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January 1

Body proportion

It is given in BWH measurement that it is expressed as circumference of circle (which means ellipse). So I want to know what is ratio of two diameters of this ellipse. Of course this ratio will be different for all 3 measurements. Also it will depend on person to person based on cup size etc. But what are those at an average. -- 2409:4043:4E82:106D:917E:C85D:86A5:17CB (talk) 10:23, 1 January 2022 (UTC)

The term "circumference" should not be understood here in its technical geometrical sense. The American Heritage Dictionary gives as secondary meanings: "The boundary line of an area or object" and "The length of such a line".^[1] Here it refers to the measurement as found by using a measuring tape. In mathematical terms, it is the length of the boundary of the convex hull of the shape of a cross section of the human body. The shape of that convex hull forms only very roughly an ellipse. Not only will the ratio of its minor to major axis vary wildly between individuals, but the averages will also depend on the age group and other population characteristics. If you must have a number, for a rough-and-ready guess for the all-out average I'd go with 4 : 5.  --Lambiam 14:53, 1 January 2022 (UTC)

[OP:] Thank you for answer. I was just wondering this question when I was sketching a female body. If say I want to make height as 5 feet (so I scale it to 5 inches on paper) and I want that BWH ratio as 36-24-36 (hourglass figure), So I was just thinking what should be length between left end to right end at these 3 points that I asked above. so I thought if I got ratio of diameters then I will use formula, circumference of ellipse = 2*pi*sqrt((R1^2+R2^2)/2), and will get radius value and then can make that drawing. I know this sounds stupid :) But now that you told that it is not ellipse then tell me what are those lengths that I want. -- 2409:4043:2B10:FE3B:70BC:AB06:5A07:E4F5 (talk) 17:14, 1 January 2022 (UTC)

Sorry, but there is no elementary function for the circumference of an ellipse in terms of the lengths of its semi-major and semi-minor axes; see Ellipse § Circumference.  --Lambiam 22:05, 1 January 2022 (UTC)

So is the formula that I wrote above, wrong? Was I taught wrong formula at school? -- 2409:4043:2B10:FE3B:9C71:27ED:72F6:7297 (talk) 05:22, 2 January 2022 (UTC)

I can't verify if you were taught this formula, but in any case, it is wrong. Set $R_{1}=1$ and let $R_{2}$ get very small. Then, as $R_{2}$ approaches $0,$, the formula approaches $\textstyle {2\pi {\sqrt {(1^{2}+0^{2})/2}}=\pi {\sqrt {2}}\approx 4.44288}\,.$ But the ellipse gets more and more like a pair of straight lines, each having length $2R_{1}=2,$ and the true circumference approaches $4.$  --Lambiam 10:52, 2 January 2022 (UTC)

I'm no specialist on female body ratios (sorry, I'm male and single, like your average Wikipedia editor), but I suggest that if you want to draw a body and want to know the ratios, just look at a number of photos, pick the ones you like, measure the ratios and calculate the average. There are even categories for photos of sparsely dressed women on Wikimedia Commons (because many editors are ... Well, you get it). Of course, there's some selection bias in those pictures. PiusImpavidus (talk) 19:03, 1 January 2022 (UTC)

By doing a Google Image search on "36-24-36" you'll find plenty of models. (Not this one.)  --Lambiam 21:30, 1 January 2022 (UTC)

OK thanks both of you. -- 2409:4043:2B10:FE3B:9C71:27ED:72F6:7297 (talk) 05:22, 2 January 2022 (UTC)

Does NPN transistor work only when circuit contain two voltage supplies and batteries?

