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November 12

Mechanical hierarchy - fixing appliances.

Q1. When it comes to HVAC, PTAC (fixing air conditioners, central air), is there a hierarchy for most-roundedness? I personally think fixing microwaves is the most well-rounded, especially if you also know how to fix washing machines. So if you know how to fix microwaves, it should be easy to learn how to fix something else?

Q2. I'd also like to know what's 2 different fields with low-overlap? For example, someone who is an electrician, does not know how to fix ovens. And someone who fixes ovens, does not necessarily know how to be an electrician? So if you train to fix ovens and an electrician, not much overlap?

So, to summarize, Q1 = someone who can fix A, can fix A and B, but someone who can only fix B, does not know how to fix A. Would microwaves be a good candidate? And Q2 = A and B do not overlap. Thanks. 67.165.185.178 (talk) 03:59, 12 November 2021 (UTC).

The term "fix" has too broad a meaning to answer this. Let's stay with the microwave. Fix it by replacing the broken turntable belt. Sure. Easy. Fix it by repairing a blown circuit board with your soldering iron - not so much. By "fix" do you mean replace a faulty component with a part you found on ebay or do you mean taking your multi-meter and oscilloscope any diving into the guts and diagnosing the the faulty component on the circuit board, remove a chip or blown capacitor and solder a new one in? 41.165.67.114 (talk) 06:56, 12 November 2021 (UTC)

Sounds like your definition of fix is the same, just a matter of components and intensity. But you could take this to the farthest level: someone who knows how to build a microwave from scratch, and this write tutorials on how it works and how to assemble 1. So, going back to my analogy, I take it people who can fix microwaves, can fix toasters, but people who only know how to fix toasters, would not know how to fix a microwave? 67.165.185.178 (talk) 12:42, 12 November 2021 (UTC).

Someone who can fix a microwave whose turntable stopped rotating by replacing a broken belt may be totally helpless with a toaster that stopped working or causes a short circuit. No one in their right mind is going to build a microwave from scratch. Today, any class of appliances requires specialized expertise. The owner of an appliance may be out of luck anyway when it stops working for some trivial reason – many are designed so as not to be fixable for a cost less than that of a complete replacement. It is somewhat pointless to seek to define a hierarchy.  --Lambiam 13:40, 12 November 2021 (UTC)

Yes, but isn't there a principle where if you can fix the circuit of a __, then you can also fix the circuit of something else? Is what I'm looking at. And therefore familiarity with what to replace. 67.165.185.178 (talk) 14:06, 12 November 2021 (UTC).

If the "something else" is really something else, and not a more specific instance, you'll be hard-pressed to find a non-trivial example. In these days of integrated circuits and mass assembly, an all-round electronic technician will generally not be able to fix any circuit other than replacing it wholesale. They may be able to diagnose and understand the problem, but that will do them little good in fixing it.  --Lambiam 14:21, 12 November 2021 (UTC)

Sure, "more specific instance." Don't appliances have these in common? So if you know how to replace 1, you can replace it in a different appliance. 67.165.185.178 (talk) 14:27, 12 November 2021 (UTC).

"What did you say your specialty is?" — "I am a fully licensed RX 6700 XT replacer."  --Lambiam 13:12, 13 November 2021 (UTC)

Ah, repair, repair, a topic I love so very much...

I think there do exist specific contexts where there are strictly defined hierarchies: environments fraught with rules, regulations, ratings, and certifications...

In an environment like a medical laboratory, a military unit, or an aviation-related industry, specific repairs are enumerated, tracked with paperwork, and are permitted to be performed only by authorized and appropriately trained individuals. (Whether any particular person or organization complies with those rules is out-of-scope, for this discussion).

One example that I am specifically familiar with pertains to aviation. The ambitiously-named "Part 43 - Maintenance, Preventive Maintenance, Rebuilding, and Alteration (also known as Title 14 C.F.R. Chapter 1 Subchapter C Part 43) lays out some rules. § 43.3, "no person may maintain, rebuild, alter, or perform preventive maintenance" ... (unless that person complies with the rules and regulations so verbosely defined). And how verbosely, exactly, is that? Well, Appendix A of this part defines what things count as "major repairs", "major alterations," and so forth. This list is literally on the test. It is not only a dumb idea, it is almost always illegal to perform those repairs except in strict compliance with the rules.

In the aviation industry, mechanics and repairmen are clearly defined: they are individuals who have been trained and tested, and hold appropriate certificates and ratings, § 65.1. The maintenance and repair work they perform is authorized, itemized, inspected, and logged. So, the bit about fixing microwaves: is the microwave attached to an airplane? If yes, there is mandatory paperwork associated with its repair! And am I simply blathering about a bunch of nonsense, as usual? Absolutely not : there really are really real reasons to be concerned about the microwave that's attached to your airplane, and you want to know that the person fixing it is qualified.

Paperwork to the rescue!

For medical devices, military equipment, and so on, you can bet that there are similar rules, regulations, and paperworks. I don't happen to be so intimately familiar with those rules, and I haven't memorized them, but along those same lines, medical people have strict rules about which people can do which tasks, sorted by which trainings and certifications and current experience requirements those people meet. Are these "hierarchies?" Well, not strictly: a doctor can do a superset of tasks that a nurse may perform, and a nurse may perform a superset of tasks compared to a Paramedic, or an EMT, or a phlebotomist, or an Ambulance Driver, and so on down the so-called hierarchy; but there are things that a technician is trained to do that a doctor is not trained on. It would be a good idea for the doctor to delegate such tasks to the trained individual, rather than pulling rank and pretending to know how to do it "better." Here is a lovely introduction from the FDA: Remanufacturing and Servicing Medical Devices: The distinction between "remanufacturing" and "servicing" is important to understand. How can one not fall in love with this degree of Government specificity? I wish everything in engineering was so well-specified!

