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August 22

Maxwell's demon

If you explained Maxwell's proposal and told me to suggest a solution, I would imagine that it was the necessity of the demon adding energy to the system by opening and closing the door. However, I see that the actual answer is different, e.g. the necessity of expending energy to learn which molecules were which, or the necessity of erasing one's memory of past molecular movements. So where do I go wrong in suggesting that the energy of moving the door is a possible answer? And has anyone published a WP:RS addressing door-energy requirements and explaining why they don't work? Nyttend (talk) 02:19, 22 August 2018 (UTC)[reply]

It's not about energy, it's about entropy.

Maxwell's demon uses entropy to produce energy. It works within the same simplified world of "light inextensible string" and "frictionless doors" as much conceptual physics. The assumption is that "filtering the entropy" would allow energy to be produced (this is uncontentious) and also that the demon can do this without consuming energy itself (that's a conceptual simplification, but it's valid because if the demon can't do it with this simplification, it certainly can't achieve it without.)

However, even with these advantages over reality, Maxwell's claim is that the demon can't function. Not for lack of energy, but for lack of entropy. Although this didn't really get a theoretical basis until the 1960s. Andy Dingley (talk) 09:48, 22 August 2018 (UTC)[reply]

The assumption is ... that the demon can do this without consuming energy itself (that's a conceptual simplification, but it's valid because if the demon can't do it with this simplification, it certainly can't achieve it without.) Doesn't that just show how meaningless the thought-experiment is? You might as well say "If I use a perpetual-motion machine to operate the door, I can generate free energy!". Iapetus (talk) 09:54, 22 August 2018 (UTC)[reply]

But the demon doesn't say that. It says, "even if I did have a perpetual motion machine, I couldnt generate free energy; so I certainly can't do it without." Andy Dingley (talk) 10:15, 22 August 2018 (UTC)[reply]

Sort-of repeating what AD said: the idea is that Maxwell's demon paradox's standard solution does not involve door-opening costs. It might well be that, if you try to build a Maxwell demon in the lab, the door-opening costs would be prohibitive even with a door in unobtanium; it might even be that matter properties at small scales forbid entirely whatever design you have in mind. However, the crux of the Maxwell demon paradox is in the information/decision part, not in the door design whose refutation will depend on the specifics (for another example of extract-work-from-single-temperature-source design, see Brownian ratchet). Tigraan^{Click here to contact me} 14:27, 22 August 2018 (UTC)[reply]

So in other words, I went wrong because my answer didn't answer the question that Maxwell posed :-) Thanks for the correction. Nyttend backup (talk) 16:20, 22 August 2018 (UTC)[reply]

Just to clarify, the solution to the problem posed by Maxwell's demon is that it is, as originally presented, supposed to violate the second law of thermodynamics by allowing the entropy of the closed system to decrease without itself increasing the entropy of the universe. This was solved by Information Theory's contribution to the idea of entropy, which is that the storage of information always creates entropy. In order to decide when to open and close the door, the demon has to store information in his "mind" (whatever that is) to allow him to decide when to open the door; every new particle creates a new memory location, which creates more entropy. Thus, the second law is still saved. This video, starting at about 7:00 minutes, explains exactly how this works. --Jayron 32 16:33, 22 August 2018 (UTC)[reply]

Why is the letter 'G' used to represent Conductivity ?

This came up on the Sieman's Talk page {1} and it got me curious, so I tried to research it myself briefly to no avail, and thought I would bring it here:
Why was the letter 'G' chosen to represent electrical conductance, the reciprocal of resistivity.
Any insight is appreciated. Cheers! --Elfabet (talk) 14:07, 22 August 2018 (UTC)[reply]

In German one may suggest G stands for Geleitfähigkeit because "Das Formelzeichen des elektrischen Leitwerts ist G (und) seine Maßeinheit Siemens" [1]. Both Georg Ohm and Werner von Siemens were Germans. DroneB (talk) 15:12, 22 August 2018 (UTC)[reply]

(edit conflict) It might not mean anything. There are a limited number of letters one can use, and the obvious letter (C) already is used for electric charge (coulomb charge), so other letters needed to be chosen for other variables; I for current and G for conductance. "G" doesn't have to mean anything per se, it just needed to be an unused letter one could press into service. Notably, G isn't always the symbol used. Sometimes, an inverted omega sign is used, (called the "mho") since the Siemens is an inverse of the Ohm. --Jayron 32 15:14, 22 August 2018 (UTC)[reply]

