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

Caloric Theory

Isn't the idea of heat being a physical substance called caloric a debunked theory? Is the current understanding of the 2nd law of thermodynamics still based on this obsolete theory? Should it be noted that mainstream science no longer considers caloric a valid model of thermodynamics?--Subversive Sound (talk) 16:07, 10 August 2009 (UTC)

Indeed, heat is not a physical substance, if by "substance" we mean collections of molecules. However, none of the laws of thermodynamics require considering heat a physical substance. David spector (talk) 22:17, 26 December 2009 (UTC)

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.

Dear unsigned, there is no such thing as an isolated system in nature, only in thought experiments and in real-world thermodynamic systems considered for finite time segments. If the Big bang is a true theory, then it is an example of this evolution from order to disorder, compact to expanded. The laws of thermodynamics have more to do with probability and order, and less with the functioning of the the real world, in which "isolation" of a system is always relative and partial. This is why the second law apparently fails in the case of a refrigerator (mentioned above). The entire universe, considered as an isolated thermodynamic system, obeys the laws of thermodynamics. David spector (talk) 22:41, 26 December 2009 (UTC)

This article misrepresents the second law

I have a question/possible need for a correction. The article says: “The second law of thermodynamics is an expression of the universal principle of entropy, stating that the entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium.” Isn’t entropy a measure of “disorder” or the distance from equilibrium? If yes, at equilibrium the value should be at the minimum value. I studied this many years ago, I prefer to ask instead of making the change myself. —Preceding unsigned comment added by Adartsug (talk • contribs) 20:14, 3 February 2010 (UTC)

"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)

About 14 billion years ago, if the Big bang theory is correct, an apparent statistical anomaly occurred in which all matter and energy "happened" to be confined to a rather small space. Such a random massively-reduced state of entropy is therefore apparently possible. This state of the universe doesn't violate the second law if the universe is not an isolated system, if the laws of physics started from the singularity that apparently occurred about 14 billion years ago, or if the second law alternates with an anti-second law. But all of this is speculation that is outside of the scope of this article; it is also mostly an area where the WP policy of WP:OR would prevent inclusion in the article. Just as Newtonian mechanics fails in certain "regimes" (very short time intervals, velocities comparable to the speed of light, temperatures close to absolute zero, energies in small multiples of Quanta (see Planck constant), and size close to the Planck length) and must be extended, you can think of the laws of thermodynamics as applying only to regimes most familiar to us. Any extension of thermodynamics for regimes in which it fails is beyond the scope of this article. Anyone is free to speculate, but WP articles must describe verified information, not speculation. David spector (talk) 23:04, 26 December 2009 (UTC)

Consider our universe as 'one point that banged', but 'other points' were all around us, and each of them banged. Since all of our universe is traveling away from itself quite quickly, we may never visit 'another point that banged'. Therefore, we must consider the second law of thermodynamics as defined by the best sources on our world, and thereby improve Wikipedia. --Sponsion (talk) 15:35, 2 May 2010 (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)

I don't believe that the second law breaks down if a system is "complex". As for application to living systems, please see the article Entropy and life. David spector (talk) 23:12, 26 December 2009 (UTC)

In it's simplest for the second law states "In a closed system, entropy approaches the maximum"

The important part here is "In a closed system". A perfectly closed system can not exist in nature, except as the universe as a whole, and possibly not even there. Kid Bugs (talk) 22:58, 5 January 2010 (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)

On An apparent paradox about the 2nd law of thermodynamics

The 2nd law of thermodynamics states, in certain way, that all physical laws' information tend to dispersion, to deaccumulation, and, in this sense, all physical laws tend to dissipation and minimization.

But, the 2nd law of thermodynamics, is also another physical law.

So, how can the 2nd law of thermodynamics "dissipate" and "minimize" itself? (in certain sense, 2nd law of thermodynamics states that physical laws are false)

(PS: btw, if there's a time arrow, then there's an arrow; and if there's an arrow, then there's order and information.) --Faustnh (talk) 14:47, 5 April 2009 (UTC)

At the heat death of the universe, it will no longer be possible to observe if the Second Law is true or not! Physchim62 (talk) 15:29, 5 April 2009 (UTC)

The second law does not state, "all physical laws' information tend to dispersion". It states that isolated physical systems tend to disorder. The second law does not apply to Newtonian mechanics, for example, a set of laws that describe everyday physical force and motion almost perfectly. It also does not apply to Brewster's Law or any other. It does not apply to itself. It applies only to isolated thermodynamic systems.

The laws of physics have all been verified through experimentation, observation, and the consistency of a large body of theoretical reasoning based on them. If you feel that a law of physics is incorrect, paradoxical, or has some other such flaw, you may simply not understand the particular law. In such a case, a WP article may need to be clarified to help remove the possibility of the misunderstanding.