For consumer goods like household appliances - in the home setting - one probably doesn't consult the paperwork for every type of repair; but generally, consumer products lay out those items of service that should be performed only by an authorized service-provider; such repairs can be certified (a word that I love very much, because carries so much more grammatical gravitas than the word we use to describe aircraft repair operations, e.g. "work performed by a certificated individual"). Those rules are there for a variety of reasons - legal liability, cost, safety, ... consumer satisfaction (... the manufacturer probably knows more about the device than the consumer, and knows that a shoddy "DIY" at-home-repair can make a broken device into a more broken device, which on the average makes the consumer less happy). There are a lot of elements in the mix. Many users today claim that modern "devices" (at large) are "difficult" to repair, and some make the further claim that such difficulties are "on-purpose" (rather, "restrictions" implemented or encouraged by the device manufacturer). A well-balanced view is warranted: sometimes, such claims are valid; sometimes, such claims are not valid. These issues show up in the news, and in popular electronics- and consumer-products- websites - a lot. It is my personal opinion that most of the inflammatory rhetoric on this topic doesn't really contribute much factual information. So:

For a fun read on the topic of Repair, here's a very recent (May 2021) book: An FTC Report to Congress on Repair Restrictions, which gives a pretty solid introduction to some of the items of concern.

Nimur (talk) 15:32, 12 November 2021 (UTC)

Capacitor questions.

Q1. A capacitor stores electrical energy in an electric field. How can we measure a fully charged capacitor? Is there devices that do that?

Q2. How do we get electrocuted from touching capacitors? Is it from touching their dielectric component?

Q3. It seems all texts distinguish capacitors from parallel-plate capacitors. Are there any common household appliances, that use both capacitors and parallel-plate capacitors? Or do most devices, just use 1 type?

Q4. "Any 2 electrodes, regardless of shape, form a capacitor." Well electrodes are conductors. What if the 2 electrodes were semiconductors, then what kind of a capacitor would 2 semi-conductor electrodes be called?" Thanks. 67.165.185.178 (talk) 19:15, 12 November 2021 (UTC).

2. From short circuiting them with your body. Some capacitors can electrocute you just from putting fingers too close, or falsely show full discharge on measuring devices then partially recover enough to electrocute again. Sagittarian Milky Way (talk) 23:22, 12 November 2021 (UTC)

1: A multimeter or any other device that measures voltage.

3: Textbook parallel plate capacitors have a rather small capacitance. Maybe very small values (picofarad range) ceramic capacitors might be plain two plates. Usually the plates are wrapped to a roll, or there are many layered plates. See capacitor types or search the web/youtube for capacitor teardown.

4: A diode. Usually diodes are constructed to minimize their capacitance as it is generally undesirable, but varicaps are used specifically for their capacitance. 85.76.74.133 (talk) 11:14, 13 November 2021 (UTC)

I generally think of the electric field as in air, so I don't know how a digital multimeter can measure it. 67.165.185.178 (talk) 13:25, 13 November 2021 (UTC).

A charged capacitor has a voltage differential between its wires. A multimeter can measure that voltage. If you want to measure the energy in a capacitor, you need to discharge it through a resistor of some kind and measure current and voltage and integrate the product of those over time. If you are a visual learner, go to YouTube and search for e.g. measure capacitor voltage to see people test capacitors. While there, supercapacitor might find interesting stuff. 85.76.74.133 (talk) 15:57, 13 November 2021 (UTC)

Wait does electrocution from capacitors also include from parallel-plate capacitors? 67.165.185.178 (talk) 17:40, 14 November 2021 (UTC).

Do human lungs have more protection against smoke compared to other animals?

Our ancestors have used fire for more than a million years. Wood smoke is not good for the lungs, but our ancestors were depended on fire. Especially in colder regions of the World like in Europe, where people would have been sheltering in closed spaces like caves or tents, the exposure to smoke would have been significant. I've been in Arctic regions and been inside tents used by indigenous people. They use woodstoves inside tents, and while there is a chimney, the concentration of smoke inside is huge. After a while your eyes start to burn.

So, the question is whether after tens of thousands of generations living in such conditions, our lungs have evolved to neutralize the damage done by wood smoke to some degree? Count Iblis (talk) 22:56, 12 November 2021 (UTC)

• Yes, and moreover I have heard that dogs heal faster from burns on their paws and mouths than wolves. Abductive (reasoning) 21:49, 13 November 2021 (UTC)

November 13

How can a wave produce even without current?

This page says: When the current is removed, the field collapses which again sends a wave.

How can a wave produce even without current? Rizosome (talk) 00:05, 13 November 2021 (UTC)

It is the collapsing field, caused by the change in current, that sends a momentary wave.--Shantavira|^{feed me} 08:34, 13 November 2021 (UTC)

The page says that the current flow builds an electromagnetic field around the wire, but for a steady direct current a better description is that it builds a magnetic field around the wire. This magnetic field contains magnetic energy. A variable magnetic field induces an electromagnetic wave (see Maxwell's equations), so when the field collapses, a wave is produced; its energy is the energy that was stored in the magnetic field but is now released. In general, when the current varies, so does the magnetic field, and a wave is emitted. It does not have to be an on–off situation; an alternating current produces a continuous wave.  --Lambiam 13:02, 13 November 2021 (UTC)

Additionally, there can exist a displacement current - a current that does not involve any movement of charge - that is a mathematical representation of the change in electric displacement over time. This "theoretical" current has identical effect on the electric and magnetic field, just as though a true charge was moving. A time-varying displacement current can induce an electromagnetic wave, with- or without- physical movement of charge. Understanding and modeling this effect, and its impact on the full wave equation, becomes critical for the practical engineering design of high-quality antennas, evaluations of interference, design of microelectronics, and so on.

This is in addition - literally, actually added - to the current (the change in charge per unit of time). This key insight is described mathematically, and in its historical context, in many of our articles including the extensded Ampère's Law; also sometimes known as the completed, or corrected, law.