@DroneB: that technically works, but doesn’t feel natural. I don’t think the term Geleitfähigkeit was ever used (the google results I found are generally typos for Gleitfähigkeit). However, Greek γ is occasionally used for Electrical conductivity, and I could imagine G could be derived from that. Cheers ✦ _hugarheimur 17:02, 22 August 2018 (UTC)[reply]

I lack a reliable relevant source for Geleitfähigkeit ("conduct capacity") because an expression such as "Die Sekretion der Bartholin-Drüsen regt die Geleitfähigkeit des Penis zusätzlich an."[2] gives a wrong (but interesting and useful to know) idea, and is probably a typo of Gleitfähigkeit. So in our fruitless search of the source of G, Geleitfähigkeit fails Wikipedia's WP:SYN and the mho character

\mho

is the unit, not the symbol of conductivity. DroneB (talk) 19:47, 22 August 2018 (UTC)[reply]

It's worse than that: C isn't even the symbol for electric charge. C was already taken for electric capacitance, so the symbol for electric charge is Q and thus conductivity had to settle for G. 97.115.66.87 (talk) 23:43, 22 August 2018 (UTC)[reply]

Really? I thought it was because C already stood for coulomb, and writing C = 0.05 C might be a little bit confusing. 78.0.242.89 (talk) 23:43, 24 August 2018 (UTC)[reply]

There is often no connection with a deeper background or higher meaning. Engineers are traditionally painfully pragmatic in such cases - they pick then "next best" letter and carry on engineering. So the letters "A-F" where already in use and thus they picked "G". --Kharon (talk) 21:24, 25 August 2018 (UTC)[reply]

If the system inherits from equation shortcuts written in the late 1800's Gutta-percha (an ubiquitous insulator back then) could have coincidentally given the initial, valued for an equivalence to zero in scale. For what it's worth, the Consultative Committee for Electricity of the CGPM was established in 1927 ( but the system as we know it of course was established in 1948 ). --Askedonty (talk) 17:18, 29 August 2018 (UTC)[reply]

What's the best estimate for Mauna Kea's all-time high height? How far in the future or past is this?

I'd be interested in any height (from base, from sea level (current or at the time), average pressure altitude..) Sagittarian Milky Way (talk) 19:31, 22 August 2018 (UTC)[reply]

Mauna Kea is towards the end of its growth, although it remains active. It is unlikely to have been higher than it is now and is likely never to get substantially higher. Mikenorton (talk) 05:52, 23 August 2018 (UTC)[reply]

Sapphire

According to sapphire, it's an aluminium oxide Al₂O₃. Does it mean that if I properly oxidize a common alumunium, I can obtain an inexpensive sapphire? 212.180.235.46 (talk) 20:18, 22 August 2018 (UTC)[reply]

Synthetic sapphires are made using the Verneuil process, the Czochralski process, or just by hot isostatic pressing, as explained later in the article. Perhaps someone else can explain how easy or how difficult this is. Dbfirs 20:23, 22 August 2018 (UTC)[reply]

You have three problems to solve:

Grow some corundum. This is fused aluminium oxide. It has been commercially manufactured as an abrasive for about 120 years. It's mostly done by the Verneuil process, which is a fairly simple process using an oxy-hydrogen flame. You now have the sort of thing that looks like a geological sample - chemically it's right, but the structure is amorphous and opaque. If you can see through it at all, that's luck.
Refine the material to make it a single crystal, thus transparent. This might use the Czochralski process. Again, this is much easier than it used to be - induction heater modules are on eBay these days, and they don't catch fire all that quickly.
Change the colour. This is a matter of adding dopants to the aluminium (yes, just like semiconductor manufacturing). Usually chromium. This is very tricky to get the colour right. One of my favourite sapphires are padparadscha sapphires, which have a red colour, somewhere from a salmon pink to an orangey red. If you see one of these for sale today, chances are they're synthetic, grown in Myanmar or Thailand - especially if they're orangey. You can also deepen an existing colour, just by heating them in a controlled manner.

