. At first glance, there are several situations where the second law appears not to hold (a refrigerator!) and I don't think this is the right place to discuss all of them. If the issue is historical i.e if this was historically proposed as a violation of the second law, maybe we can include that but only if the appropriate historical references are added. Jacob2718 (talk) 14:26, 11 September 2008 (UTC)

Entropy And Gravity

"In simple terms, the second law is an expression of the fact that over time, ignoring the effects of self-gravity, differences in temperature, pressure, and density tend to even out in a physical system that is isolated from the outside world. Entropy is a measure of how far along this evening-out process has progressed."

I have some queries concerning this statement : 1) What motivates the caveat whereby the effects of self-gravity must be ignored in order for the differences in temperature, pressure, and density to even out with time. I severely doubt that the author of this passage meant to imply this, but are we to take it that the effects of a self-gravitating system (where, hopefully, an example could be provided of such a self-gravitating system) somehow preserve the overall entropy of a system so that either the entropy of the system remains unchanged OR that the entropy may even be reversed?

Perhaps I should rephrase this question - how did the author envision that gravity interferes with the progression of entropy as per the Second Law of Thermodynamics? Surely, gravitation and self-gravitation should not be expected to alter whether the Second Law of Thermodynamics holds? If so, why include the phrase "ignoring the effects of self-gravity,"?

ConcernedScientist (talk) 11:17, 29 September 2008 (UTC)

Hmm, self-gravity will lead to differences in composition between non-miscible phases: Structure of the Earth is a good and well-known example. I don't like the phrase quoted, but I can't immediately see that its wrong. Physchim62 (talk) 11:34, 29 September 2008 (UTC)

The problem lies with the "simple terms", not with self-gravity. While the second law always holds true, the simplified formulation does not, as can be seen in many real (but maybe somewhat uncommon) systems. Take for example a crystal. The density is not uniformly distributed, but nevertheless a crystalline state can be the equilibrium state of a system. I propose to remove the reference to self-gravity and add some caveat in the form "Often (but not always) the second law can be seen as an expression of the fact [...]" Hweimer (talk) 12:46, 30 September 2008 (UTC)

I am a physicis (long time ago that I did thermodynamics in first year university though), but I wouldn't have the slightest clue about the self-gravity remark. Can there be at least a link to an article explaining it better, or move it to a section going into more detail. I wouldn't expect an opening remark to contain this kind of vague remarks to little known effects. —Preceding unsigned comment added by 85.18.14.0 (talk) 21:49, 2 October 2008 (UTC)

Consider a nebula, a vast cloud of cold dispersed gas in space. Assume that the system is in complete equilibrium, with gravity force which is holding the nebula together equal to gas pressure (thus, the gas doesn't expand or contract). If this system were isolated, it obviously has maximum entropy and nothing can happen in it anymore - but in reality, it's not isolated and even a slight shock wave (e.g. a nearby supernova exploding) can break the equilibrium, contract the gas and help gravity overcome gas pressure. The gas condenses and makes new stars, drastically reducing entropy of the system. Can someone explain me how this phenomenon corresponds with "second law of thermodynamics"? As I see it, either the law does not work, or there is no such thing as an "isolated system" in reality.

This article misrepresents the second law

"In a system, a process can occur only if it increases the total entropy of the universe."

The second law is a generlization, and a *tendency*. A process that does the above *can* occur. It is not the second law of thermodynamics. Putting these similar, but misleading quotations up will only lead the reader in the wrong direction. Comments? Fresheneesz (talk) 01:46, 5 October 2008 (UTC)

I don't see the problem. Just because a process can occur doesn't mean that it will occur in a given finite time period, but all processes which do occur lead to an increase in the entropy of the universe. Physchim62 (talk) 09:40, 5 October 2008 (UTC)

The problem is that processes that decrease the total entropy of the universe *can* spontaneously occur, and that is at odds with that sentence. Fresheneesz (talk) 10:11, 5 October 2008 (UTC)

Give me one example of a spontaneous decrease in the total entropy of the universe… Physchim62 (talk) 10:32, 5 October 2008 (UTC)

