Welcome to the science section
of the Wikipedia reference desk.

Select a section:

WP:RD/S

Want a faster answer?

Main page: Help searching Wikipedia

How can I get my question answered?

Select the section of the desk that best fits the general topic of your question (see the navigation column to the right).
Post your question to only one section, providing a short header that gives the topic of your question.
Type '~~~~' (that is, four tilde characters) at the end – this signs and dates your contribution so we know who wrote what and when.
Don't post personal contact information – it will be removed. Any answers will be provided here.
Please be as specific as possible, and include all relevant context – the usefulness of answers may depend on the context.
Note:
- We don't answer (and may remove) questions that require medical diagnosis or legal advice.
- We don't answer requests for opinions, predictions or debate.
- We don't do your homework for you, though we'll help you past the stuck point.
- We don't conduct original research or provide a free source of ideas, but we'll help you find information you need.

Ready? Ask a new question!

How do I answer a question?

Main page: Wikipedia:Reference desk/Guidelines

The best answers address the question directly, and back up facts with wikilinks and links to sources. Do not edit others' comments and do not give any medical or legal advice.

October 30

Analog to digital conversion process

I didn't understand the portion: Sampling in Analog to digital conversion process.

I read this sentence from here: To sample the input signal the switch connects the capacitor to the output of a buffer amplifier. The buffer amplifier charges or discharges the capacitor so that the voltage across the capacitor is practically equal, or proportional to, input voltage. In hold mode the switch disconnects the capacitor from the buffer. The capacitor is invariably discharged by its own leakage currents and useful load currents, which makes the circuit inherently volatile, but the loss of voltage (voltage drop) within a specified hold time remains within an acceptable error margin for all but the most demanding applications.

It mentioned "input signal" clearly, but it didn't mention term output signal anywhere in the paraphrase. It didn't even mention output signal is binary type. So how binary type here achieved ? Rizosome (talk) 04:36, 30 October 2021 (UTC)[reply]

The description you give is just for sampling. A sample of the input voltage appears on the capacitor. This then allows measurement of a constant value, rather than the time varying input. You could say that the output is the voltage of the capacitor. To convert to binary, you need the analog to digital converter. (perhaps you can compare the capacitor voltage, to a voltage ramp, and measure the time it is equal. ) Graeme Bartlett (talk) 11:15, 30 October 2021 (UTC)[reply]

To be more explicit, the process of transforming an analog input signal to a digital output signal is best understood as a two-step process. The amplitude of the input signal is a continuous function of continuous time to a continuous range of values. In the output signal, both the time domain and the range are discrete. Sampling only makes the time discrete and leaves the range continuous. Using R for a continuum and Z for a discretum, and "→" as is usual for the direction of a mapping from domain to range, we have:

input signal: R → R

sample: Z → R

output signal: Z → Z

Our article Sample and hold describes only the first step. --Lambiam 11:47, 30 October 2021 (UTC)[reply]

Can non lipid ointments diffuse into the skin?

Maybe there is an article about such non lipid ointments/creams/lotions/Colloids?

Thanks, 182.232.129.86 (talk) 05:54, 30 October 2021 (UTC)[reply]

Calling a substance an ointment suggests that its main ingredient is a lipid. Glycolic acid is used in skin-care products for its capability to penetrate skin. A non-lipid solvent that is known for its ability to penetrate the skin without damaging it is dimethyl sulfoxide (DMSO), which can be used for topical delivery of solutes. Its use has several hazards, some of which can be serious; see the section DMSO § Safety. Our article Absorption (skin) does not discuss any specific substances other than DMSO, and I did not find an article with a more general treatment of non-damaging skin penetrating substances. --Lambiam 11:23, 30 October 2021 (UTC)[reply]

Very many substances are toxic by skin absorption and this reference for acetonitrile is typical. Hence most lab chemists handling solvents wear appropriate gloves of nitrile rubber or other similar material. The OP's term "ointment" is, as Lambiam said, normally reserved for things that are intended to be rubbed on the skin, not necessarily to penetrate it, otherwise described as topical applications. Mike Turnbull (talk) 12:12, 30 October 2021 (UTC)[reply]

