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September 14

Red colored fabrics

Goodmorning! Why do red colored fabrics (e.g. a t-shirt) tend to fade more over time when exposed to light than those of other colors? Thank you--93.43.187.226 (talk) 05:42, 14 September 2021 (UTC)[reply]

Who says they do? ←Baseball Bugs ^{What's up, Doc?} carrots→ 05:47, 14 September 2021 (UTC)[reply]

There is a book called Red: The Art and Science of a Colour that discusses this issue. If you go to Google Scholar and search for "Fading of red dye" or "paint" or "objects" or "fabrics", you will get many hits in the academic literature. One I found began, "The irreversible fading of organic red colorants in art is well documented and greatly affects the perception of masterpieces from antiquity to the present day." So, there's that, Bugs. Cullen³²⁸ Let's discuss it 06:05, 14 September 2021 (UTC)[reply]

Thanks for the citation! ←Baseball Bugs ^{What's up, Doc?} carrots→ 06:09, 14 September 2021 (UTC)[reply]

I was looking around WP for a discussion of the lightfastness of red pigments and didn’t’t find much. However, both red pigments and more particularly, red dyes (which would be the choice for fabrics) are notoriously fade-prone. Looking around elsewhere, this is commonly attributed to preferential absorption of UV, since red light is reflected for it to appear red. However, I think there’s more to it than just that. Acroterion (talk) 06:12, 14 September 2021 (UTC)[reply]

In connection with the fading of outdoor printed material (such as advertising posters) which, as commonplace observation reveals to anyone who actually looks, lose reds fastest and blues least fast, I once read an explanation that disruption of redder pigments inherently require less light energy than that of bluer ones, not unconnected with the fact that higher frequency (= shorter wavelength) blue photons are more energetic than lower frequency (= longer wavelength) red photons. However, I can't remember all the details or find the original source. Is there an optical physicist in the house? {The poster foremrly known as 87.81.230.195} 90.200.67.3 (talk) 07:42, 15 September 2021 (UTC)[reply]

Some time ago I read an explanation. I don't remember where I read it and can't guarantee it's true, but it made sense. Red pigments reflect red light and absorb green and blue, blue pigments reflect blue light and absorb red and green. Blue light, due to its higher energy per photon, does more damage when absorbed than red or green light, so the red pigments, absorbing more of the higher energetic radiation, are damaged faster. PiusImpavidus (talk) 08:32, 16 September 2021 (UTC)[reply]

How sweat comes out of human skin at the same time water can't go inside the skin?

I didn't understand the mechanism of human skin towards fluids. Rizosome (talk) 08:01, 14 September 2021 (UTC)[reply]

Who says skin does not absorb water? ←Baseball Bugs ^{What's up, Doc?} carrots→ 08:27, 14 September 2021 (UTC)[reply]

One Dr Hoffman says: "The epidermis, the outermost layer of skin, provides a waterproof barrier" [1] Alansplodge (talk) 20:03, 14 September 2021 (UTC)[reply]

In any case, read Sweat gland for some answers on the first part of your question. ←Baseball Bugs ^{What's up, Doc?} carrots→ 14:29, 14 September 2021 (UTC)[reply]

Does it depend on whether or not you have anhidrosis? Martinevans123 (talk) 14:32, 14 September 2021 (UTC)[reply]

Courtesy link: Anhidrosis. hydnjo (talk) 16:47, 14 September 2021 (UTC) [reply]

Alansplodge "The epidermis, the outermost layer of skin, provides a waterproof barrier" is enough for me. Rizosome (talk) 00:09, 15 September 2021 (UTC)[reply]

That's why your fingertips don't wrinkle when they've been in water for a long time. ←Baseball Bugs ^{What's up, Doc?} carrots→ 01:23, 15 September 2021 (UTC)[reply]

Obviously you know that they do, and are presumably being sarcastic. However Alansplodge is correct – the epidermis does indeed prevent water from penetrating deeper into the dermis. However, the very outermost layer of the epidermis is made up of dead, dry cells made mostly of keratin; when these get wet they absorb water (which does not penetrate beyond them) and swell up, but since their attachment to the underlying dermis prevents them from expanding like a balloon, their increased volume instead forces the skin into wrinkles.