I'm not going to answer that because it is a statistical improbability. And that is the point. Entropy and the second law of thermodynamics are about the tendency toward more likely states of matter. Heat transfers to cold because of simple statistics - the hot particles are more likely to give more heat to nearby particles, than cold particles are. However, statistics (and the second law of thermodynamics) does not prohibit that statistically improbable things happen. In fact they happen all the time (in small amounts of course). Please read this explanation. Fresheneesz (talk) 06:37, 6 October 2008 (UTC)

The classic thought experiment is Maxwell's demon. In practice, any Maxwell's demon has to do work to separate the two systems so as to lower the entropy, so increasing the entropy elsewhere. As for the blog link you posted, firstly, evolution by natural selection does not require a decrease in entropy in any closed system (the systems described are neither closed nor decreasing in entropy). Secondly, if entropy did spontaneously fall in a closed system, we would never know about it: if we did, the system wouldn't be closed, and our act of measuring the supposedly lowered entropy in the system would increase the entropy of the surroundings! Physchim62 (talk) 08:23, 6 October 2008 (UTC)

Two examples. The first is equilibrium fluctuations of macroscopic quantities about their maximum entropy values. If from S = k ln W we infer S = k ln p + const., where p is the probability, we can invert that to give p ~ exp (S/k). Now consider the entropy function S(x) for some macroscopic variable x. The equilibrium value of x will be that which maximises S(x). Sufficiently close to this maximum, we can assume that S(x) will be quadratic, so p ~ exp (S(x)/k) will have a Gaussian bell shape. Hence x will typically not take the value which completely maximises S(x), but will fluctuate in a band of slightly lower entropy close to this value. For example, the local air pressure in part of a room will mostly be close to the average air pressure - this is the value of the local pressure which maximises the entropy. But there is a random chance that very slightly more molecules will be in that part of the room at a particular time - a pressure fluctuation, exploring a state of slightly lower entropy. Often, of course, the standard deviation of these fluctuations is very small (though calculable). But sometimes the fluctuations Δx can become really quite large compared to x, particularly at parameter values close to phase transitions -- see for example critical opalescence.

A second example is entropy fluctuations in non-equilibrium systems on their way to equilibrium. The system entropy will usually increase; but there is a calculable probability that due to a fluctuation it will actually fall, and the system will (temporarily) explore a further-from-equilibrium state. See fluctuation theorem for the formula. Such excursions away from entropy increase have actually been observed in sufficiently small mesoscopic systems - see eg G.M. Wang, E.M. Sevick, E. Mittag, D.J. Searles & Denis J. Evans (2002). "Experimental demonstration of violations of the Second Law of Thermodynamics for small systems and short time scales". Physical Review Letters 89: 050601/1–050601/4. doi:10.1103/PhysRevLett.89.050601. -- Jheald (talk) 09:45, 6 October 2008 (UTC)

Exactly Jheald. The point of my complaint is that entropy does not always fall, but because of statistical anomalies will not always decrease at the same rate and might even increase. It is a very nitpicky complaint, but one I think need to be addressed. If I'm correct in my thinking, the statistically improbable decreases in entropy will even more rarely cause equilibrium to be reached later in time. Fresheneesz (talk) 04:08, 7 October 2008 (UTC)

Applications to living systems

It is stated that:

However it is incorrect to apply the second law of thermodynamics to any system that can subjectively be deemed "complex".

Is this correct, in general? I would for instance expect a closed system to behave according to the second law of thermodynamics, even if it is complex. -- Crowsnest (talk) 12:27, 24 October 2008 (UTC)

Entropy?

Let me state just two strange things that, to me, seem to be implied by the second law of thermodynamics:

If I run either my (1) refrigerator or my (2) air-conditioning for an extended period of time (BILLIONS of years), what would happen? Entropy? Or does the law enable me to keep running my fridge and AC forever? 97.103.81.29 (talk) 18:16, 2 November 2008 (UTC)

Anything you do will increase the total entropy of the universe. In this case you would get an astronomical electric bill and help accelerate the heat death of the universe! --Itub (talk) 09:24, 4 November 2008 (UTC)

One consequence of the Second Law is that you can't run a refrigerator or air conditioning without an external power source: if you ran them for billions of years, your external power source would run out. On a related note, I was living in Paris during the 2003 European heat wave, when temperatures reached 44 ºC (112 ºF)… at the time, French newspapers ran commentaries on the Second Law, reminding people that keeping the fridge door open was a very expensive way of making the room even hotter! Physchim62 (talk) 21:39, 4 November 2008 (UTC)