Drag on Satellites

To quote from an article in "New Scientist" ; "...a warming planet might ..reduce.. the drag on satellites, keeping them in orbit for longer." This seems to imply that as the atmosphere warms it causes less drag on orbiting satellites. I can't understand how this could be. Wouldn't the upper atmosphere reach further into space if it warmed up and expanded? And wouldn't this cause MORE drag on satellites in low earth orbit, bringing them down sooner? Any references or comments on this would be much appreciated!! 49.197.130.6 (talk) 07:01, 30 October 2021 (UTC)[reply]

A warmer atmosphere would presumably become less dense (unless additional atmospheric gases were released from the lithosphere and/or hydrosphere), and atmospheric material that expanded higher than the orbit in question would no longer exert a net gravitational attraction on a satellite in it; perhaps these would more than offset increased drag in some circumstances.

Can you cite the issue and/or date and page? I subscribe to New Scientist (in paper form) and would like to study the full passage in context. {The poster formerly known as 87.81.230.195} 90.200.65.29 (talk) 07:16, 30 October 2021 (UTC)[reply]

(OP) it's in the leader of the latest edition, 30 october 2021 49.197.130.6 (talk) 07:31, 30 October 2021 (UTC)[reply]

I'm having trouble finding the article (I only have an database subscription, can't easily browse a whole issue). But [1] is an explanation of an anomalous cooling in the upper atmosphere as a result of CO2 released at lower levels. The cooling causes contraction: pulling the atmosphere "down" a bit leaves it less dense (==less drag) up where the satellites are. DMacks (talk) 07:48, 30 October 2021 (UTC)[reply]

Ah, right! The New Scientist leader points to a Features article on pp 42-46, 'Space Jam' by Robin George Andrews, which I hadn't yet reached (my copy arrives no earlier than Friday).

The relevant passage (p 44) reads:

"One saving grace so far has been that space junk often gets dragged into the lower atmosphere and burns up. Unfortunately, as we have recently discovered, climate change means the rate at which this happens may decline, making orbital overcrowding worse.

In the upper atmosphere, the sun's extreme ultraviolet radiation splits molecular oxygen into two oxygen atoms that collide with carbon dioxide molecules, releasing infrared energy. Adding more carbon dioxide to the upper atmosphere increases this effect. As more infrared energy is released, it escapes into space and so the upper atmosphere cools. In this more frigid environment, the particles lose energy and the upper atmosphere contracts.

This phenomenon was predicted in 1989, and has been observed for some time. But [Hugh Lewis, a space debris expert at The University of Southampton, UK] and his colleagues recently realised it is influencing the lifetime of space junk. When falling orbital debris meets atmospheric particles, the object experiences drag. This causes the size of its orbit to shrink, bringing it closer to the denser, lower atmosphere in which it will eventually be incinerated. The climate change-induced contraction of the upper atmosphere will reduce the drag that debris experiences as it spirals towards us. This means it will stay in orbit for longer.

A recent paper, co-authored by Lewis, found that objects in low Earth orbit will stay up there for 30% longer even if we restrict carbon dioxide emissions to successfully keep the global average temperature rise to 1.5°C this century."

Hope this is useful. {The poster formerly known as 87.81.230.195} 90.200.65.29 (talk) 12:17, 30 October 2021 (UTC)[reply]

You compare Earth with Venus, which has CO2 atmosphere. Whereas Earth has a hot thermosphere with the temperature in excess of 1000K, at Venus the thermosphere is rather cool at 300-400 K during day and at about 100K at night. As a result the atmosphere of Venus is rather compact. Ruslik_Zero 20:18, 30 October 2021 (UTC)[reply]

Grinding plant-parts with oils

If we want to use certain natural products available in plants, a good way to do that is to grind relevant plant-parts (flowers/fruits/roots/seeds, etc.), often after drying them, with some oil.

Are some oils better at capturing the natural product out of the plant-part better than others?

For example, should "saturated" oils be better than "non saturated" oils and alike?