So, not easy. But if you have a glassworking workshop, or even a ceramics workshop, you're halfway there. Or you can grow them directly as crystals, using epitaxial growth. But CVD processes, let alone MO-CVD, tend to involve horribly toxic chemistry. Andy Dingley (talk) 20:51, 22 August 2018 (UTC)[reply]

We have Sapphire § Padparadscha. Might want to make a redirect. --47.146.63.87 (talk) 06:45, 23 August 2018 (UTC)[reply]

Done Tigraan^{Click here to contact me} 07:57, 23 August 2018 (UTC)[reply]

This video may be of interest: https://www.youtube.com/watch?v=a0N4JYN9lFI --Guy Macon (talk) 21:02, 22 August 2018 (UTC)[reply]

Or just buy a synthetic one outright, which will be cheaper than the equipment necessary to make it (as detailed above), unless you're already in the jewelry business. Synthetic gemstones are cheap because gemstones are rocks. They don't involve rare materials. "Natural" gems are only expensive because of a combination of factors that are an object lesson in behavioral economics: the gem mining industry is an oligopoly, and said industry heavily promotes "natural" gems as somehow fundamentally different from "fake" synthetic ones and the only suitable material for demonstrating your love for someone, or from showing off how rich you are, which justifies their exorbitant cost. At least in the Western world the public largely goes along with this because propaganda works. Industrial and scientific use of gemstones is almost universally synthetic stones because in those contexts the people using them aren't trying to impress others; they know the stones are cheaper and just as good. (Sometimes better, because the properties of the stone can be carefully controlled, which is important for some applications.) --47.146.63.87 (talk) 06:35, 23 August 2018 (UTC)[reply]

Nitpick: Synthetic gemstones are cheap because gemstones are rocks. They don't involve rare materials. - Partly true. Ruby and sapphire are waaay cheaper to manufacture (in a marginal cost sense once you have all the equipment) than the natural thing, but synthetic diamonds cost almost as much as natural ones even though it is pure carbon. It depends on the temperatures etc. needed in the production process. Tigraan^{Click here to contact me} 08:03, 23 August 2018 (UTC) [reply]

Are you sure? I thought gem-quality diamonds could be made pretty cheaply (but for the reasons discussed previously people prefer expensive natural ones). The linked article doesn't say anything about the cost to manufacture, just the selling price. Iapetus (talk) 08:14, 23 August 2018 (UTC)[reply]

Hmm, I did base this on the article sentence synthetic diamonds sold as jewelry were typically selling for 15–20% less than natural equivalents, and my knowledge that synthetic rubies are cheap enough that a crazy friend of mine bought some to forge his own proposal ring (here's a retailer at $40 for 200 pieces). However our article also says De Beers started selling synthetic diamonds to the general public at a couple of hundred dollars each, so the manufacturing cost cannot be super high either. Tigraan^{Click here to contact me} 11:03, 24 August 2018 (UTC)[reply]

I've a few 3/4" diameter rubies. One or two I've used, set into sword pommels. I've no real use for 3/4" rubies, but when you see them listed and they're so cheap, it's hard to resist. Much silversmithing I've done has involved padparadscha sapphires, just because I like the colour.

This isn't to say that synthetic means cheap though. Laser rods, or the three foot discs in some of the big lasers, can cost as much as Liz Taylor's diamonds. I've a small piece of a three footer, which one day "just broke". Most of the bigger pieces were worth so much as scrap they were re-ground as rods or discs for smaller lasers. Andy Dingley (talk) 12:03, 24 August 2018 (UTC)[reply]

Isn't the high cost of those laser rods in large part due to their being precision cut to tighter tolerances than jewelry? 202.155.85.18 (talk) 05:57, 28 August 2018 (UTC)[reply]

Certainly in part. The ends have to be polished, and to be both flat and accurately parallel (the sides are less critical). When this comes to a disc laser, the amplifiers are both large and the emitting faces which have to be carefully polished are much bigger. So yes, serious money! Andy Dingley (talk) 00:32, 29 August 2018 (UTC)[reply]

Even for diamonds, it's cheap to make bulk polycrystalline diamonds, and these have been part of the industrial abrasives trade for decades. It's when you need monocrystalline (i.e. transparent) diamonds in large sizes that it gets expensive. Andy Dingley (talk) 08:20, 23 August 2018 (UTC)[reply]

Cheap is relative. The cheapest industrial abrasive is Sand. Compared to Sand, industrial diamonds are still very expensive. The question of worth is nicely pointed out in the article Opal. (cite) The internal structure of precious opal causes it to diffract light, resulting in play-of-color. (citeend) Ofcourse that includes how common these colors are. Thus like the rare pink and blue diamonds are traded highest it seems blue and orange-pink are the most expensive sapphires. Else they become industrial parts like in case of sapphires as bearings in watches or as part of cutting tools in glass works. --Kharon (talk) 21:14, 25 August 2018 (UTC)[reply]

Carborundum abrasives are actually cheaper than abrasive sands. Sand isn't usually very abrasive, so the grades used as abrasives have to come from particular locations. Like building sand (which we're now running short of), the price is going up and poor grades are being passed off as better stuff. Parts of India now have "sand pirates" who are stealing river sandbanks (of good sand) and causing havoc in some deltas.