Nimur (talk) 13:45, 13 November 2021 (UTC)

Lambiam says: an alternating current produces a continuous wave. But where exactly this continuous wave produce in radio circuits which was the main topic of this question? Through radio speakers ? Rizosome (talk) 09:06, 14 November 2021 (UTC)

The original question was about producing a wave without current. The answer to the new question is that the radio waves used in communication are produced by a transmitter; our articles Radio wave and Transmitter give further explanations.  --Lambiam 10:49, 14 November 2021 (UTC)

The line: "the radio waves used in communication are produced by a transmitter" solved my doubt. Rizosome (talk) 00:10, 15 November 2021 (UTC)

Resolved

Why are very complex things like geoids modeled as a weighted sum of millions of spherical harmonics?

Why not just make a table? Pixel number 1 is 3, pixel number 2 is 4, pixel number 3 is 4 and so on. If you have enough pixels (RAM is cheap) you don't even need to care about distributing them evenly, it could just be x pixels per circumference in both latitude and longitude.

Sure with the sum of unrelated harmonic shapes way all ∞ points have defined values, to any false precision you want but if you must have that then just define the model value of points not listed as linearly interpolated.

I read that they're only up to the 360th level of spherical harmonics till 2008 (>130,000 coefficients giving only 55-110km resolution) and are barely at 10 nautical mile resolution now in spherical harmonics, can that even show Mount Everest? It must take a painfully long time to brute force 4 million (!) coefficients till it's not worse than the actual data, can someone explain why they bother? For 4 million bytes they could make a lookup table even better than 10 nautical miles to a height precision they can't even measure. Sagittarian Milky Way (talk) 04:21, 13 November 2021 (UTC)

You write "bubbles that are repulsed by each other and attracted to the origin and maybe have size limits or surface tension(?)". I think you misunderstand the nature of mathematical functions. The sine and cosine functions are neither repulsed by or attracted to each other or anything else, and the same holds for the spherical harmonics. Where did you read that the spherical harmonics representation of the geoid only gives you barely a 10-mile resolution? This statement does not make sense. To cover the surface of the geoid with pixels about the size of a circle whose diameter is 10 nautical miles, you already need some 60 million pixels. A geoid is supposed to model what is basically a gravitational equipotential surface, on which Mount Everest does not make a bulge that is noticeable without magnification. EGM2020 gives a 10 cm (3.9 in) accuracy. What more do you want?  --Lambiam 12:43, 13 November 2021 (UTC)

I know what basic trig is, I use 90° minus inverse tangent all the time. Cause I don't have a cotangent button. It's the sphere harmonics I'm very fuzzy on. My chem teacher showed electron cloud shapes and said the shape is cause they can't collapse from attraction cause quantitized energy level reasons and the negative charges repel. For all I know they rarely look like sphere harmonics cause clouds of different energy levels distort each other or something. So what is a circle or sphere harmonic? Why are these shapes the harmonics of a sphere and no others? Shouldn't there be one with 4 eggs in a tetrahedral orientation? Why is there a double tetrahedral thing with 8 eggs? Why is 6 eggs in a plane on the same level as that? Why is level two 3 copies of the same thing, just aligned with x, y or z? Why do some shapes get cloned (even down to orientation and polarity)? It's right there in the section, it says 1/6th degree horizontal resolution. Which would only need 2160*1080 pixels by the way, not 60 million. Or ~41,252*36 if you make them equal area. There are only 200 million square miles on the entire Earth. Sagittarian Milky Way (talk) 00:41, 14 November 2021 (UTC)

This is a why question; the answer is astonishingly easy! People who use geoids prefer using harmonic series. It is literally a matter of preference, colored by the practical ease of use. If you work the math using these mathematical forms, the math is easier. We can use more difficult math to describe the same shape - but this kind of infinite series is the best and easiest math for certain jobs. It is the math we are familiar with, and that we train for, and that we use in many other places, so it works well.

When I learned Calculus, using this famous and popular book, the first several chapters were about sequences and series. My inclination was to skip these and go "straight to the integration and derivatives." I specifically recall one very smart professor telling me that I would not be very good at most of mathematics if I didn't spend a little time learning to love the methods of studying infinite sequences... it is such a succinct and accurate statement, and it rings true. Series representations show up everywhere and they are very helpful for getting real work done.

Nimur (talk) 13:55, 13 November 2021 (UTC)

What kinds of things do model users need to do that's inconvenient even with today's fast computers if done with a lookup table but less so if done with a sum of 4 million individually bad electron cloud shapes? (or large numbers of other shapes in other fields, it's cool that if you had computing time to spare you could find a sum of trig curves that reproduce heyjude.mp3 to better than 1-sigma but the file already exists, would the trig equations be helpful for anything like frequency analysis?)

If millions of mutually-correcting terms is the easiest way to represent a geoid or Google Earth's exact world elevation map with math (I suspected that) then math is just not good at representing it. Especially compared to many other shapes like conic sections of revolution or Pringles®.

Another question: If you used a better resolution model of a hairless standing nude man to model his outer surfaces (skin, eyes, nostrils or ≤~1 nostril diameter into the nose etc) with 4 million 3D graphics triangles and 4 million trig curves and 4 million sphere harmonics how would the 3 fidelities compare? Sagittarian Milky Way (talk) 16:13, 13 November 2021 (UTC)

As it happens, approximating the audio signal as a series of trig functions is exactly how a MP3 file works… because that’s a very efficient and meaningful representation of the data. Your ear does something similar before sending the nerve signals to your brain. —Amble (talk) 17:17, 13 November 2021 (UTC)

I guess that makes sense since a sine wave is the simplest sound and each mp3 curves are apparently only fitted to very short blocks, not the entire song (how hard would it be to brute force a set of sine waves that play for the entire song?) Sagittarian Milky Way (talk) 18:56, 13 November 2021 (UTC)

That would be the DFT of the entire time sequence. Not difficult, but the block structure makes it much easier to do things like on-the-fly encoding and decoding, streaming, and seeking, and it avoids the infinite impulse response (IIR) problem where a big transient like a pop or bang could result in “ringing” through the entire song. —Amble (talk) 19:21, 13 November 2021 (UTC)