This is the basic diagram of NPN transistor; Image

It has two voltage supplies and two batteries. Can it won't work if only one voltage and resistor present in the circuit? Rizosome (talk) 11:04, 1 January 2022 (UTC)

This is an amplifier circuit using an NPN transistor with one voltage supply and resistors. See Transistor#Transistor_as_an_amplifier for details. Philvoids (talk) 11:48, 1 January 2022 (UTC)

I got answer from this: This is an amplifier circuit using an NPN transistor with one voltage supply and resistors. Rizosome (talk) 13:17, 2 January 2022 (UTC)

Resolved

Snake Found

A long time ago, I am too old to remember when, I asked about a very long black snake seen near one of my rental cabins in the piedmonts of South Carolina. Last week, a renter ran over a very long black snake that appears to be the exact same as the one previously seen. It was just shy of eight feet long. I took it to Clemson to get it identified. It is an indigo snake. They are not normally found this far north, but the identification is certain. They are popular pets, so it is possible that this is a pet that got loose and didn't travel very far. I don't really have a question. I just wanted to give an answer to the question I asked previously as there was a lot of discussion about how long the snake actually was. My personal opinion is that because the cabin is only a few miles from Hollywild animal park, the snake likely escaped from there. Now that it is gone, I will go back to only warning the renters about rattlesnakes (who don't make noise here), copperheads, and moccasins, just in case they are not smart enough to leave snakes alone. It is nice to know that the huge snake was not a overgrown moccassin. 97.82.165.112 (talk) 17:22, 1 January 2022 (UTC)

The range of the Eastern Indigo Snake as seen here borders the South Carolinian part of the Piedmont. With temperatures rising, the ranges of cold-blooded reptiles may be expected to begin extending further North.  --Lambiam 21:17, 1 January 2022 (UTC)

I think you were looking at the map of the swampsnake. The indigo map is farther down in the page. It is coastal and rarely further north than Georgia. That's probably why it stayed near the cabin. Plenty of critters like to stay under it because it is semi-insulated to keep the pipes from freezing and the humans leave plenty of scraps around to eat. I've tried attracting owls and chipmunks, but I mainly just have snakes, spiders, and the occasional white squirrel. 97.82.165.112 (talk) 17:44, 2 January 2022 (UTC)

I was looking at the map with the legend "Eastern Indigo Snake (Drymarchon couperi)". The northmost point of the black area is less than 15 miles from Augusta, Georgia, which (at least according to the map in Piedmont (United States)) is at the border of the Piedmont in South Carolina.  --Lambiam 20:38, 2 January 2022 (UTC)

I see. "piedmonts" is not a scientific term. Locally, it refers to the low mountains along the North Carolina/South Carolina border. It is also referred to as the "upstate." Regardless, the herpetologist said that she never saw one so far north and that it was likely an escaped pet. The entire point was just to identify the snake and point out the actual length. 97.82.165.112 (talk) 15:23, 3 January 2022 (UTC)

"Piedmont" means "foot of the mountain(s)" and Piedmont links to a similar region in Italy. --←Baseball Bugs ^{What's up, Doc?} carrots→ 03:50, 4 January 2022 (UTC)

January 2

What are the main ways to do virtual reality?

I understand that "modern virtual reality games" are typically played with an headset and with one to four motion controllers (one for each limb), rather than with a keyboard and a mouse.

Is there anything else needed to play "modern virtual reality games" besides that?

And not less importantly, what is the "next stage" which is possibly already described deep in the engineering literature, I mean, what might be even more "realistic" than headset and motion controlling? Is there some "brain activity reading device" already under at least theoretical, development?

About the first question, I assume that Haptic suit and a Joystick may also come handy (if indeed, a joystick can't be virtualized), but about the second question I don't know. 109.64.238.180 (talk) 23:24, 2 January 2022 (UTC)

In a laboratory setting, researchers have successfully used a non-invasive brain–computer interface (BCI) to make a mouse control a cursor.^[2] For the state of the art in human BCI research, see Brain–computer interface § Non-invasive BCIs. Apart from practicality (is it wearable?) and cost, it is not immediately clear that applying this to controlling a virtual avatar will result in a more immersive experience; the proprioception is absent. What is missing most in current virtual reality game set-ups is haptic feedback, and the awkward existing technology that requires donning a full-body suit only provides a sense of touch with subtle forces. A player in a haptic suit may try to open a (virtual) door that is stuck by pushing hard, but there is no force pushing back, so then they cannot push hard either.  --Lambiam 11:22, 3 January 2022 (UTC)