Sand is also a very poor abrasive (it's both weak at cutting and also hazardous). I have a 17th century book (reprint) which discusses the problem of people selling fake "sandpaper" to woodworkers, instead of the real glasspaper, coated with powdered glass. Andy Dingley (talk) 12:40, 26 August 2018 (UTC)[reply]

I think you confused Carborundum with Corundum aka aluminum oxide or Al₂O₃, which is relative abundant in Nature, used everywhere in industry and manufacturing as main abrasive and only valuable in its rare variations called ruby and sapphire, which of course thus is Al₂O₃ too. So to answer the original question, if you would oxidize Aluminum industrially to Al₂O₃ you would not end up with valuable sapphire but cheap Corundum. But with some advanced tricks you can get sapphires and rubies too. You can read about it under Corundum#Synthetic_corundum --Kharon (talk) 19:26, 26 August 2018 (UTC)[reply]

No, carborundum. Admittedly this is "seconds", and it rather surprised me too, but in fine grits, carborundum grit is available more cheaply than corundum (and normal size carborundum). The grit is manufactured by a different process than for solid pieces and as I understand it from the supplier, this also produces a near-waste stream of undersize particles (they're actually grown as particles in a gas-phase process, so they start out small). Andy Dingley (talk) 20:03, 26 August 2018 (UTC)[reply]

Rice foamed after being soaked for a couple of days, probably less

I put the rice in the rice cooker but never got to cooking it. It's summer, and after more than a day, the water had quite some foam on the top with a hint of fermentation. Is the rice still safe for cooking? 104.162.197.70 (talk) 21:44, 22 August 2018 (UTC)[reply]

see Rice wine which is what is forming. Rice is quite cheap, chuck out, wash it out and start again. Graeme Bartlett (talk) 21:58, 22 August 2018 (UTC)[reply]
Going by this NHS page, I'd strongly suggest not eating it. As Graeme says, even if it is actually safe, why risk it? › Mortee _talk 22:36, 22 August 2018 (UTC)[reply]
- Could be Bacillus cereus which can survive boiling and is a known pathogen of cooked rice. Heaviside glow (talk) 22:40, 22 August 2018 (UTC)[reply]
  - Yes but the OP isn't referring to cooked rice. There are significant differences between cooked rice and uncooked rice, especially regarding nutrient availability. Germinated brown rice is a common thing, and while it's generally suggested to change the water after a while, there doesn't seem to be great concern over food illnesses from Bacillus cereus. Of course nutrient availability is likely to be reduced further with brown rice although interestingly this [3] mentions that brown rice has higher Bacillus cereus load in the first place. Germinated brown rice seems to end up with an overall load of a bit over 1 order of magnitude higher load although they didn't test enterotoxin levels. Nil Einne (talk) 01:21, 23 August 2018 (UTC)[reply]
    - All true, and I'm interested in the white vs brown rice distinction you raised. Still, given that the bugs are there and inclined to grow in a damp environment at room temperature, unless OP fancies experimenting on themselves, it's probably best that they just get some new rice. Per our article, germinated brown rice is left for 4–20 hours, not "a couple of days". At a guess, brown rice is also more resilient to soaking because its husk is intact, so I'd expect B. cereus to get started rather faster with white rice, regardless of the initial load (not that we know what colour OP's rice is). I don't have any expertise here at all, but I'm still very much inclined towards caution. › Mortee _talk 01:35, 23 August 2018 (UTC)[reply]
      - No that's for 30-40 degrees C water. The OP appears to come from New York, and while a lot of the northern hemisphere is having hot summers, i find it unlikely that water sitting there. Most sources including our article recommend longer for lower temperatures although I can't find good sources discussing recommended times for more reasonable temperate country room temperatures (say 20 degrees C). Although this source mentions one commercial processor using 72 hours at 30 degrees C [4]. Various sources e.g. [5] [6] mention GABA or other levels peak at 48 hours, so they seemingly didn't think these such dangerous levels to be not worth testing. This isn't exactly surprising since as the sources also mentioned, soaking grains and other seeds for such periods isn't exactly uncommon. This source actually suggests up to 72 hours may be best [7]. (As an aside it also hints at as do other sources another thing namely that you probably also get better results at higher temperatures (well the 30-40 degrees) than you do at lower temperatures.)
        I'm not suggesting there is no risk or the OP does it, but we also have to be realistic here that there's very limited evidence either way. The risks with cooked rice have very little relation to the risks with cooked rice.
        BTW an an interesting point, some sources recommend soaking rice overnight to reduce arsenic levels and I'm pretty sure this includes white rice which I assume is what the OP is referring to. Unfortunately the best source I could find is some BBC show [8]. (Since it's just a summary from the show, it doesn't say if this soaking was at room temperature or in the fridge or what.) This source which is discussing the results of the show includes a quote from an epidemiologist endorsing soaking [9]. While it's impossible to know if she gave some dire warnings the source neglected to mention, none are mentioned there.
        Weirdly this group carried out fancy percolator tests for cooking but didn't seem to test such a basic step. However I don't believe this is likely to be due to health concerns from soaking rice overnight. More likely that percolator makes for a more popular paper than simple soaking and also as fancy as it sounds, it's probably a more cost effective method for industrial and commercial kitchen usage. Of course 48 hours is quite a bit longer than overnight, however I don't think sources are going to recommend leaving cooked rice out overnight or even for 4 hours. (This is of course a common problem. Simple solutions which have the potential to work well in lesser developing countries where the problems may be significant are not well considered.)
        Nil Einne (talk) 04:25, 23 August 2018 (UTC)[reply]
        