Even more than that, a single continuous DFT would not be an efficient representation of the song Bea use it would require a large bit depth (very high numeric precision on every sine and cosine) to avoid degrading the audio. So you would either get a much larger file, or a worse-sounding one, or (most likely) both. A format like MP3 is based on a sophisticated psychoacoustic model so that the digital representation of the signal in both time and frequency is similar to how you perceive it in time and frequency. That way, the file can keep the components you hear, and drop other components that you would not notice anyway. That’s why it is able to reduce the file size and still sound good (or at least OK). —Amble (talk) 19:38, 13 November 2021 (UTC)

• The answer is harmonics are easily modeled using fourier series. Indeed, any function in any number of dimensions can be modeled as an infinite series of harmonic functions. Calculating these by hand is hard, but not for computers. --Jayron32 12:04, 15 November 2021 (UTC)

I think our OP might benefit from reading about convergent series, about sampling, and about coordinate transforms. Raster formats (the sort of things we commonly intuitively understand as "pixels") are only one way of thinking. We can be much more efficient - whether we're working by hand or with a computing machine - by being creative and opening up our minds a little wider. If the connections between these concepts, and the OP's questions about geoids, still aren't obvious - I'd recommend stepping away from the computer, grabbing a copy of the book I recommended earlier, and drawing some curves on paper until the first glimmer of insight emerges. The opening question asks, "why not just make a table?" To this, all I can answer is... why not?

Nimur (talk) 17:50, 16 November 2021 (UTC)

I'm not smart enough to deduce what a lot of the math symbols I never learned stand for (like u and counterclockwise δ), so the alienese can't help me know how the fuck they make these interesting shapes. In fact the lead image made me think it looked like lobes, which is surely a useful interpretation of the equations for some purposes but if it had been the simplest possible line drawing of the pyramid of spheres cut into bands and gores, with a pyramid of spheres with the red/white/blue shading underneath then I would've understood instantly what the fuck these are. Not totally of course but a core idea. And buried in the alienese (I never did like learning languages) is a sentence or two saying these are also sphere vibration modes and "string/sine vibration modes but 1-dimension higher" other very interesting ways of saying what these are that I couldn't have otherwise figured out except possibly after going down a rabbit hole of links full of dense math. Eventually I plan to learn some of the math I never did to understand a bit more of the alienese in science but not yet. Having finally learned what sphere harmonics are the truncated series of the 4.6 million simplest of the infinity sphere harmonics converging on partially non-symmetric geoid (or other semi-random sphere maps like Big Bang radiation) data thing seems a bit more "pure" to me. Sagittarian Milky Way (talk) 19:52, 16 November 2021 (UTC)

If you're not interested in learning the answers to your questions, you should stop asking them. You not wanting to put in the work to understand the answers is not our problem. --Jayron32 13:07, 17 November 2021 (UTC)

Wouldn't it take me at least ~a year to learn the math scientists know? Sometimes I want to know things like "is there an equation to assign a strength of continentality number to a climate?" or "how quickly would the ground track of a plane aligned with the Leaning Tower of Pisa deviate from the ellipsoid geodesic with the same initial azimuths?" but the math looks like hieroglyphics to me. I'm too uneducated, I left before calculus and didn't retain everything after math quickly went from easy (signed PEMDAS, slope intercept, FOIL, basic trig..) to painful stuff I only retained enough to avoid summer school without cheating. Well I "cheated" with $x={\frac {-b\pm {\sqrt {b^{2}-4ac}}}{2a}}\ \$ ≥90% of the time just to finish homework faster.

If you know what's the closest thing to a flowchart or categorization system of alienese by visual appearance that would be nice. Especially if doesn't just say e.g. "it's vector calculus" but also shows a "tech tree" or "Mohs scale" of narrow subfields you must know to comprehend, e.g. it could mention vector calculus, integration and/or differentiation, vector conventions etc. If there are any common uses for something (e.g. find area under a curve) or possibilities for what a symbol stands for (e.g. if this equation has a τ it could mean time) listing those would be nice too). (Sub)categories might have names like "equation with lone big Greek letter on left edge", "equation with lone big uppercase pi on left edge (signature of a ... equation)", "symbols resembling Latin letters but not close enough to be mistaken for them (listed alphabetically by resemblance)" and "symbols that either are Latin letters or resemble them close enough to possibly be mistaken for them" Sagittarian Milky Way (talk) 21:20, 17 November 2021 (UTC)

Don't get too hung up on the "lobes" – the spherical harmonics only look like that when plotted individually, and they look like that because they are zero in some directions. When you're modelling the geoid (or the CMB temperature fluctuations) you are starting with the sphere (the lowest-order spherical harmonic) and then you're adding small amounts of the higher harmonics to that. Take the next row of harmonics, these are the dipoles, there's three of them for the spatial directions – by summing them with the correct coefficients you can make dipoles in any direction. If you add a bit of one of them to the sphere, you increase the radius of the sphere in one direction, and decrease it in the opposite direction. Where the dipole is zero (call that the equator) the sphere is left unchanged. As I said above by using all three basis dipoles you can distort the sphere in any direction you want. The next row is the quadrupoles, add one of them again to the sphere (remove the dipoles for the moment) and the sphere will bulge out in the direction where the quadrupole is blue, bulge in where the quadrupole is yellow, and be unchanged in those directions where the quadrupole is zero (for the left one that would be in longitude 0, 90W, 90E and 180, and of course in the direction of the north and south poles). It's not as easily explained why there are five basis quadrupoles, but as with the dipoles you can create any quadrupole with any orientation by suitably combining those five. Do the same for the higher spherical harmonics, the octupoles etc., and you distort the sphere on ever smaller scales. The whole process is analogous to a Fourier series, there's a similar convergence theorem (any function on the sphere, i.e. any function of two angles, say longitude and latitude, can be written as an infinite sum of spherical harmonics), but for modelling you always use a finite number of terms, so a finite sum, no need to worry about convergence. Spherical harmonics have mathematical properties that make their use very convenient - the basis harmonics as they are shown in the article are orthogonal to each other, i.e. maximally independent. They are actually an orthogonal basis of a infinitely dimensional vector space (the three dipoles for instance are the basis vectors of a three-dimensional vector space). I don't think it's necessary to learn a lot of calculus to get a bit of an intuitive feel for how this works... --Wrongfilter (talk) 18:19, 17 November 2021 (UTC)