Please elaborate a tiny bit more about "there is no force pushing back", what exactly did you mean physicswise and what do haptic-suit engineers are researching aiming to develop a solution for this problem? Thanks, — Preceding unsigned comment added by 109.64.133.223 (talk) 18:14, 4 January 2022 (UTC)

Initial high resistance to movement that becomes less when a button or lever is moved, simulating a return spring with negative spring constant, is a desirable characteristic of pushbuttons since it gives the operator an Illusion of control. Philvoids (talk) 23:25, 4 January 2022 (UTC)

Newton's third law: "To every action there is always opposed an equal reaction"; see Reaction (physics). This applies to an equilibrium situation. If (in reality) you push against a body, the force should normally cause the body to accelerate (according to the equation $F=ma$). If the body cannot move, as with a door that is stuck, $a=0,$ which implies $F=0.$ Your force is cancelled by an opposite force that is equal in magnitude. To make virtual reality feel realistic in this scenario, the player needs to experience an opposite force, a force that is actually pushing back, and pushing harder as the player pushes harder. Imagine that you are in a virtual reality environment that is pitch dark. This you can easily simulate by standing in the middle of a dark room – you won't notice the difference. Now imagine that you are standing before a door – in reality there is no door. Push hard against it. You can‘t. Neither could you if you had a headset and a haptic suit.  --Lambiam 23:33, 4 January 2022 (UTC)

Of course with doors, the player's imagination can usually fill in the blank, but it becomes more serious in situations where, in real life, you depend on that force feedback.

I've worked on training simulations that involve putting a wrench into a tight spot in an engine. In real life it's a little tricky, but you can mostly do it by "feel". But in VR, without feeling the wrench hitting the engine it's virtually impossible. The simulation creator often as to "cheat" and make the tool 'jump' to wherever it needs to be, or pass through obstacles. Depending on the type of training being done, that can be a problem. We don't realize how much we depend on our sense of touch until we have to do tasks without it.

A more game-realated example is swordplay. If you swing your sword, and the bad guy blocks it, your real-life hand keeps going, because nothing blocked your real-life hand. Now your simulated sword, and your real-life hand are in different positions. There are different ways of dealing with that in-game, but none are perfect. It would be nice if a real force could be applied to your sword hand. ApLundell (talk) 05:52, 5 January 2022 (UTC)

See Holodeck accident.  Card Zero  (talk) 01:04, 6 January 2022 (UTC)

Did you mean Holodeck malfunction?  --Lambiam 14:59, 8 January 2022 (UTC)

January 4

What is Knip et Prév. Pig.?

Alright, so I've been doing a bit of digging through taxonomic records for a couple of pigeon articles, and I keep seeing references to Knip et Prév. Pig. Does anybody know what exactly this is a reference to? I've tried looking it up, but I can not find anything on it outside of more abbreviated references in 1850–1900ish taxonomic sources. AryKun (talk) 12:34, 4 January 2022 (UTC)

They were French scientists who worked extensively to characterise pigeon species in the early 19th century. This pdf (in French) gives many references to their work. Prév was actually Prévost. Mike Turnbull (talk) 13:30, 4 January 2022 (UTC)

There is an article about Knip at Pauline Rifer de Courcelles (she married Joseph August Knip). Mike Turnbull (talk) 13:42, 4 January 2022 (UTC)

Prév. is Florent Prévost and Pig. is an abbreviation of the standard multi-volume work Les Pigeons, [par Madame Knip]. Prévost was listed as (co-)author on later editions. See entry 1838–4- in this bibliography.  --Lambiam 23:08, 4 January 2022 (UTC)

January 5

Finding a scheduling algorithm (project management)

Hey! I am a student of industrial engineering and as part of a project I'm working on I'm trying to minimize the number of tardy jobs in a production floor, where I have a constant number of non-identical working stations (the "machines"). My data shows that ~60% of my jobs at the current situation are tardy. In addition, Managers in the organization prioritize customers using numbers (i.e. The most preferred customer gets the score 1, followed by the second customer with the score of 2, and so on). Unfortunately, I can't pinpoint the exact scheduling method that works on non-identical m machines, and can combines the priority matter.