With reference to the implied question in the title: sweat does not just ooze out of the skin randomly, it is formed in sweat glands and expelled out by internal pressure via sweat pores. Due at least in part to surface tension, this is a one-way process – water cannot enter the body via the pores. {The poster formerly known as 87.81.230.195} 90.200.67.3 (talk) 07:33, 15 September 2021 (UTC)[reply]

Your fingertips do not "swell up" when in water - if they did, your fingertips would be less wrinkled, not more wrinkled. And in fact "prune hands" has nothing to do with water being absorbed or released at all (cite). Matt Deres (talk) 17:43, 15 September 2021 (UTC)[reply]

I didn't say that the fingertips swell up, I said that the outermost keratinous cells swell up but do not expand outwards because of their attachment to the deeper epidermal (and indirectly dermal) cells, and since they can't cause the skin overall to expand, they must accommodate their larger size within the same lateral span, which forces the skin into wrinkles, thus: ---------- —> /\/\/\/\/\.

This is what your reference says scientists used to think, and I concede that you have found a more up-to-date reference. I'm reminded that the "half-life" of "facts" on the well-researched programme QI is said to be about 18 months (or something similar). {The poster formerly known as 87.81.230.195} 90.200.67.3 (talk) 22:50, 15 September 2021 (UTC)[reply]

18 months ago it was about 18 months. Now it is more like 9 months. Soon we'll have the 24/7 science cycle. --Lambiam 08:21, 16 September 2021 (UTC)[reply]

That's fair. Apologies for misrepresenting your post. Matt Deres (talk) 13:53, 17 September 2021 (UTC)[reply]

September 16

Why there is no vent for medical oxygen tanks?

I remember seeing a vent on top of oxygen tanks at industries, but why there is no vent for medical oxygen tanks? Rizosome (talk) 03:49, 16 September 2021 (UTC)[reply]

Maybe the venting oxygen tanks you saw were for holding liquid oxygen, whereas medical oxygen tanks contain compressed gas. Here you can read about the need to vent liquid oxygen cylinders. --Lambiam 07:05, 17 September 2021 (UTC)[reply]

A great link. Just to add that liquid oxygen would be far too cold to use in a medical setting, e.g. to assist breathing.--Shantavira|^{feed me} 08:28, 17 September 2021 (UTC)[reply]

For iron's metallic bond, what type of iron ion is present to bond with the delocalized electrons?

For sodium, each Na will lose 1 e- to form Na+. Then sodium ions are attracted to the delocalized electrons, and therefore there is metallic bond. However, for iron (and other transition metals), how many electrons will each iron atom lose? Will they lose all of the 4s electrons? Or, is the ion Fe(II) or Fe(III) ion? Thank you so much!!!! Jocosus2000 (talk) 15:04, 16 September 2021 (UTC)[reply]

NaCl has ionic bonds. Those can be simplified as "one atom loses one (or more) electron(s) to another atom, and both stick together due to electromagnetic attraction". (For the longer explanation, see the linked article.)

Iron, on the other hand, has metallic bonding. A gross oversimplification is that iron atoms pool electrons together into a sea of free-moving electrons. Individual atoms do not really "lose" electrons in that process. How many free electrons per atom are yielded by that process is a good question, but not one I can answer - researching this probably goes through our article Valence and conduction bands; based on a sentence in Drude model I assume usually as many as the valence number (so, probably 3?).

Finally, many compounds have covalent bonding, where close-by atoms share some electrons, and shared electrons count for both atoms towards the "having a complete electronic shell" stability criterion. The difference with metallic bonding is that the electrons are not free-moving so the bond is really local. Tigraan^{Click here for my talk page ("private" contact)} 15:52, 16 September 2021 (UTC)[reply]

Thank you so much!!! By the way, is band similar to orbitals? Thanks:)Jocosus2000 (talk) 10:04, 17 September 2021 (UTC)[reply]

No. Atomic orbitals are a concept linked to a single atom (the "location" of a given electron, basically). The conduction/valence bands are macroscopic (or mesoscopic) concepts related to energy levels of a large number of electrons around a large number of atoms. Tigraan^{Click here for my talk page ("private" contact)} 12:06, 17 September 2021 (UTC)[reply]