Yes I agree, it's a lot simpler than the math looks. I thought lots of square degrees' radials pierced the lobe twice which confused me. And negative lobes means you shave the sphere there, now I see what the lobe representation means. And the coefficient can be negative which is why you need 3 dipoles, not 6. Sphere harmonics are more aesthetic than they seemed! This must also be why deflection of the plumb bob is given as deflection N-S & E-W but no actual deflections or azimuths anywhere: math people must like splitting up stuff into a sum of orthogonal subcomponents Sagittarian Milky Way (talk) 22:15, 17 November 2021 (UTC)

November 14

Sensitivity to coffee

When someone says he or she is sensitive to coffee, what does that mean?2603:7000:8102:657B:412:4DDE:E611:E730 (talk) 01:58, 14 November 2021 (UTC)

Probably about their sleep schedule. 67.165.185.178 (talk) 02:06, 14 November 2021 (UTC).

It could mean a LOT of things. It could mean sleep schedule, as suggested by 67.165.185.178, or it could mean digestive irritability to small amounts of coffee, it could mean they taste even a small amount of coffee added as an ingredient (I don't often notice a small amount of espresso added to chocolate baked goods to boost the chocolate flavor, but my wife does and hates it), and probably several more things. --OuroborosCobra (talk) 16:49, 14 November 2021 (UTC)

There are many adverse effects to the caffeine in coffee any one of which might be described as "sensitivity". Mike Turnbull (talk) 17:13, 14 November 2021 (UTC)

Or perhaps they are sensitive to the plight of the coffee plant and its subjugation by Homo sapiens who covet its precious beans? nagualdesign 23:24, 14 November 2021 (UTC)

November 15

Deja-vu dreams

So throughout my life, I have been dreaming about scenes that later happen. When I have the dream, it will be an exact recreation of the scene, and I will not remember it when I wake up. Later on, sometimes its the next day, sometimes its a year or more, the scene will occur and i'll feel like i've already been in it and realised I dreamt about it before. An example would be me reading a book and thinking about one of the illustrations, and then a week later doing that exact thing. This seems to run in the family, as my grandfather had the same kind of dreams. I have not been able to find any scientific explanation for this phenomenon. Does anyone know one?

Thanks, Luke — Preceding unsigned comment added by 81.178.75.46 (talk) 13:43, 15 November 2021 (UTC)

Yes, those are coincidences. 67.165.185.178 (talk) 14:25, 15 November 2021 (UTC).

Déjà vu, the Wikipedia article, has some information on dreams as well. Though not linked, it does list several studies for further research you could explore. --Jayron32 14:27, 15 November 2021 (UTC)

See, in particular, the "Déjà rêvé" subsection and the study cited at the end of it. Deor (talk) 15:55, 15 November 2021 (UTC)

The cited study in PDF is free to download from the Heidelberg University Library site in here. Alexcalamaro (talk) 20:03, 18 November 2021 (UTC)

Someone having a déjà vu experience (I experienced this before), realizing there was no such prior experience in real life, may easily be inclined to think their false sense of familiarity stems from an earlier dream experience.  --Lambiam 15:04, 15 November 2021 (UTC)

1) if the dream happens first, it isn't a recreation (exact or otherwise), 2) are you sure you remember the dream accurately enough to compare it to the reality? (Heck, are you sure you have observed the reality accurately enough to compare it to the dream?). --Khajidha (talk) 16:47, 18 November 2021 (UTC)

@Khajidha: Sorry for the late reply, but yes, I do believe that the reality is completely accurate to the dream in every way I can think of, even to the point of me remembering I had a dream about it (In the dream, I have deja-vu about having the dream, if that makes any sense).

Thanks you for helping, Luke

November 16

Herd immunity thresholds

When calculating herd immunity for a given disease, what geographical level is typically considered - a country, a city, etc.? Particularly, does it make sense to calculate herd immunity for city only? Say, if per Herd_immunity#Theoretical_basis, 82% of people are immune to pertussis, but only within a city, would it be considered safe even if on a country level the value is much lower? 212.180.235.46 (talk) 18:37, 16 November 2021 (UTC)

• The idea behind herd immunity is that even as the virus or bacteria is still being transmitted, it is not being transmitted in a self-sustaining way. So if a person is infected, they will infect, on average, fewer than one other person. So the disease will die out. So then the question is, how to you define a herd? By the degree of isolation from other herds, one supposes. Is the population of a city a herd? I suppose it really depends on how much members of the population travel outside the city, and how many travelers visit the city. Abductive (reasoning) 04:28, 17 November 2021 (UTC)

Many experts now believe that Delta is so infectious (think measles), even with the best vaccines we will not achieve herd immunity, which I think of as human sacrifice. With lesser diseases, it's possible. Imagine Reason (talk) 16:58, 17 November 2021 (UTC)

on that. Also, vaccines are not just about "never being infected". They also lessen the severity of disease, which appears to be the greatest value in the current pandemic; people are having many more asymptomatic or mild disease because of the vaccines. Regarding vaccines and the Delta variant, per the CDC: [1] Vaccines continue to reduce a person’s risk of contracting the virus that cause COVID-19, including this variant. Vaccines continue to be highly effective at preventing hospitalization and death, including against this variant. Fully vaccinated people with breakthrough infections from this variant appear to be infectious for a shorter period. --Jayron32 17:19, 17 November 2021 (UTC)

I have read everywhere in the news some variation of the claim that vaccinated people, if infected, have less severe forms of the disease than non-vaccinated people; whether for the Delta variant specifically or for all strains of sars-cov-2. That claim seems plausible both from a biological point of view (the vaccine gives your immunity system a "head start" even if it failed to entirely prevent infection) and from a cursory look at the statistics (the ratio of hospitalization to cases, or deaths to hospitalizations, is lower for vaccinated people). Nonetheless, many plausible things are false, so I would very much like to see a proper statistical study (which the CDC page does not link to). Tigraan^{Click here for my talk page ("private" contact)} 14:11, 18 November 2021 (UTC)

