Wikipedia:Reference desk/Archives/Science/2016 March 12

= March 12 =

Interrupted Dreams
Sometimes Im having a really good dream, then I have to get up and go to the toilet. When I get back to bed, I cant get back into my dream. Why is this?--31.109.183.147 (talk) 02:07, 12 March 2016 (UTC)


 * Lucid dreaming is the process of being aware of your dreams as they happen and potentially controlling their content. If you could remember your dream, maybe by writing it down immediately, then perhaps you could get back into it. StuRat (talk) 03:52, 12 March 2016 (UTC)


 * As a person who occasionally experiences lucid dreaming, though, I have this advice to give you: lucid dreaming is NOT restful, in fact when you wake up from a lucid dream you might feel like you didn't get any sleep at all. 2601:646:8E01:515D:5537:81A4:208D:1E74 (talk) 11:45, 12 March 2016 (UTC)


 * I know of no source besides our articles already linked, as well ass the main one, dream. But my experience is that if I actually have to get up out of bed, negotiate my way to the toilet, make the proper motions, wipe, and flush, then get back to bed, the "train of thought" is gone.  Whereas if I wake up and roll over and go back to sleep, it is much more likely the dream will continue.
 * As for lucid dreaming, with me it usually only occurs in unpleasant dreams, where I can tell myself "this is a dream" and the direction of the dream will become less troubling (e.g., I am being chased through a building by a bear > I am walking through a natural history museum) but I still can't actively choose the direction or outcome. μηδείς (talk) 17:47, 12 March 2016 (UTC)


 * I didn't read our article, but the area of lucid dreaming actually is likely helpful to your question. In particular, the obvious question in response to your question is "how do you know?" This isn't being facetious but is actually an important issue that Sturat has hinted at above. Are you sure you're remembering exactly what you dreamed after you went back to bed? Unless there's something really unusual about your sleep and dreaming, if you have enough experience and knowledge about how dreaming works, you probably recognise it's quite likely there are many more dreams than you remember. (Notably it seems you're a lot less likely to remember your dream if you don't wake up soom after or when it's happening.) And even for those few dreams you do remember when waking up, unless you write them down or try very very hard to remember them long term, you probably won't. And in fact, you can even be sure if you memory of what you dreamed is correct. In other words, it's difficult for you to be sure your dream didn't continue since maybe it did you just don't remember it happening particularly if you slept for a while afterwards. And even if you by some magic do remember all the dreams you had after you went back to sleep, you'd need the same magic or some other method (like writing it down which unfortunately may either help or disrupt you dreaming the same thing) to be sure you're remembering the first dream you're trying to get back to. Again I didn't read our particular article but I know lucid dreaming does deal with this memory problem and suggestions as to how those who want to experience lucid dreaming should deal with it. Nil Einne (talk) 18:11, 12 March 2016 (UTC)


 * To come at this from the opposite way: on a number of occasions I've woken up remembering a dream that took place in a non-real location, and with the definite impression that I've dreamt of being in the same dreamed locale before. However, that impression might actually be only a part of the dream I've just had. (Keeping a Dream diary might be a way of obtaining proof of such recurrences, but I choose not to do that.)
 * I suspect that such dreamed impressions might be common: for example, H. P. Lovecraft wrote stories utilising the idea of voluntary return to a dreamed location, the Dream Cycle.
 * I myself have on several occasions been able to return to and continue a dream, but usually when deliberately daydreaming during the day rather than when in bed at night. This doubtless borders on the lucid dreaming mentioned by others above.
 * I fully sympathise with the OP in his frustration at being unable to return to an interesting 'immersive drama'. I suspect quite a few fiction writers began by continuing such 'unfinished stories' in writing. {The poster formerly known as 87.81.230.195} 5.66.243.108 (talk) 23:27, 12 March 2016 (UTC)
 * This is pure anecdote, but while almost all my dreaming follows the same pattern as the OP (i.e. once you wake, you can never continue it), I have on occasion only half-woken up, remained groggy for a few minutes, and then gone back to sleep and either continued the dream or repeated it. I have even on one occasion continued dreaming despite waking up otherwise fully.  Iapetus (talk) 21:24, 13 March 2016 (UTC)

Does any one know?
Hi friends ,where i can get a carbon-ceramic matrix,air interface film coefficient.i want to use it to solve one disk brake static thermal simulation.please guide me 112.133.223.2 (talk)  —Preceding undated comment added 08:33, 12 March 2016 (UTC)
 * You made that up all by yourself :D? --Kharon (talk) 13:03, 12 March 2016 (UTC)
 * See Heat transfer coefficient for our article. Unlike the thermal conductivity, the heat transfer coefficient isn't a property of the materials, but depends on the geometry of the system and the characteristics of the airflow. Tevildo (talk) 14:11, 12 March 2016 (UTC)