My own opinion: just because some WP articles may be too advanced for a casual reader to understand is no justification to claim that such an article is incorrect. People reading WP are expected to be educated enough to understand the scope of any article. On the other hand, WP articles should be written as clearly and completely as possible within a reasonable amount of space. David spector (talk) 23:29, 26 December 2009 (UTC)

On another strange question about the 2nd law of thermodynamics

Hi, I'm considering another special question about this physical law:

If the Universe doesn't suffer losses, then how come the 2nd law can be fulfilled at universal scale? (The second law bases on radiation arrow of energy; what happens when this arrow meets the border of Universe? should it reverse somehow? And if the Universe is a borderless sphere, what implications should this have with respect to that arrow?) --Faustnh (talk) 22:23, 8 May 2009 (UTC)

See WP:TALK: this page is for discussing improvements to the article, based on verifiable sources, not your original research. If you want to blether or get all existential, follow the arrow elsewhere. . . dave souza, talk 23:48, 8 May 2009 (UTC)

I agree with Dave Souza's comment, but I would have expressed it far more politely and gently. Foustnh is clearly sincere and interested in a discussion exploring implications of the second law of thermodynamics. Unfortunately, WP, as Souza points out in an unnecessarily rude way, is an encyclopedia of information, not a discussion forum. David spector (talk) 23:33, 26 December 2009 (UTC)

Rigorous treatment based on statistical physics

The derivation of the equation dS = dQ/T for reversible processes should be derived in this article from first principles, as it is an essential part of the second law. I gave a derivation (similar to given in the book by F. Reif) here, but it obviously belongs to this article.

I mentioned in the previous section of the article in the fundamental relation that dS = dQ/T for a reversible process is part of the second law, but even this statement is not emphasized in this article.

So, I think this article needs an appendix in which the derivaton of dS = dQ/T can be moved to. And a new section is needed to explain how you get from the definition S = k Log(Omega) to the conclusion that the entropy of an isolated system can only increase. Count Iblis (talk) 03:16, 11 August 2009 (UTC)

I agree.. the heat expression is in fact a special case of the general Boltzman equation S = k Log W (W = thermodynamic probability, which should also be defined) and W is replaced with Wp (most probable microstate).

Vh mby (talk) 10:05, 31 May 2010 (UTC)

easier explanations

Is it possible to include an easier explanation of this subject (and related)? Especially for childs or people with no background in physics jargon. Are there already some introductory articles on the subject?

Dear unsigned, I agree that WP should include in each article explanations that can be more or less understood by everyone as well as compact explanations that use jargon, mathematics, and other such mechanisms that permit eloquent description in a small amount of text.

In this case, the portion of the article just after "In simple terms..." gives the intuitive heart of the second law.

Web searching (or reading books in a library) can almost always uncover easy-to-understand information on any subject. WP is not a place to receive a complete, well-balanced education. David spector (talk) 23:41, 26 December 2009 (UTC)

What a foolish thing to say. The point of reading an encyclopedia article is to learn about a topic that you don't already understand. This is simply a very bad article, which badly needs improvement. Certainly no one who isn't already well-grounded in thermodynamics can make sense of this mish-mash. Even the statements of the 2nd law are couched in weasel-words. "Generally"? "Tend to"? Sheesh! What exactly do you think the purpose of this article is then, if not to educate? Yappy2bhere (talk) 06:44, 31 December 2009 (UTC)

I'm afraid I must agree with Yappy2bhere, this article is a hopelessly overcomplicated mish-mash that needs a ground-up re-write.

From the very first paragraph it falls apart. That paragraph could have been stated simply as:

"In a closed system, entropy approaches the maximum"

After that, definitions of the terms "closed system", "entropy" and the maximum value of entropy would complete the needs for casual research. The remainder of the article could then happily wander into areas for the more serious researcher.

Furthermore, the section "Applications to living systems", intended to correct the common misrepresentation of the second law by creationists, is probably much more than is necessary. The easiest rebuttal to their argument is to point out that a key phrase in the second law is the first four words, "In a closed system", which creationists leave out of their argument as a deliberate lie of omission. It is clear that they do not leave that part out by accident, as they are not stupid, and doing so would flunk anyone from grade 10 physics. It's pretty hard to convince someone that something that they desperately want to believe is wrong. Kid Bugs (talk) 21:36, 3 January 2010 (UTC)

Miscellany

I've gone ahead and deleted the following miscellany. None of it serves to improve the article.