Here is a synopsis from Johns Hopkins. Here is a short article published in JAMA which links to a NEJM published paper. Here is an article from Yale University citing a Lancet published paper. Here is a NEJM paper that comes to the conclusion "Most breakthrough infections were mild or asymptomatic, although persistent symptoms did occur." Here is an article from the University of Minnesota citing three more studies that show "COVID vaccines protect very well against the Delta (B1617.2) variant, with unvaccinated people having 5 times the risk of infection and more than 10 times the risk of hospitalization or death compared with vaccinated people." There's many more out there, as a google search will show you. --Jayron32 12:06, 19 November 2021 (UTC)

Thanks. This paper one click away from your second link was the sort of thing I wanted, but could not find by an online search, because I did not know the keyword "breakthrough infection". It does shows lower viral load and lower symptoms, and I see no obvious selection bias in the methodology. Tigraan^{Click here for my talk page ("private" contact)} 16:47, 19 November 2021 (UTC)

November 17

Instrumental analysis in the biology department?

So I've never taken a course in biology before, what is the biology equivalent for the chemistry course instrumental analysis? In chemistry, I'm familiar with GC for gases, HPLC for liquids, and UV-vis for liquids and gases and fluorescence for liquids, and flame AA for solid ions. What instrumentation are in biology department, I guess they use a Eliza system. So, suppose I wanted to take a sample of tap water, and what to identity what organisms are in there? What would they use, just a microscope? A microscope can never visually see chemical compounds, but can it work for larger organisms like bacteria and fungi? 2601:249:8200:A640:B03D:42C6:D0C4:CF22 (talk) 05:13, 17 November 2021 (UTC).

• Biology laboratory instruction takes a more incidental approach to the tools needed, usually introducing them with whatever is being taught the same day. For example, pipetting is introduced in the first year, as part of the lab on gel electrophoresis, maybe with a couple of practice runs using water dyed with food coloring. Abductive (reasoning) 05:22, 17 November 2021 (UTC)
  • Automated cell counters are becoming increasingly common. --Jayron32 13:05, 17 November 2021 (UTC)

...and are very unreliable in my experience.Fgf10 (talk) 13:57, 17 November 2021 (UTC)

There is literally a near infinite suite of analysis techniques used in biology, including GC, HPLC and UV-Vis. Biology encompasses a good chunk of the techniques from in chemistry in particular (hence biochemistry), and depending on the field quite a bit of physics too. You could start with the by no means exhaustive Category:Laboratory_techniques. For your example, it depends on what you're after, but you could start with a visible light microscope yes, but these days you'd be more likely to get a more comprehensive answer with PCR Fgf10 (talk) 13:57, 17 November 2021 (UTC)

Many eukaryotic (animals, plants, fungi, and "protists") organisms could be identified with a simple light microscope, not always to the exact species. In many cases, several species of microorganisms are possible but which exact one is present might make little difference. Bacteria could be identified by culturing on or in various media (gels and broths) to see 1) if it grows at all, 2) how much it grows, 3) what colors and shapes its colonies take up, 4) how it affects the surrounding medium (for example, some bacteria will produce pH changes that interact with indicators in the media to produce color changes). --Khajidha (talk) 16:53, 18 November 2021 (UTC)

Respirator thickness redux

Extending a previous question, I have just received KF-94 respirators for the first time, and they're noticeably thinner than KN-95s, which as I said previously seem to be thinner than N-95s. Imagine Reason (talk) 17:01, 17 November 2021 (UTC)

Is there something we can help you find references for? --Jayron32 17:09, 17 November 2021 (UTC)

I've used lots of different respirators and n95's tend to be thicker. If you are suspicious of your kf94's, there are ways to test them that are borderline feasible for DIY'ers. Basically you spray an aerosol of salt particles through the mask, and use an air quality monitor on the other side to check how many particles get through the filter. There is a guy on youtube who tests various masks that way but I no longer have the url for his channel. Maybe you can find it. These days though, real n95's are plentiful enough that I have not felt a need to mess around with dubious masks. 2602:24A:DE47:B8E0:1B43:29FD:A863:33CA (talk) 06:05, 18 November 2021 (UTC)

Is it better to boil tap water, then cool it to room temperature?

Hot tap water contains calcium carbonate that regular tap water does not have. So if I boil regular tap water to just below 100 C, then cool it back to room temperature, is it better than using regular tap water?

I know the answer for 2 variables, but I don't know if that is all the variables. For bacteria, yes, and chemicals, mostly yes. Bacteria generally do not have a cardinal temperature of more than 40 C range, (so E. Coli for example, ranges from 8 C to 48 C). Boiling from 25 C to 90 C will definitely destroy/deactivate it. However, tap water, I believe, has more of a chemical problem than a biological problem. I went to the EWG website, and searched for my jurisdiction, to what chemicals are found in my tap water. From that list, only 2 chemicals will be boiled to a gas: chloroform (bp 61 C) and bromodichloromethane (90 C), but will not boil these chemicals: dibromochloromethane (120 C), dichloroacetic acid (194 C), trichloroacetic acid (197 C), and dibromoacetic acid (decomposes 234 C). But, what I don't know is false positives. What if boiling tap water, causes something to decompose, and decomposes to 2 or more, that are more toxic than what it was before, so therefore it should have been left at room temperature? And maybe other factors you guys can think of? Then, for chemicals that are metals found in tap water, of course boiling it then cooling it makes no difference to them. So sounds like there probably is no disadvantage to cooking tap water? I'd also like to know if boiling water can cause 2 chemicals to form combination reactions, and that new product is more toxic and has a even higher boiling point. 2601:249:8200:A640:F9D6:6DF0:48CF:530E (talk) 21:48, 17 November 2021 (UTC).