The purpose of the question is to understand if some oils are better than other for creating cosmetics grade / food grade essential oil products by means of fragrance/flavor/nutritional content.

Thanks, 182.232.129.86 (talk) 09:24, 30 October 2021 (UTC)[reply]

It is hard to discuss in generality – what is the nature of the "natural products available in plants" you want to "capture", and what is the purpose to which they will be put after extraction? The process of extraction by immersion in a liquid is known by the more technical name "maceration". If you want to extract essential oils, used for example in aromatherapy, this article on how essential oils are made contains a detailed description of extraction by maceration, but mentions, generically, using carrier oils as solvent. The absence of a discussion of which oils to use as carrier suggests that it does not make a big difference. This article states that commonly used solvents are olive oil and sunflower oil. Another carrier oil, advocated if the product is to be rubbed in the skin, is jojoba oil. --Lambiam 10:54, 30 October 2021 (UTC)[reply]

For interest, Shea butter is also used as an ointment base, as well as on its own. {The poster formerly known as 87.81.230.195} 90.200.65.29 (talk) 12:30, 30 October 2021 (UTC)[reply]

It is likely that vegetable oils are selected on the basis of their having little to no odor of their own, and not being too quick to go rancid. The color of the oil may also be important. Abductive (reasoning) 01:27, 31 October 2021 (UTC)[reply]

This recipe for chili oil says "High temperature oil can simulate the strong aroma while lower temperature oil brings us the bright red color". Alansplodge (talk) 12:51, 1 November 2021 (UTC)[reply]

October 31

If I mimic some person's voice, am I matching his frequency of sound wave or wavelength of sound wave?

If I mimic some person's voice, am I matching his frequency of sound wave or wavelength of sound wave? What exactly am I mimicking here in terms of a wave? Rizosome (talk) 03:59, 31 October 2021 (UTC)[reply]

The two are directly linked. wavelength (m)*frequency(Hz)=speed of sound(m/s). Greglocock (talk) 05:54, 31 October 2021 (UTC)[reply]

I am not asking the relation between wavelength and frequency. I am asking about physics behind vocal mimicry. Rizosome (talk) 05:59, 31 October 2021 (UTC)[reply]

OK, to put it another way, wavelength and frequency are inextricably linked (different ways of expressing the same thing), so the answer is, both.--Shantavira|^{feed me} 09:28, 31 October 2021 (UTC)[reply]

It's also unlikely that you would be precisely matching it. ←Baseball Bugs ^{What's up, Doc?} carrots→ 10:21, 31 October 2021 (UTC)[reply]

A human voice is far too complex to be reduced to a single sound wave. It is a complex mixture of sound waves, both fundamentals and harmonics created by the physical characteristics of that person's entire vocal tract and by resonances in their sinuses and other head and neck anatomy, plus the unique way they use their voice in terms of volume and pitch variations, speech rhythms and pauses, their idiosyncratic, social class and regional accents, and the register they consciously or unconsciously employ.

Just to allow an estimation on effort for any response: Have you got the basic answers to your recent question? --91.47.21.246 (talk) 15:22, 31 October 2021 (UTC)[reply]

And many others before that. Rizo almost never follows up. ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:30, 1 November 2021 (UTC)[reply]

See also the article Spectrogram. The spectrogram of a person speaking is also called a voiceprint. They are used in speaker recognition, but also in forensic analysis much like fingerprints.^[2][3][4] If someone can produce intelligible speech while matching another person's voiceprint, the timbre of their voice will sound like that other person's. --Lambiam 20:50, 31 October 2021 (UTC)[reply]

Baseball Bugs I am following every response to my questions. This is the line I am waiting for: If someone can produce intelligible speech while matching another person's voiceprint, the timbre of their voice will sound like that other person's. Rizosome (talk) 04:26, 1 November 2021 (UTC)[reply]

I didn't say "follow", I said "follow up". It would be nice if, once you get an answer, you could follow up by saying something, such as "I understand" or "Thank you." ←Baseball Bugs ^{What's up, Doc?} carrots→ 04:34, 1 November 2021 (UTC)[reply]