English is not my native language, so feel free to edit this question if the English is incorrect. Thanks! — Preceding unsigned comment added by 2A0D:6FC7:20E:B0DA:702D:566B:124B:245D (talk) 17:05, 5 January 2022 (UTC)

For a given job, should it be scheduled to be performed at a specific machine among the m machines, or is there some freedom of choice? Is the only difference the processing speed of the machines? Can different jobs be processed concurrently at different machines or is there at any time only one job being processed that may need several machines in some order? Optimal job scheduling and job-shop scheduling are both optimization problems with many variations, and can have a number of different objectives to serve as the objective function – minimizing the number of tardy jobs is a bit crude as not all occurrences of tardiness are equally bad, which is obvious in this case because all customers are preferred, but some are more preferred than others. You need to give a more detailed description of the concrete problem and the objective before an expert (which I am not, I'm afraid) can (hopefully) direct you to a good scheduling method.  --Lambiam 23:11, 5 January 2022 (UTC)

Sorry, I also can't give a specific recommendation. Lambiam has suggested some good articles for reading; I would also suggest takt time. If I'm understanding you correctly, part of the problem seems to stem from there not being takt times assigned to the machines. If nothing else, that may at least help narrow down your searching. Matt Deres (talk) 21:53, 6 January 2022 (UTC)

blood oxygenation

Questions inspired by usual stories about covid patients.

1. A typical symptom of covid pneumonia is that the person has very low blood oxygen (like 50% even) but is awake, talking, and in comparatively little distress. Normally someone with such low O2 would be unconscious. How is it that Covid patients avoid this?
2. Covid treatment (besides meds) centers on getting O2 into the person through the lungs, first with supplemental O2 through the nose, then intubation, and EXMO is a last resort. Intubation itself seems horribly drastic though. Is there a big obstacle to using EXMO sooner, i.e. is it way more drastic than something like kidney dialysis? Is that because of the high volume of blood that it has to move to deliver enough oxygen?

Thanks. 2601:648:8202:350:0:0:0:9435 (talk) 20:32, 5 January 2022 (UTC)

The article about Extracorporeal membrane oxygenation (ECMO or EXMO) describes a number of side effects and complications. Philvoids (talk) 23:16, 5 January 2022 (UTC)

These possibly not all RS hint at another barrier besides risk/benefit to the patient i.e. resourcing (especially staff time) [3] [4] [5] [6] [7]. From what I can tell ECMO is more "expensive" treatment than intubation in terms of the demands on highly trained and experienced staff for initiation and monitoring.

From [8] I found [9] where they call a similar strategy "ECMO first". Although it's only a single case report of a specific case where there were concerns for initiating mechanical ventilation so doesn't actually tell you anything useful about which is better but there is a list of the end of possible disadvantages of such a strategy, and perhaps searching for ECMO first will find a bit more. I did also find [10] from the real early days which suggests even then there was great disagreement about such a strategy.

Nil Einne (talk) 05:13, 6 January 2022 (UTC)

Re (1): I don't know, but from Oxygen saturation (medicine):

"Oxygen is more readily released to the tissues (i.e., hemoglobin has a lower affinity for oxygen) when pH is decreased, body temperature is increased, arterial partial pressure of carbon dioxide (PaCO2) is increased, and 2,3-DPG levels (a byproduct of glucose metabolism also found in stored blood products) are increased."

Perhaps some of the body's responses to Covid-19, including fever, help to offset the effects of low blood oxygen. Is there a doctor in the house? {The poster formerly known as 87.81.230.195} 90.208.89.176 (talk) 08:18, 6 January 2022 (UTC)