I am not sure that it is true that hot tap water contains more calcium carbonate than cold tap water. Calcium carbonate is unusual in that its solubility decreases as the temperature of the water increases. Also, some calcium carbonate in your drinking water us not necessarily bad: no adverse health effects of hard water are known; it may even be beneficial to heart health. Boiling water and then letting it cool back will not appreciably affect the concentration of dissolved salts and other minerals. Boiling the water may kill or deactivate pathogens, but if the concentration of any chemicals poses a health risk, I'd switch to bottled water.  --Lambiam 07:29, 18 November 2021 (UTC)

Any calcium in tap water will be there from the outset and boiling will not alter that. As Lambiam said, the main reason to boil water is to destroy pathogens or because you are making a hot beverage. Your IP address shows you are in the US, where municipal tap water is of high quality, so for the sake of the planet (and your energy bill) I suggest just using the water as supplied and certainly not using bottled water, which is much less environmentally friendly and relatively expensive. Mike Turnbull (talk) 10:56, 18 November 2021 (UTC)

The drinking water supplied by US municipalities is not necessarily safe coast-to-coast; see e.g. Lead contamination in Washington, D.C., drinking water.  --Lambiam 12:10, 18 November 2021 (UTC)

True, but boiling doesn't remove lead. I believe that the above advice against boiling was meant to imply that US water supplies are generally free from pathogens that boiling water can fix. Other than temporary "boil water" orders that happen when water supplies are contaminated by line breaks and floods due to natural disasters, I am not aware of any widespread cholera outbreaks in the US. The US water supply is generally safe from that sort of thing. --Jayron32 12:49, 18 November 2021 (UTC)

Next to the advice against boiling water, there was also a plea to not use bottled water – presumably in reaction to my suggestion to switch to bottled water when the concentration of any chemicals presents a health risk. Such a risk would generally not be mitigated by boiling.  --Lambiam 14:37, 18 November 2021 (UTC)

It is true that carbonate content in municipal water is not detrimental to humans (or at least, not in general; consult your doctor if you have medical issues that make it different for you). However, the OP might well be trying to use that water for some appliance (e.g. a coffeemaker) which does have problems with water hardness. Of course, Lambiam’s answer still stands (any water you get from the tap, hot or cold, comes from a single municipal water pipe, whatever heating-cooling cycles occur at your home are unlikely to affect the carbonate content in any significant direction). Tigraan^{Click here for my talk page ("private" contact)} 14:04, 18 November 2021 (UTC)

Isn't calcium carbonate found in hot tap water because of the hot-water boiler? If you pour hot tap water and let it stand still, you will see it is cloudy/foggy, compared to cold tap water that is clear. That foggy substance is calcium carbonate. Anyways, I just look at the decomposition reaction article and saw solids will have a decomposition point in place of a melting point. What about for liquids? Can there be liquids that have a decomposition point in place of a boiling point? 2601:249:8200:A640:F9D6:6DF0:48CF:530E (talk) 14:14, 18 November 2021 (UTC).

Certainly their are. But not water or calcium carbonate; the cloudiness is not because new calcium carbonate is being created out of thin air; it is merely becoming more visible because it is precipitating very slightly at the higher temperature, heating and cooling water with calcium carbonate in it does not change the amount of calcium carbonate; to remove it you would need to use some sort of water softener. --Jayron32 14:21, 18 November 2021 (UTC)

There will generally be an equilibrium when using a boiler: as much calcium carbonate flows out as flows in. The build-up of limescale in the vessel, mainly calcium carbonate deposit, means that actually less flows out than flows in, although the difference is truly minute.  --Lambiam 14:37, 18 November 2021 (UTC)

Okay, I checked my notes. Says calcium carbonate is formed from calcium ions and bicarbonate ions when heated. But this is irreversible in the sense that cooling it, will not decompose it, cooling it will just keep it as a solid. 2601:249:8200:A640:F9D6:6DF0:48CF:530E (talk) 15:03, 18 November 2021 (UTC).

Quite possible, Carbonate equilibrium is complex, in solution there will always be an equilibrium between CO₂, CO₃^2-, HCO₃^- and H₂CO₃, the exact ratios are dependant on things like temperature, pH, and other dissolved ions. The only one of those that significantly precipitates with Ca²⁺ would be CO₃^2-. Indeed, if I were to fathom a guess, the formation of cloudiness at high temperatures is probably largely due to the increase in CO₃^2- ions due mainly to the increased solubility of carbon dioxide at those higher temperatures; as hot water absorbs more carbon dioxide from the air, more carbonate ions form, causing more calcium ions to precipitate out as calcium carbonate. --Jayron32 15:14, 18 November 2021 (UTC)

Actually for your last part, gases are more soluble in colder water than hot water, and I don't believe CO2 is an exception. So boiling water will cause CO2(aq) to part into a gas, or from the HCO3-. 2601:249:8200:A640:F9D6:6DF0:48CF:530E (talk) 16:26, 18 November 2021 (UTC).

You're correct, of course. Mea culpa. --Jayron32 11:55, 19 November 2021 (UTC)

November 19

How can a circuit breaker allow me to reset ?

A circuit breaker in my home was triggered because I turned on the multiple high amp devices how can I am able to reset it without turning off the devices? Rizosome (talk) 01:29, 19 November 2021 (UTC)

Maybe it's not "idiot-proof". ←Baseball Bugs ^{What's up, Doc?} carrots→ 03:45, 19 November 2021 (UTC)

If I try to understand your question. If you turn off your circuit breaker and turn it back on, vs. reset your circuit breaker without turning it off, of what difference is that? 2601:249:8200:A640:BC94:25D0:A27B:E49B (talk) 04:21, 19 November 2021 (UTC).