Yes, in order for me to speak in such a way that I can be mistaken for you, I must match your timbre. Whether I can do that is much more complex than matching pitch (frequency/wavelength), which normally fluctuates within every sentence. —Tamfang (talk) 02:55, 2 November 2021 (UTC)[reply]

November 1

Berberine for diarrhea

This involves the following two discussions:

Do the sources listed in the above discussions support a claim that Berberine is an effective treatment for diarrhea? My conclusion is that the sources supporting that claim do not meet the requirements of WP:MEDRS. Am I correct? --Guy Macon (talk) 04:43, 1 November 2021 (UTC)[reply]

If the issue is the interpretation of WP:MEDRS, then the talk page of that guideline may be a better place for a discussion. Just refer to the specific sources allegedly supporting the claim; the prior discussions seem to be more about the appropriateness of the guideline rather than its interpretation, and so are not particularly relevant. --Lambiam 09:37, 2 November 2021 (UTC)[reply]

Can activated carbon destroy acetic acid?

I want to thicken distilled vinegar (so to make a vinegar paste), with activated carbon.

Can activated carbon destroy/sabotage the vinegar's acetic acid molecules (or, at least, is activated carbon a chemical base)?

Thanks, 182.232.42.121 (talk) 12:36, 1 November 2021 (UTC)[reply]

We had another IP user wanting to thicken vinegar, with answers here. I have no idea why you would use activated charcoal but it will be inert to the vinegar, being neither an acid nor a base itself. Mike Turnbull (talk) 14:54, 1 November 2021 (UTC)[reply]

I think that this other question from me as a person and not as an IP :) isn't related. I thought that activated carbon may have medicinal properties and is worth checking. I found activated carbon as a wonderful vinegar thickner, actually the smoothest and least residual I have tried, but it can be very "messy" to work with due to color stains. 2001:44C8:42CD:E478:AC74:4933:5B45:8BAB (talk) 04:05, 3 November 2021 (UTC)[reply]

Solar radius instead of diameter

Why solar radius rather than solar diameter has been chosen as standard? One can assume that diameter gives a better idea of the star's size, whereas radius gives only half of the value. Thanks. 212.180.235.46 (talk) 15:18, 1 November 2021 (UTC)[reply]

By convention, spheres are defined by their radius; as one convenient definition of a sphere is every point in three dimensions located a distance, r, away from a single point. That distance, r, is the radius, and needs to be manipulated by multiplying it by 2 to get the diameter. Since 1) It is entirely arbitrary whether to use the radius or the diameter and 2) we already have definitions that use radius, it seems perfectly fine to use the radius. All other measures (surface area, volume, etc.) can also be defined from r. They could be defined from d too, but it isn't any easier or harder to do so, so we stick with one measurement (radius) instead of 2 (radius for some measures, and diameter for others). --Jayron 32 15:22, 1 November 2021 (UTC)[reply]

The NASA ephemeris gives angular size in diameter not radius and not everyone knows that 2pi is tau, it's not radius always. I suppose using r but pi makes these more elegant than the other 3 conventions: .5τr² πr² τ/8d² π/4d² • 2τr² 4πr² .5τd² πd² • .6τr³ 4/3πr³ 1/12τd³ 1/6πd³. Sagittarian Milky Way (talk) 19:52, 1 November 2021 (UTC)[reply]

Yes, angular size is in diameter, and most people understand how to multiply or divide by 2. It's not difficult math. --Jayron 32 11:22, 2 November 2021 (UTC)[reply]

Yes it doesn't matter which one gets the name. I was mentioning that it was circumference over diameter that got the famous Greek letter for some reason (I'm not sure why) and not circumference over radius which would be more "pure". Sagittarian Milky Way (talk) 17:09, 2 November 2021 (UTC)[reply]

The conversion is pretty easy: if the radius of a star is 3 solar radii, then the diameter of that star is 3 solar diameters. --Amble (talk) 18:23, 1 November 2021 (UTC)[reply]

November 2

What's the term for prioritys.