Wind mill blades efficiency
How is the most efficient number of blades determined. I can read that 3 blades is the 'most efficient' for fans, what about windmills or propellers? Is it as simple as 3 blades is 'usually most efficient (moves the most air), or are there other factors? Thanks if you can help? — Preceding unsigned comment added by 94.210.130.103 (talk) 11:52, 12 March 2016 (UTC)


 * Old-fashioned windmills - for grinding corn - generally had an even number of blades for balance. If one was damaged and had to be removed, the opposite one could also be removed and the mill operate temporarily with just two. If you had three blades, and lost one of them, it was unbalanced and useless. 109.150.174.93 (talk) 12:41, 12 March 2016 (UTC)


 * Ofcourse there are other important factors like material-, production-, assembly- and maintenance-costs. In Engeneering the approach is often not from perks like "moves most air" or "generates most energy" but instead if one additional blade is worth its investment, efford and potential trouble. --Kharon (talk) 13:00, 12 March 2016 (UTC)


 * It is not just about efficiency, but practicality that determines the number of blade. All engineering is an assemblage of compromises. Take a wind turbine. A single bade has the utmost efficacy of capturing the energy from the wind and it requires the least amount of material in its construction.  Yet the strains  on the tower and  bearings etc, take there toll – requiring more costly maintenance. Two bladed turbines have better balance but create a resonances in the supporting tower. Three bladed mitigate much this, but cost more...  yet  have a longer life span.  So at present, it appears that three bladed wind turbines demonstrate the best compromise, which  in the long run produces cheaper megawatts over the economic lifespan of the turbine, compared to  other configurations. P.S. More blades does not  mean the more capture of available  wind energy.--Aspro (talk) 13:33, 12 March 2016 (UTC)


 * We have Wind turbine design although it's largely unsourced. Nil Einne (talk) 14:33, 12 March 2016 (UTC)


 * Another factor is that with more blades, each can be lighter, making it easier to handle. StuRat (talk) 15:48, 12 March 2016 (UTC)

It's a matter of defining "efficiency" since it depends on what you're trying to efficsh. In engineering, you can be trying to minimize various things such as weight or maintenance cost, thus narrowly defining "efficiency". In economics, you add up all the costs, because cost is what you're trying to minimize, which makes the question, "What is cheapest?" Jim.henderson (talk)
 * User:Jim.henderson is exactly correct. The optimal thermodynamic efficiency might not provide the optimal economic value proposition.
 * If we want to formalize this problem so that we can apply engineering methods to find the most efficient solution, we must define a cost function that meaningfully accounts for all of our subjective ideas about what makes the system "better." The cost function can be a composite of all sorts of diverse items, like material use, energy cost, dollar value, risk, and so on.  Defining that cost function is typically the most difficult part of the problem: it requires deep understanding of many totally different disciplines, and requires an ability to turn fuzzy subjective ideas into quantitative decisions.  These kinds of problems are formally studied as industrial engineering, process optimization, and related disciplines.
 * If all you care about is thermodynamic efficiency, we can direct you toward some great books on wind turbine design. Start here: How do wind turbines work? from the Department of Energy.  At a high-school science level, we can say that more blades are more efficient at harnessing energy, but only if there is a lot of wind; more blades are heavier, so if the wind is slow, the extra efficiency isn't enough to make up for the extra weight.  From there forward, additional detail requires a very abrupt transition into some really hard problems in fluid mechanics and statistics.  We have an article on wind turbine design, §blade count; but if you want the real math, you'll have to grab one of the books listed in our section on "Further Reading."
 * Nimur (talk) 17:38, 12 March 2016 (UTC)

Single blade propellers are the most efficient, by the usual definition, but they have a lousy power density. In the case of wind the incoming energy is 'free' so the usual definition of efficiency is misleading, your goal is to extract the optimum energy from the windstream, taking many other factors into account. Greglocock (talk) 22:12, 12 March 2016 (UTC)


 * You just need one blade for harvesting energy to produce electricity. But wind turbine designs are not just about the energy efficiency. Llaanngg (talk) 22:38, 12 March 2016 (UTC)


 * It's a trade-off between the number of blades and the height of the tower. If you have a lot of short blades, you have a complicated and fault-prone hub - if you have fewer blades then they have to be longer and that means building a taller tower. SteveBaker (talk) 03:42, 14 March 2016 (UTC)

Eczema herpeticum
What is the mechanism by which herpes and eczema can interact to cause a systemic infection? Surely, if a localised area of eczema was infected with herpes, the infection would only remain there. 82.132.212.108 (talk) 12:42, 12 March 2016 (UTC)

STDs and alcohol
It's often argued that sex involving alcohol is more likely to transmit stds because it causes poor judgement, people are less likely to use condoms and it's More likely to be casual but could it also be argued that alcohol weakens the immune system hence increasing std transmission risk? 2A02:C7D:B907:6D00:8911:F3BA:77F4:29CB (talk) 13:34, 12 March 2016 (UTC)