"Idiot-proof"? I am asking how circuit breakers allow me to reset without turning off high amp devices? Rizosome (talk) 07:47, 19 November 2021 (UTC)

If the circuit breaker trips, devices on it are likely to shut down. Are you saying they don't? ←Baseball Bugs ^{What's up, Doc?} carrots→ 07:51, 19 November 2021 (UTC)

Baseball Bugs It is very obvious that they are asking about putting the switches in the "off" position before resetting the breaker, not about devices magically being powered. Tigraan^{Click here for my talk page ("private" contact)} 10:00, 19 November 2021 (UTC)

The only way to do that would be to have your devices on different circuits. Be aware that some devices could be damaged if they are not turned on using the switch on the actual device.--Shantavira|^{feed me} 09:29, 19 November 2021 (UTC)

Are you concerned about the inrush current tripping the breaker again? The only way to avoid this is either turn the devices off before the breaker is turned on, and then slowly turn the devices back on. Or permanently put the devices on a different breaker. LongHairedFop (talk) 09:39, 19 November 2021 (UTC)

• To answer the OP seriously, the answer is you probably cannot. What usually trips a circuit breaker is that there is an excess of current on that circuit. If you make no changes to the circuit, such as trying to "reset it without turning off the devices", the breaker will likely just trip again immediately. If you are tripping a breaker, the best solution is to remove one or several of the high-amperage devices from the circuit that was being tripped, and plug them in on a different circuit. --Jayron32 17:03, 19 November 2021 (UTC)

There are a lot of situations where a marginal overload might not immediately trip a breaker. Appliances may require slightly more or less current depending on what they're doing at that moment. So if you're right on that line, you might be going above and below the breaker's rating without actually turning anything on and off. Besides that, depending on its technology, the breaker itself may take a few minutes to trip if it's right on the edge between good and too-much-current. This is actually considered a feature of home circuit breakers and is deliberately designed in.

Check out the illustration (and description) at Circuit_breaker#Low-voltage. It actually has separate fast and slow mechanisms for very over-current and slightly over-current situations. ApLundell (talk) 20:33, 19 November 2021 (UTC)

The most common reason that a circuit breaker trips, in my home, is that a light bulb fails. The circuit breakers are in a single unit but these are clearly marked as being for light circuits or for the ring mains/cooker socket and so on. In the case of a bulb failing, it isn't going to light up again when switched on, so I just reactivate the circuit and go look for the bulb that's now off and needs changing. For any other case, I'd advise "safety first" and always switch off the high-current devices individually before resetting the circuit breaker. Mike Turnbull (talk) 22:17, 19 November 2021 (UTC)

It's pretty rare for a bad light bulb to trip a breaker. Personally, I've never even seen it happen. If that happens regularly you may have some kind of problem. ApLundell (talk) 00:52, 20 November 2021 (UTC)

Anecdote: in the days just after the Second Great Northeast Blackout, the power supply was considered unstable at first and we were told to minimize usage during peak-usage hours. One night at 10 pm, I started watching a TV show, with the lights and air-conditioning off. At the first commercial break, I walked over to my computer terminal and changed the power authority's web site to confirm that the evening usage peak was over, which it was. I then turned on the A/C and the light switch nearest me. We still had incandescent lights on those days and that switch controlled two 150-watt bulbs. There was a momentary flash, and then the room was dark again — no lights and no TV either! I panicked for a few seconds: had I really triggered a new blackout? Then I noticed the terminal was still working, and realized the lights and the TV were on the same circuit as each other. One of the 150 W bulbs had blown when turned on, as they would so often do, but in such a way that the broken piece of filament caused a short-circuit within the bulb, which tripped the breaker. I wrote at the time: "I've probably experienced this three or four times in my life... But it's the first time when the lights I was switching on were going to be the only ones that were on! That was dramatic!" --184.144.99.241 (talk) 03:34, 20 November 2021 (UTC)

Spiroshell

I'm reading a trashy spy novel set in the 1990s UK. One of the characters suffers respiratory arrest and is put in a Spiroshell to recover. Web search finds that a Spiroshell is a breathing assistance apparatus, basically an iron lung for the upper body only. But there are remarkably few references online. Has anyone heard of this thing? By the 1990s, wouldn't they use a ventilator instead? (Late 1990s I think, since the characters have mobile phones). If you think it's appropriate, maybe make a redirect from Spiroshell to iron lung? Thanks. 2602:24A:DE47:B8E0:1B43:29FD:A863:33CA (talk) 11:44, 19 November 2021 (UTC)

I guess that novel might be this one (a mention is on that page). It's also known as a "Curiass ventilator"; and "Shell ventilator". see e.g., [2]. [3] (p. 454), [4], [5]. Wtmitchell (talk) (earlier Boracay Bill) 12:22, 19 November 2021 (UTC)

Whereas "cuirass" or "shell" refer to the type of respirator, closely fitted to the uper body, "Spiroshell" looks like a trade name, formed from "respirator" + "-o-" + "shell". I found two non-fictional uses in medical publications, next to those in Forfeit, enough to make clear this is not a fictional trade name. Neither was particularly informative; both appeared to assume that the readership was familiar with the term.  --Lambiam 15:09, 19 November 2021 (UTC)

More about cuirass ventilators here. Apparently they give greater control than other methods and "maintain and redevelop the respiratory muscles which may weaken with respiratory failure and mechanical ventilation, this allows patients to be weaned from a ventilator". Alansplodge (talk) 15:41, 19 November 2021 (UTC)

Yeah I also found the reference to Forfeit, but the novel I'm reading is much different.[6] (I'm not recommending it, it's entertaining enough to keep reading if you're already into the genre, but it's absurdly slow paced and has other serious weaknesses.)

Thanks for the find about cuirass respirators. Any idea if they are in widespread use? I had never heard of one before, and the iron lung article indicates that iron lungs are near obsolete now. I guess Spiroshells are much less invasive than ventilators. The person can talk, read, etc. while enclosed in one. So those are significant advantages. 2602:24A:DE47:B8E0:1B43:29FD:A863:33CA (talk) 21:10, 19 November 2021 (UTC)=

I'm no expert but I've been sort-of following this discussion out of curiosity. See change over time in this graph and the table in Polio § Epidemiology. Also, the Mechanical ventilation has related info. Wtmitchell (talk) (earlier Boracay Bill) 21:41, 19 November 2021 (UTC)

Oddly, we had redirects from Biphasic cuirass ventilation and Biphasic Cuirass Ventilation to iron lung but not one from the term that people would be more likely to search, namely cuirass ventilator. I'm just about to create that redirect. Mike Turnbull (talk) 22:01, 19 November 2021 (UTC)