What do you call this kind of situation: if something is both C and B, we call it C. If something is both B and A, we call it B. And so forth. I know this is a heavily CS question, but I know we have this in science, especially organic chemistry. For example, if a compound is both an alcohol and an amine, we call it an alcohol, but if something is both an alcohol and an amide, we call it an amide. So, some kind of precedence. What do we call this in CS, and, what do we call this in science? 67.165.185.178 (talk) 23:25, 2 November 2021 (UTC).[reply]

I remember asking a pharmacology scientist, if something is both an anti-biotic, and an anti-viral, what do we call it? He didn't know there was a thing, or a special word that emphasized both. 67.165.185.178 (talk) 23:27, 2 November 2021 (UTC).[reply]

How about "antiseptic"? ←Baseball Bugs ^{What's up, Doc?} carrots→ 01:42, 3 November 2021 (UTC)[reply]

I'm not sure there is a general rule stating that a compound that is both an alcohol and an amine should preferentially be referred to as an amine. An example is 2-aminoethan-1-ol, for which the infobox in our article lists a dazzling array of names, including β-aminoethyl alcohol. The suffix "-ol" in the preferred IUPAC name classifies it as an alcohol; the hydroxyl group is considered the functional group with the highest priority. In general, I think "priority" is a good term for indicating which one of a choice of two candidates is to be preferred; for a choice among a larger group, "highest priority" is more explicit. I can't think of a good situation where two names are equally appropriate, given their definitions, but where one conventionally takes priority. A cat is both a pet and a predator; whether we refer to Molly as a pet or as a predator depends on the context. Sometimes both are appropriate.^[5][6][7] --Lambiam 11:42, 3 November 2021 (UTC)[reply]

Sigh. Here's the priority, straight from my organic chemistry textbook. ~~And I properly indented your comment~~. Carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene > alkyne > alkane > ether > halides. 67.165.185.178 (talk) 12:55, 3 November 2021 (UTC).[reply]

My contribution was properly indented, because it was solely in response to the original question and had nothing to do with names for substances that are both antibiotic and antiviral. --Lambiam 17:10, 3 November 2021 (UTC)}[reply]

I think that the term the OP seeks is a classification and that article gives a lot of the background. In organic chemistry the series quoted is that established by Friedrich Konrad Beilstein in the 19th century and now implemented in the Beilstein database and elsewhere.^[1] The history of science is full of attempts to classify things. For example, the periodic table was developed when it was realised that classifying elements by their atomic number (rather than, say, their atomic weight) was useful. Such classification is a human construct: the appropriate classification depends on context — so, for example Beilstein's system does not include inorganic compounds and in some respects is not "logical". Any such systematic approach gains traction because it is useful and widely applicable. Mike Turnbull (talk) 14:12, 3 November 2021 (UTC)[reply]

Is the OP referring to the Cahn–Ingold–Prelog priority rules? That's only used in a specific application, though... --Jayron 32 16:31, 3 November 2021 (UTC)[reply]

This is about the IUPAC nomenclature of organic chemistry. Crucial to its application is determining the parent functional group, if any, with the highest order of precedence. I think the Blue Book calls this "seniority order". The precise rules are very complex; one needs expertise to apply them. If I can believe our article, the alcohols (priority 7) come before amines (priority 9). Some textbooks, however, while referring to the IUPAC rules, present a different priority sequence.^[8] --Lambiam 10:57, 4 November 2021 (UTC)[reply]

References

^ Luckenbach, Reiner (1 May 1981). "The Beilstein Handbook of Organic Chemistry: the first hundred years". J. Chem. Inf. Comput. Sci. doi:10.1021/ci00030a006.