January 6

Disease-related deaths for illnesses preventable by vaccine

Boy, that's some awkward wording in the header; sorry about that! It's less common than in the US, but there's no shortage of anti-vaxxers here in Canada. One of the things they whine about is that the COVID-19 vaccine is essentially mandated if you want to do any of several common activities. When it's pointed out that this has always been the case - school boards have long required the full suite of childhood immunizations - they protest that those were serious diseases, not silly over-blown ones like COVID. Now, that false-ness aside, it did get me curious about the deaths associated with those diseases (measles, mumps, rubella, diphtheria, whopping cough, etc.) in Canada. The time frame would preferably be in the area of the 1970s or 1980s, but other years would be okay too. This is the closest I've found so far for measles, though it's cases, not deaths, and the format of the chart is not reassuring (it seems to go from sample years: 1930, 1940, to year by year for no reason). And, to access the base data you have to register (and pay for the good stuff). This is more scholarly, but still focuses on cases. I guess I was assuming - perhaps wrongly - that some of the basic metrics we now see every day for COVID would have been done (retro-actively) for other serious illnesses, if only for uses like this. On the other hand, I simply might not be searching effectively. Any help? Matt Deres (talk) 21:47, 6 January 2022 (UTC)

Just noting that "whopping cough" is one of the more interesting typos I've seen lately. I guess it is a whopping cough at that. --Trovatore (talk) 20:16, 7 January 2022 (UTC)

Rubella is an interesting case. AFAIK and this seems to be supported by our article, it's generally a fairly mild disease. The big concern is for Congenital rubella syndrome which is very serious when it occurs but as maybe obvious from the name, is something that arises in a developing fetus when rubella is contracted during pregnancy especially in the first trimester. So while there is some small advantage to the individual who is vaccinated, AFAIK preventing CRS is a far bigger community reason for vaccination. If you do find any figures, you'll need to make sure it differentiates between who is affected. The CDC for example only mentions deaths of newborns and miscarriages. [11] Nil Einne (talk) 13:50, 7 January 2022 (UTC) 00:26, 8 January 2022 (UTC)

The case of diphtheria is pretty clear-cut. This WHO paper cited in our article reveals that the case fatality rate in the past reached up to 50%. I assume that the WHO website would be a good place to look for other statistics. Mike Turnbull (talk) 14:08, 7 January 2022 (UTC)

Boy, I'm really sorry you have to spend time arguing with those idiots. Hey, they're in EVERY country but far fewer in countries that have more recent experience with epidemics or death from preventable newborn infections etc. I don't know if they'll listen to any reason, so even the best answers you get here might not count for anything with them. And I'm not a medical doctor or researcher and cannot give you expert information off the top of my head.

But one thing I can point to is this: which communicable diseases are there NOT vaccines for, or being strongly sought for. I could mention the common cold. Most people (before the recent health restrictions) would get one every year, and only a tiny few would die or suffer long-term complications. And that is why no large funding has been devoted to vaccine development against the pathogens that cause what gets classified as the common cold (but considerable resources towards flu vaccines, since those ARE more serious). So unless your anti-vax friends don't admit that covid-19 is much more serious than a simple cold (in which case they really are beyond help!) this is an example of why some vaccines are more needed than (a few) others, and resources thereby devoted (at least a bit) wisely. Interferometrist (talk) 20:12, 7 January 2022 (UTC)

That's not really why there isn't massive funding going towards a "common cold" vaccine. The reason is that there is no such thing as the "common cold," just like there is no such thing as "cancer." There are hundreds of pathogens that cause what we call the "common cold," and unlike influenza, these aren't closely related strains of one virus family. Common cold causing viruses are as distantly related to each other as we are to birds. Maybe even more distantly related than that. That is similar to cancer, where pancreatic cancer and breast cancer are entirely different illnesses that both happen to manifest with malignant tumors, and thus we call them "cancers." There is not much funding to develop a universal "cancer" vaccine or therapy, however, there has been a lot of funding (and success) at developing therapies and even vaccines against specific cancers. The HPV vaccine, for example, has cut cervical cancer by about 95%. --OuroborosCobra (talk) 20:43, 7 January 2022 (UTC)