• Flanders and Swann produced a setting of a statement of the Second Law of Thermodynamics to music, called "First and Second Law".
• The economist Nicholas Georgescu-Roegen showed the significance of the Entropy Law in the field of economics (see his work The Entropy Law and the Economic Process (1971), Harvard University Press).
• Creationist Duane Gish incorrectly used the Second Law of Thermodynamics to argue that evolution was impossible, although stand-up comedian Dave Gorman has pointed out that Gish misunderstood the definition of a closed system.
• The Last Question, a science fiction short story by Isaac Asimov, is centered around the question of how to reverse the Second Law of Thermodynamics, or entropy.
• One of acclaimed comic writer Alan Moore's short stories, chronicled in a collection called Wild Worlds, depicts indestructible and/or immortal characters facing down the unstoppable entropy at the end of the universe.

Chemeditor (talk) 19:32, 11 October 2009 (UTC)

IMO, these items deserve to be moved somewhere where they can be preserved and found. Although they are less WP:Notable (or important) to be included in WP, they do appear to be true and clearly relevant for some WP users. David spector (talk) 23:44, 26 December 2009 (UTC)

QM derivation

Worth mentioning?

• http://link.aps.org/doi/10.1103/PhysRevLett.103.080401
• http://arxiv.org/abs/0802.0438

--Dc987 (talk) 07:40, 8 December 2009 (UTC)

Yes, but then this article has already been criticized, so we need to then mention the criticism also... Count Iblis (talk) 15:10, 8 December 2009 (UTC)

Where can I find it? I've only seen a few comments on the blogs. --Dc987 (talk) 19:52, 8 December 2009 (UTC)

arXiv:0909.1726, arXiv:0911.2610, arXiv:0912.1947 Hweimer (talk) 15:12, 4 February 2010 (UTC)

Heat radiation

When we speak of heat traveling in the form of radiation it seems to me we are really talking about net heat. As I understand it, two bodies of different temperatures are both radiating heat. The net effect is for the cooler body to warm up and wamer body to cool down, but heat travels from the cooler body to the warmer body at the same time heat travels from the warmer to the cooler body. The net effect does not contradict the 2nd law but in the process, heat is traveling from cooler to hotter. Jojalozzo 13:58, 2 May 2010 (UTC)

If in a vacuum, you had a microscopic, hot body separated 1 millimeter from cooler heat lamps surrounding at all angles, may be possible that heat can flow from cold to hot, and the reason is while the heat flux density of the cooler lamps is by definition lower than that of the hotter body, the effective area of radiation reduces as the heat from the heat lamps concentrates toward the center. Therefore, if the lamps are not too cold, it is conceivable that the heat flux from the lamps may be concentrated at the microscopic body, resulting in a heat flux density higher than the hot body, thereby warming it up.Kmarinas86 ^(6sin8karma) 19:28, 2 May 2010 (UTC)

A macroscopic example is an array of reflectors set to point at a particular spot, such as the PS10 solar power tower, where it is evident that heat in the form of radiation is reflected by the cooler solar reflectors into a spot that is heated to 285 degrees centigrade.Kmarinas86 ^(6sin8karma) 19:37, 2 May 2010 (UTC)

Introduction

The rules for editing and writing encyclopedia entries require the knowledge to be verifiable (ie true as far as possible to determine) and stated with clarity. Which excludes philosophical speculation.. reference to the 'equal a priori probability postulate to the future' have no relevance to the introduction of this topic, is unclear and inadequately referenced. Vh mby (talk) 03:53, 31 May 2010 (UTC)

log vs. ln

Shouldn't the equations containing "log" be changed to "ln" so that they don't get confused with logarithm of base 10? Compare with, for example, the entropy article. /Natox (talk) 07:56, 4 June 2010 (UTC)

Proposed deletion

Under the definition proposed by Lord Kelvin, the statement:

"This also means that it is impossible to build solar panels that generate electricity solely from the infrared band of the electromagnetic spectrum without consideration of the temperature on the other side of the panel (as is the case with conventional solar panels that operate in the visible spectrum)."

Is unclear at best, and incorrect at worst. If the phrase it accurate, it needs supporting explanation to justify the claim. Infrared and visible light are the same thing, just at different frequencies. Why then would it be necessary to consider the other side of the panel in one case but not the other? Perhaps the statement assumes that the hypothetical solar panel works off the net IR radiation, in which the radiation from the panel must be subtracted from the incoming radiation.

I personally don't see the relevance of this statement and suggest it be deleted. 65.112.42.84 (talk) 20:34, 24 June 2010 (UTC)

Well, it certainly needs clarification, but there is a grain of truth in there somewhere. At a temperature of, say, 3000 K, black body radiation is well into the visible. So its like saying you cant have a solar panel at 3000 K that generates electricity from the visible without at least worrying about a temperature gradient. If there is no temperature gradient, then the solar panel will be acting like a black body in the visible, emitting as many visible photons as it absorbs, whatever the mechanism, and there won't be any net gain of energy. PAR (talk) 00:44, 25 June 2010 (UTC)

Nuclear Fusion contradicts Thermodynamics

In nuclear fusion, matter is converted into energy which means energy CAN be created and contradicts the second law.