November 3

terminology about luminescence and fluorescence

I'm curious to know if biologists, chemists, and physicists have different words for the same thing? I wonder if physicists have their own word for what chemists call chemiluminescence and fluorescence. Like, what do physicists call the concept behind glow-in-the-dark toys? If inorganic semiconductors do fluorescence, we all call them quantum dots. 67.165.185.178 (talk) 14:27, 3 November 2021 (UTC).[reply]

Not as far as I know. There are two different physical processes that can lead to luminescence known as fluorescence and phosphorescence, which are different processes, and not two different names from different fields. It should also be noted that chemiluminescence is yet another different process. AFAIK, there are not different terms in use among different fields for these processes. --Jayron 32 15:52, 3 November 2021 (UTC)[reply]

Sometimes you'll see things just called "luminescence spectra" as a catchall term, but I've seen that from chemists, biologists, and physicists. If they don't want to get into specific details like intersystem crossing of electronic states and spins, lifetime effects, etc., it can often be OK to just use "luminescence." An example would be an more simple analytical application that just makes use of the wavelength and intensity of the luminescence, but doesn't necessarily care about the specific electronics of the system undergoing transitions. --OuroborosCobra (talk) 16:22, 3 November 2021 (UTC)[reply]

Also, as the OP mentioned quantum dots, it should be noted that this is also a distinct phenomenon on its own, and not a synonym for luminescence used by one branch of science. --Jayron 32 16:29, 3 November 2021 (UTC)[reply]

Afaik, quantum dots is simply something that absorbs UV and emits light, but only when that something is an inorganic semiconductor, possibly set at a small but defined length. And now, they're inventing ways to make quantum dots absorb IR, and still emit light, which is now anti-Stokes shifts. 67.165.185.178 (talk) 00:17, 4 November 2021 (UTC).[reply]

In biology, it's bioluminescence. -- 107.15.157.44 (talk) 08:06, 4 November 2021 (UTC)[reply]

Our article Luminescence contains a long list of types. --Lambiam 10:59, 4 November 2021 (UTC)[reply]

November 4

Potentiometer and Zener diode are interchangeable in eletrical circuits?

I discovered that both are somewhat voltage regulators. So those two are interchangeable? Rizosome (talk) 02:09, 4 November 2021 (UTC)[reply]

Rather obviously not. They perform comletely different functions. They are both somewhat resistive and somewhat mettalic too.--Shantavira|^{feed me} 09:04, 4 November 2021 (UTC)[reply]

It requires a very firm grasp to rotate a Zener diode by 90º, and the resulting change in resistance may be permanent. --Amble (talk) 14:00, 4 November 2021 (UTC)[reply]

I discovered that lettuce and poison ivy are both leafy plants. So those two are interchangeable? --Jayron 32 15:41, 4 November 2021 (UTC)[reply]

Goats eat them both, so in a sense I guess? I've done some tests making salads with them, but I will have to leave it to others to do fuller experimentation as I have no interest in pursuing it further. I guess you could say [puts on sunglasses] I've scratched my itch. DMacks (talk) 15:46, 4 November 2021 (UTC)[reply]

Zener diode is so thin in nature why does it need very firm grasp to rotate 90º? Rizosome (talk) 02:12, 5 November 2021 (UTC)[reply]

I'm fairly sure that Amble was pointing out potentiometers are generally used as variable resistors and after connection to a circuit their resistance is generally adjusted by rotation. Trying to rotate a zener diode when it's connected to a circuit is difficult and will likely destroy the diode if successful or at least disconnect it from the circuit. (Although it's true there are potentiometers which are adjusted via sliding and trying to rotate one will likely be equally destructive.) Nil Einne (talk) 07:36, 5 November 2021 (UTC)[reply]

Ok, I understand potentiometer and Zener diode very well now. Rizosome (talk) 04:04, 6 November 2021 (UTC)[reply]

So you have obtained a very firm grasp of the subject matter. --Lambiam 10:15, 6 November 2021 (UTC)[reply]

Was stealth technology for US airplanes based upon on previous technology?

Something I was discussing in the bar tonight. Someone was talking about the Roswell crash and how a bunch of new technology for the US air force came from that.

Apparently the stealth material for fuselage of the planes as used by the USAF just appeared out of nowhere, not based on any previous research? Is that right? And the Russians/Soviets were trying to replicate it and failed until a US stealth bomber was shot down in the Bosnian war and the FSB took samples of the wreckage back to Moscow to be reverse engineered?