Well I hardly think we disagree. I realize the "cold" isn't a single disease (why I said "pathogens" in plural, above) but most colds are from a few families of viruses and I'll bet their differences in a lot of cases are not much greater than between covid strains that are (more or less) protected by the same vaccines. Which is also why I understand there is interest (perhaps not possible) in a vaccine that would provide immunity to ALL coronaviruses (thus all covid-19 and a lot of cold viruses). What you said is true, but even so I believe what I said is right: there is no strong motivation for immunity against this year's main cold viruses (as they do for this year's main flu viruses) or the most common classes of cold viruses which might respond to the same vaccine, for the very reason that colds do not pose a serious health risk except in a very tiny portion of cases. And that was to answer the original question of why covid vaccines are so much more needed to protect people (even with a death rate <1%, much higher than colds or flu, and long-covid rate several times that) AND to stop the spread and end this awful pandemic, which I fear governments are now giving up on. Agreed? Interferometrist (talk) 21:11, 7 January 2022 (UTC)

Actually it seems fairly possible that covid is destined to become just another cold virus reasonably soon. The so-called Russian Flu of 1889 may have been caused by the OC43 coronavirus, which is now generally grouped with the common cold. It may have decreased in virulence over the years, or it may just be that most people get exposed as children when the illness it causes is usually mild, so few people are immunonaive. This time around, we're getting a jump start on that with the excellent vaccines. --Trovatore (talk) 21:20, 7 January 2022 (UTC)

Well we're way off the topic here, but I have to answer. Many intelligent experts (nowhere near all) are saying that, but I don't think that even omicron is so mild that everyone getting it won't matter too much, nor do I like the fact that many promoting that view think so because the costs of lockdown (etc.) is greater, that's not a choice we should be making! It's not clear that it will become milder or that previous exposure will make it that mild. One version is that everyone will be vaccinated regularly to solve the problem, but the idiot anti-vax movement might preclude that, and unless vaccine availability becomes much greater than the last year when we REALLY needed it, it means covid will be a serious endemic disease in poor countries. If due to childhood exposure (as I gather you're saying) then it's hard to tell, and we won't find out for some time. Most talk of covid becoming "simply" endemic has the result of not treating the problem and tolerating a rate of death and debilitating long-covid rather than fighting it. The world could have eliminated the pandemic after the initial lockdowns and with greater solidarity between rich and poor nations (and shunning the Trumps, Bolsaneros, and internet quacks) and vaccinations produced in maximum quantities (look how fast a country can produce bombs in the months following the beginning of an unexpected war!) with all patent protection eliminated so that didn't reduce their supply. This might all sound theoretical but then I look at China, a big chunk of the world's territory and population, which has been so successful. Yes maybe what you say will come to pass, but I'd never resign the population of any country to the "endemic scenario" as long as we know that there are ways to eliminate it as China has done. Anyway, that's my view, and if it were to be properly acted upon, we'd never find out how serious endemic/non-treated covid will become. Interferometrist (talk) 01:34, 8 January 2022 (UTC)

We're getting into opinion here obviously, but you gave yours so I'm going to give mine. I think what you're hoping for is totally off the table. Not gonna happen. Covid is going to be endemic sooner or later. The window to avoid that probably closed in early 2020.

Luckily, for the vaccinated, the risk is small. You're (almost) certainly going to be exposed; you're very likely to get infected, and fairly likely to have symptoms at some point, but they are unlikely to be very different from other illnesses you've had and not made a big deal over.

I seriously doubt China has done anything remotely resembling "eliminating" covid. You'll need to provide evidence for that claim. Ideally independent of the Chinese Communist Party.

Finally, it's almost unbelievably good luck that the first variant to substantially "escape" the vaccines, omicron, turns out to be so much less virulent than the previous strains. We should really be celebrating that, not tearing our hair. --Trovatore (talk) 01:45, 8 January 2022 (UTC)

Chickenpox was generally considered (at least by the general public) to be so minor that families would intentionally have their kids infected at parties just to get it out of the way. That is, until the vaccine was available. My son was required to get vaccinated for chickenpox, and when I went back to college, I had to get a titer test to prove that I'd either had chickenpox as a child (I did), or had a vaccine in the past (I didn't). Prior to vaccination, the number of cases per year was roughly the same as the number of people born that year (obviously they were born earlier, but that's just to say that basically 100% of people eventually got chickenpox). It had a fatality rate of 0.0017%, far lower than COVID19 (prior to omicron, COVID19 hat a roughly 2% fatality rate globally, or 1200 times greater than chickenpox. Despite that far lower mortality rate, chickenpox vaccination is required in the United States. --OuroborosCobra (talk) 20:38, 7 January 2022 (UTC)