        P.S. I initially gave Bangladesh as an example in my last sentence but removed it as in retrospect it may be a poor one give the frequently high arsenic levels in their water levels meaning that you could perhaps make the problem worse or at least get a lot less improvement than may be observed in countries with better water supplies. Nil Einne (talk) 04:33, 23 August 2018 (UTC)[reply]
        
        Interesting, I'd never heard the arsenic advice. › Mortee _talk 09:30, 23 August 2018 (UTC)[reply]
        Personally, I think there's far too much personal speculation going on here, and I wish this matter had been left at the initial advice of Graeme and Mortee. This is an inquiry that is close enough to a request for medical advice (it is, afterall, an inquiry about the safety and health implications of a course of action) that the editors here should have refused to answer this question outright, and if any answer were put forth, it should be the one which clearly endorses precaution and safety. A protracted discussion about whether or not food poisoning is actually likely to occur only creates the possibility that the OP may decide to consume something at least potentially dangerous on the basis of opinions (or a sense of doubt among them) here. That is exactly the kind of outcome our no medical advice rule was established to guard against. Frankly, some of the laissez-faire opinions above display a marked lack of understanding of the topic and a pretty free-wheeling willingness to discount the risk of contamination. For starters, one doesn't just have to consider common pathogens associated with under-prepared foodstock of this sort; so long as there is standing water involved, the question of what was in that water initially, what was on the surface of the cooker, and in the immediate environment (in the addition to heat and other factors) all create risk of bacterial and/or fungal growth after that period of time, with potential toxicological consequences. Indeed, the fact that the OP mentioned the heat and apparent fermentation are additional signs of why it is imprudent to say anything tot he OP which might induce them to have doubt as to whether to pursue the more cautious course (in the present case or in the future). And even if I were not familiar with these basics microbial concerns, I'd still be saying as a matter of policy that opining on this topic is not appropriate use of the desk. The OP should be advised to seek their health and safety advice from appropriate sources, and this discussion should be closed. Props to Mortee, however, for maintaining a consistent tone of discouragement to the notion of reheating already fermenting food. Snow ^{let's rap} 06:18, 24 August 2018 (UTC)[reply]
        Some medical advice ("don't risk it. Throw it out and thoroughly clean the cooker") is OK, and is vastly preferable to "ask your doctor if eating rotten food is OK", Other medical advice ("go ahead and eat rotten food") is definitely not OK. --Guy Macon (talk) 07:38, 24 August 2018 (UTC)[reply]
        More on this in the section below. --Guy Macon (talk) 10:39, 26 August 2018 (UTC)[reply]