Not trying to conspiritard. Just looking for more info. I find it interesting. 146.200.107.70 (talk) 02:24, 4 November 2021 (UTC)[reply]

The idea of radiation-absorbent material predates the alleged 1947 Roswell incident. Those pesky Nazis were doing research in World War II. German submarine U-480 had a special rubber coating to try to defeat sonar. Reimar Horten, one of the designers of the Horten Ho 229 flying wing, claimed he intended to add radar-absorbent material to its surface. Clarityfiend (talk) 03:47, 4 November 2021 (UTC)[reply]

See also the history section in the Stealth technology article. -- 107.15.157.44 (talk) 04:56, 4 November 2021 (UTC)[reply]

It's a blindingly obvious development after radar was invented. No need to involve extraterrestrials. Now if you were to suggest a certain ex-POTUS obtained alien mind-bending technology to use on his endless rallies ... Clarityfiend (talk) 05:36, 4 November 2021 (UTC)[reply]

Stealth: The Secret Race to Invent an Invisible Airplane by Peter Westwick says in the first chapter, Roots of the Revolution, that during WWII, the Germans, British and US all experimented with radar absorbent or reflective coatings, called Schornsteinfeger ("Chimney sweep"), Salisbury Screen and Halpern Anti-Radiation Paint (HARP) respectively. Early work on reducing an aircraft's radar signature by modifying its shape were unsuccessful, mostly because the science behind how radio waves bounce off an object did not begin to be understood until Ohio State University applied computers to the problem in the early 1960s. Air forces worked around the problem with chaff, decoys and electronic countermeasures, and it wasn't until Vietnam and the 1973 Yom Kippur War showed that a better approach was needed. Alansplodge (talk) 12:56, 5 November 2021 (UTC)[reply]

In my list of radar countermeasures, I forgot nap-of-the-earth intrusion, which is flying underneath radar beams. This is the option that the RAF went for, even with their enormous Avro Vulcan bombers. Low flying military training is a whole lot cheaper than spending a gazillion dollars on a totally new technology. Alansplodge (talk) 12:36, 6 November 2021 (UTC)[reply]

B cell

"BCRs allow the B cell to bind to a specific antigen, against which it will initiate an antibody response." This is before somatic hypermutation happens. My question is how can BCRs recognize the "specific" foreign antigen if this is the first time ever they encounter it? 156.142.197.92 (talk) 22:26, 4 November 2021 (UTC)[reply]

If a BCR binds to some antigen (after negative selection in the development phase), it means the antigen is foreign (except when something goes awry, as in autoimmune diseases). It does not need to recognize it. --Lambiam 12:43, 5 November 2021 (UTC)[reply]

user:Lambiam Let's say there is already memory B cell for that specific antigen. If a BCR gets to it before a memory B cell does then the whole immune response (with hypermutation) seems redundant. Am I missing something? 2601:444:4100:F709:392C:B1D8:B5D7:362B (talk) 19:32, 5 November 2021 (UTC)[reply]

Without a parent cell with a BCR recognizing the antigen as foreign, the memory B cell reacting to the antigen would never have come into existence. Also, while memory B cells are often long-lived, they do not persist forever. After a long period, not enough may be left to mount a forceful immune response; it is then a good thing that the first line of defence is still there. --Lambiam 20:24, 5 November 2021 (UTC)[reply]

Oh, and the memory B cells, when encountering the antigen, quickly trigger a strong response. In comparison, the first line fires up slowly. --Lambiam 20:27, 5 November 2021 (UTC)[reply]

user:Lambiam Alright, let me get this straight. Assuming this is the second time the body is encountering a specific antigen, memory B cell would most likely get to the antigen way before a BCR would? Therefore, hypermutation does not happen twice for the same antigen. Am I correct? 2601:444:4100:F709:392C:B1D8:B5D7:362B (talk) 23:03, 5 November 2021 (UTC)[reply]

In an infection, there are usually many foreign elements with a specific given antigen. It is not very likely that the memory B cells recognizing it will get rid of all these antigenic elements before some B cells also encounter one and recognize the antigen. These B cells will do whatever they would have done without the presence of the antigen-specific memory B cells. --Lambiam 00:04, 6 November 2021 (UTC)[reply]