I think it was not so much "to get it out of the way" as that chickenpox is typically worse the older you are when you get it. --Trovatore (talk) 20:56, 7 January 2022 (UTC)

Which is another way of saying "to get it out of the way" --OuroborosCobra (talk) 22:50, 7 January 2022 (UTC)

Sure, I suppose you could put it that way. To me it sounded like you meant something like just wanting not to have to worry about it anymore, rather than that the potential severity was increasing. --Trovatore (talk) 23:11, 7 January 2022 (UTC)

January 7

Doubt on armature in DC motor.

If current experience reverse direction why can't armature experience the same i.e zero degrees?

Right after armature making 90 degree turn, then direction of the current go reverse direction using split ring but why can't armature go back to 0 degrees ? Rizosome (talk) 01:22, 7 January 2022 (UTC)

See Brushed DC electric motor with its animated graphics. Philvoids (talk) 16:00, 7 January 2022 (UTC)

Your question illustrates why a normal brush-DC motor's commutator needs to include at least 3 different current flow positions. It WOULD be possible to make one with only 2, after the armature moved 180 degrees (or 180/N degrees) the current would reverse. But that motor would rotate in either direction (dependent on some rotational inertia for smooth motion), equally likely depending on which way it started.

An analogous situation concerns synchronous (AC) motors needing at least 3 (thus 4) poles fed by AC in different phases, not just 2 opposite poles with one oscillating magnetic field, so that it will run in a particular direction (but most of the power can be in one with the other just for starting it in the right direction). A notable exception is the typical synchronous motor that turns the platter in a microwave oven (a so-called "magnetron" where I live). That only has 2 opposite poles (one phase) which is why it can start in either the clockwise or counter-clockwise directions, and the food doesn't care. :-) Interferometrist (talk) 19:21, 7 January 2022 (UTC)

@Interferometrist: Your question illustrates why a normal brush-DC motor's commutator needs to include at least 3 different current flow positions. I am asking about a position of an armature not about current. Rizosome (talk) 06:01, 8 January 2022 (UTC)

If I understand your question (and I'm not sure if I do – your English is very unclear), it's because the armature is a real object with mass and therefore (angular) momentum that is already spinning in a particular direction. When the current and therefore the induced magnetic field (and therefore the force on the armature) drops to zero, the armature's angular momentum keeps it moving past the 'zero point'.

At the 'zero point' there is no force acting on the armature, so why should it change direction (i.e. velocity: see Newton's First Law of Motion)?

When the current, magnetic field and force then reverse, the armature has passed the 'zero point' and the reversed force acts upon the armature in the same rotational direction as before. The physical design of the armature and split ring ensures this.

It seems to me that the article linked by Philvoids above explains this very clearly. If you cannot understand that explanation, perhaps you should be using the Wikipedia for your own language, rather than the English Wikipedia.

(NB: Don't expect any further response from me on this query, as I have yet to be convinced that you are not feigning ignorance.) {The poster formerly known as 87.81.230.195} 90.208.89.176 (talk) 12:24, 8 January 2022 (UTC)

object moving from point A to point B with no outside force

In the new so called "Live Science" publication an article appeared yesterday titled: "Wild video shows goldfish 'driving' a water-filled car in weird experiment" [12] . I will describe the idea briefly. It is done in Israel. They made a light "vehicle" consisting of a small aquarium made of thin glass positioned on a piece of plywood with four wheels underneath. They then put a goldfish (eventually 6 goldfish, but it is not important) and using some behavior tricks enticed them to move the carriage, they call it a car, forward. They claim that goldfish showed intellect, moved the car around the obstacles, etc, but this is not what I am interested in.