 * Dear anonymous. It could be argued that alcohol weakens the immune system. However, lets stick to the most likely. Loss of judgement due to alcohol. The effects Ecstasy and Cannabis appear to have a  more lovey effect on  couples, who  will tend to seek out any contraceptive barriers that may be readily available in respect of their lovey feelings toward each other. Alcohol has the thumbs down. Maybe this is because because alcohol depresses the outer cortex of the brain wherein  our higher intellect is situated. Cannabis and ecstasy enhances it. The instance of STD has been coming down in resent year. Do you think that that is because teenagers have all of a sudden changes their ways?!  Drink a bottle of Bourbon every day, for long enough and ones immune system may suffer but a teenagers metabolism appear not to get get going until a half bottle has entered the blood stream. The cheaper option to alcohol these days,  is illicit drugs and politician should wise-up and rationalize  the drug laws so that all drugs are properly controlled. --Aspro (talk) 14:09, 12 March 2016 (UTC)
 * I'm hatting this. Your text has nothing to do with the question asked and is mere proselytizing. Take it elsewhere. Matt Deres (talk) 17:54, 12 March 2016 (UTC)


 * Our article at immunosuppressive drug and immunosuppression do not mention alcohol, but this piece by the NIH says "In both males and females, alcohol exposure suppresses immune responses..." and "An overwhelming amount of evidence reveals that both acute and chronic alcohol exposure suppresses all branches of the immune system, including early responses to infection and the tumor surveillance system..." with several links and citations. Matt Deres (talk) 18:02, 12 March 2016 (UTC)

C4H10O
Hey,

I try to answer for this question:

$$A\xrightarrow{E} B \xrightarrow{F} C \xrightarrow{HCl or ZnCl_2} D$$

A is an isomer of C4H10O.

Find A, B, C ,D, E, F.

I know that C most be alcohol & I also find 4 isomers of C4H10O but not sure how should I find solution to this problem.

thanks. — Preceding unsigned comment added by 31.210.177.225 (talk) 15:06, 12 March 2016 (UTC)
 * Think of a structure (or maybe several possible ones) of what C could be (you say you know it's an alcohol--not sure how you know this, but assuming it's true...); there are only a few isomers of 4-carbon chemicals that have that functional group). If that is C, what would the structure of D be? Note that not all alcohols react with HCl. So now you have some ideas for A and some ideas for C, you just need to figure out some intermediate that looks like the partial conversion. For example, if A has the OH on C2 and B has the OH on C1 instead, what are ways you know to get "OH on a primary position" and what are ways you know "to get OH off a secondary position"? Or pick an isomer you think A could be and list all the things you can make from it in one step (various E/B possibilities). Which, if any, of those B can you transform by one step into any of your C possibilities (the F method). That is, either work forwards-and-backwards and find the middle, or work from one end and see how close you can get to the other. DMacks (talk) 15:21, 12 March 2016 (UTC)
 * I know it because I learned that only alcohol reaction with HCl and Z_nCl...it seems to be so hard...I don't know. I will ask my teacher on Monday. Best.
 * I fixed the formula and spelling for zinc chloride. DMacks (talk) 18:10, 12 March 2016 (UTC)
 * I want to make this comprehensible, besides I'm getting rusty on my own chemistry... a quick search for alcohol "zinc chloride" gets this helpful demonstration (well, I can't view the video but the text tells me a classroom exercise)  There they use 1-butanol, 2-butanol, 2-methyl-2-propanol.  (Note butane = 4 carbons with 2 H per carbon plus two more on the ends = C4H10, and 1- or 2- butanol adds an oxygen under one of the hydrogens.  2-methyl-2-propanol has three CH3s and an OH linked to a central carbon, again C4H10)  Anyway, the tertiary alcohol in the demonstration reacts immediately; the secondary alcohol reacts after 6.4 minutes, and the primary alcohol doesn't react.  So we know that C is the secondary or tertiary alcohol.  Now D, we are told, is the alkyl chloride produced by an SN1 reaction from whichever C is.  That leaves us to riddle out what isomerizations you can do to C4H10 in two steps to end up with one of these... Wnt (talk) 16:35, 12 March 2016 (UTC)
 * Now that we know ZnCl2 is an alternative for HCl, it makes sense to assume that C would be an alcohol. With only "HCl" as the original question, it could easily have been alkene or alkyne, etc. Lucas' reagent is a great demonstration of the reaction of (and reactivity difference among) various alcohols that Wnt notes. There's one more isomer of the original than the three already mentioned (but 4 is not a lot of choices anyway). Note that there are several possible answers I see, even restricting the range of reactions to SN1/SN2/E1/E2 and simple reactions of alkenes. That sounds like how much you have covered so far in your class?--knowing "what you know" would help us keep from steering you into reaction topics you have not yet learned. DMacks (talk) 18:10, 12 March 2016 (UTC)
 * Did the question actually say "HCl or ZnCl2"? Or did it say "HCl/ZnCl2"?  The second would mean HCl and ZnCl2, i.e. Lucas reagent.  I don't know what to do if it's the first. Wnt (talk) 21:24, 12 March 2016 (UTC)