A B-cell usually needs an additional signal from a helper T-cell for its activation. Since T-cell are already selected not be autoreactive (in thymus), it follows that autoreactive B-cells are not activated. Ruslik_Zero 20:31, 5 November 2021 (UTC)[reply]

November 5

UK bird flu restrictions

With recent events in the UK and the new biosecurity restrictions, does this mean that I should stop flying my pet parrot outside (always on leash) and keep her inside for the present time? Or does this not apply to parrots? I might email the govt for clarification myself, but does anyone know? I don't even know if the goffin is susceptible to avian flu. --Iloveparrots (talk) 00:55, 5 November 2021 (UTC)[reply]

You should read the latest from GOV.UK, and decide for yourself:

"Bird flu - Latest situation: Avian influenza prevention zone declared across Great Britain". GOV.UK. 3 November 2021. --107.15.157.44 (talk) 02:03, 5 November 2021 (UTC)[reply]

Mohs hardness for iodine

Is there a reported value? --- Sandbh (talk) 03:18, 5 November 2021 (UTC)[reply]

It's really hard to find, but I dug it up in this CRC Handbook. Page 713-714 contains "Physical Properties of Iodine", and has a reported Mohs hardness of 2. --Jayron 32 12:22, 5 November 2021 (UTC)[reply]

November 6

Chemistry reactivity questions.

Suppose A is more reactive than B, as in, A reacts with a bunch of other substances faster than B does. Then can there be cases where B reacts with something better than A does? Note that, I imagine to make this question useful, we must make A and B in the same category of something. Like, A and B should both be metals. So if A reacts with 10 different acids better than B does, can B react with an acid that A doesn't? Or B reacts with a base that A doesn't? Can it be such that B reacts with gases better than A does?

Since this is such a specific question, I'll throw in another 1. Our article on chemiluminescence says the 1st accidental discovery was in 1877. But the source is in German. Just wondering if anyone compiles a list of discoveries of chemiluminescence in order, I'm curious to know what the 1st discovery is by theory and not by accident. I'm also wondering if anyone discoveries chemiluminescence in say, the past 20 years, by accident, or and by theory still. 67.165.185.178 (talk) 14:51, 6 November 2021 (UTC).[reply]

for your first question the answer is yes. Consider aluminium and copper. When exposed to water and air copper reacts much faster to make a green covering. But aluminium dissolves much better in acid than copper. This is because aluminium forms a protective film. Sometimes the compound produced can be unstable, so for example, lithium reacts with nitrogen, but potassium does not. Or sometimes a stable complex can be formed. For gold, it is pretty tough to react or dissolve, but with air and cyanide it can dissolve. Graeme Bartlett (talk) 21:29, 6 November 2021 (UTC)[reply]

Ducks and geese don't have crops?

See the recent edit to crop (anatomy) here. The anon rightly points out in his/her edit summary that the article states that ducks and geese don't have crops, but the article is illustrated with an image and a video showing a duck and a goose, apparently with a bulging crop. I have found some websites (e.g. this) that say that ducks and geese don't have crops too. So I have no idea. Anyone able to help? I only really know about parrots to any great extent. --Iloveparrots (talk) 20:26, 6 November 2021 (UTC)[reply]

[...] waterfowl do not possess a true "crop," or widened portion of the esophagus. Instead, their esophagus is capable of expanding to accommodate substantial amounts of food. --"Understanding Waterfowl: Duck Digestion". www.ducks.org. --107.15.157.44 (talk) 22:20, 6 November 2021 (UTC)[reply]

Fun fact from the same article:

In 1911, a gold rush was spurred in western Nebraska after hunters found small gold nuggets in the gizzards of ducks they had shot. The source of these gold nuggets, however, was never discovered. --107.15.157.44 (talk) 22:24, 6 November 2021 (UTC)[reply]

[1] Luckenbach, Reiner (1 May 1981). "The Beilstein Handbook of Organic Chemistry: the first hundred years". J. Chem. Inf. Comput. Sci. doi:10.1021/ci00030a006.

[1]