I wonder what Sir Isaac Newton would have said about it. Some motion of an object can be expected when something explodes inside of it, but it is not a directional motion. Imagine there is a soccer ball on the grass and a player kicks it. The ball will fly forward. If you can measure a force applied and know a mass of the ball you can write s simple equation, calculate speed of the ball, etc. But here there is no external force. How does it work? They don't say if the fish touches the opposite wall of the aquarium with their noses or simply "swim." What is a secret here? Looking at their video one has to conclude that there is no secret. The fish swim and the car moves in a given direction! Is it a new mechanics? AboutFace 22 (talk) 19:40, 7 January 2022 (UTC)

Are you asking how the car is powered? If so, it's motorized. Cannolis (talk) 20:05, 7 January 2022 (UTC)

(ec) The car is motorised and equipped with a camera that detects the fish's position in the tank and a control system that translates those position measurements into commands to the motor. The fish doesn't actively propel the car but pilots it. I guess Isaac would have been quite puzzled because he didn't know about electric motors... --Wrongfilter (talk) 20:10, 7 January 2022 (UTC)

I missed it. Thanks. Now everything is on its place. I got scared for Sir Isaac Newton :-) AboutFace 22 (talk) 22:54, 7 January 2022 (UTC)

January 8

COVID-19 particles

How big is a COVID-19 particle and can that particle just float around in the air by itself or MUST it be affixed to some sort of larger droplet in order for it to remain airborne? Essentially what I’m trying to approximate is what “size” is the airborne threat, as surely this is an important question in determining what PPE is appropriate etc. --Uhooep (talk) 03:17, 8 January 2022 (UTC)

The average diameters of most coronavirus virions fall in the range of 80 to 120 nm.^[13] For SARS-CoV-2, the coronavirus species that causes COVID-19, a diameter of around around 120 nm has been reported.^[14] A normal cloth face mask will not filter these out; quoting from the article: "One 2010 study found that 40–90% of particles in the 20–1000 nm range penetrated a cloth mask and other fabric materials.^[16]" However, an individual's immune system can easily beat a single particle. The main risk is inhaling a drop of fluid with a high viral load. Most airborne drops of fluid will be caught by a cloth mask, and the larger (and more risky when inhaled) they are, the more likely they are to be caught. Cloth masks do not offer the same protection as N95 masks, but definitely help, when used systematically, to lower the reproduction number, which needs to get and stay below 1 to end this pandemic.  --Lambiam 10:42, 8 January 2022 (UTC)

Field strength and inverse-square law

The first sentence of Antenna_(radio)#Arrays_and_reflectors begins, "The field strength from a transmitting antenna at a specified distance is reduced according to the inverse-square law..." Is there really a sense of "field strength" in which this can be correct, or is it a mistake? My understanding otherwise is that both electric field strength and signal strength decrease with inverse distance, not inverse-square [EDIT: in the context of EM radiation, I mean]. Do I have it right? -Amcbride (talk) 16:41, 8 January 2022 (UTC)

Yes, the electromagnetic fields fall off as 1/r, not 1/r^2 in case of electromagnetic radiation. The power that escapes to infinity is then finite, because the Pointing flux, which is proportional to the square to the fields, then falls off like 1/r^2, and multiplied with the area of a sphere of radius r, this then tends to a finite limit. In case of slowly varying fields, like if you move a charge and push another charge away, the energy that escapes to infinity will be zero, because Pointing flux times surface area tends to zero in the limit of infinite radius. The energy transfer from the first to the second charge comes with a feedback on the first charge. In case of radiation the energy moves away and can hen affect charges without these charges causing a feedback on the original charge. Count Iblis (talk) 16:52, 8 January 2022 (UTC)

Thanks! I'd edit the article, but there are several places it uses the term field strength, and for now I'm not sure which are correct and which should be intensity instead. -Amcbride (talk) 18:42, 8 January 2022 (UTC)

Another radiation question

Is there a word for the sum of the magnitudes of irradiance/intensity from all directions? That is, the power received by an ideal isotropic antenna divided by its effective aperture? (Or am I confused and these aren't the same idea? Or am I so confused that one or more of these ideas is not even coherent?) -Amcbride (talk) 19:36, 8 January 2022 (UTC)