Where an atom in a compound loses an electron to some other place in the structure, and it is just a single electron, that is called an electride. They're different to metals as they are insulators. Graeme Bartlett (talk) 12:41, 17 September 2021 (UTC)[reply]

Thank you so much! Jocosus2000 (talk) 15:20, 17 September 2021 (UTC)[reply]

September 17

Position of black hole event horizon

Is the event horizon (i.e. absolute horizon) of a black hole in the same place for all objects, or does it vary according to their mass or some other factor(s)? PaleCloudedWhite (talk) 07:17, 17 September 2021 (UTC)[reply]

The event horizon depends on the mass, charge, and angular momentum of the black hole. Dja1979 (talk) 17:09, 17 September 2021 (UTC)[reply]

But is it in the same position relative to all other objects? i.e. if a large planet and a tiny asteroid approach the black hole, would they both cross the event horizon at the same place? PaleCloudedWhite (talk) 17:45, 17 September 2021 (UTC)[reply]

As the two objects involved (the black hole and the approaching object) warp spacetime conjointly, the definition of the spatial metric required for a notion of being "in the same place" is tricky. Quoting from Binary black hole § Shape: "As two black holes approach each other, a 'duckbill' shape protrudes from each of the two event horizons towards the other one." (This is observed in numerical simulations based on the GR equations, but there is no reason to assume this mathematical prediction is not faithful.) If the approaching object has a mass that is almost enough to make it collapse into a black hole, continuity implies that the receiving black hole also extends somewhat of an expecting duckbill for its kiss of death. For a planetary mass like that of Jupiter, there must then also be a bump in the shape, but (I suspect) so tiny that it is negligible. --Lambiam 07:04, 18 September 2021 (UTC)[reply]

Thanks. So if your suspicion is correct, any difference would be in terms of degrees of negligibilty. I admit that whenever I try to get my head around the concept of the warping of spacetime, my head starts warping as well....... PaleCloudedWhite (talk) 11:59, 18 September 2021 (UTC)[reply]

Getting your head around anything tends to warp it. It stretches the imagination and is a good aid to grokking the Einstein field equations, but take care not to warp your brain into a singularity. --Lambiam 21:08, 18 September 2021 (UTC)[reply]

September 18

If KE generated is tiny due to an object's size, what's the purpose of avoiding it here?

In here it says: The amount of work energy needed to move a unit of electric charge from a reference point to the specific point in an electric field with negligible acceleration of the test charge to avoid producing kinetic energy or radiation by test charge.

If Kinetic energy is due to an object's motion. then an object (in this case, charge) tiny in size, so is necessary to avoid producing KE? Rizosome (talk) 02:54, 18 September 2021 (UTC)[reply]

While the mass of the electron is tiny, the velocity and thereby the kinetic energy that an electron can obtain by using an electric potential difference to move them – as is done in particle accelerators used in high-energy physics – can get very large. It can go up to several GeV, a billion times the work of moving an electron around in a low-voltage circuit. The avoidance clause is part of the definition, and is not a matter of a practical set-up for measuring potential. --Lambiam 06:39, 18 September 2021 (UTC)[reply]

How exactly GeV getting avoided here? If it not a matter of a practical set-up, then why it is included in definition ? Rizosome (talk) 04:31, 19 September 2021 (UTC)[reply]

The definition (like any definition) describes an idealised situation where the charge is moved infinitely slowly from one point to another. Only in this idealised situation is the difference in potential equal to the work done on the charge. This makes for a clean precise definition, but describes a situation that in practice can only be realised approximately. In any real life situation, the energy balance is more complicated as it has to take into account the difference in kinetic energy and the amount of energy radiated away. Real life is messy, definitions are abstractions that reduce things to the essential aspects. A similar thing happens in thermodynamics, where theory demands that changes of the state of a system occur infinitely slowly so that the system is always in equilibrium. In reality systems always go through non-equilibrium states that are much harder to describe.

Having said that, it seems to me that for the lead sentence of a Wikipedia article this one is rather cluttered. It might be less confusing if that sentence did not mention KE and radiation; the definition could be made more precise further on in the article. --Wrongfilter (talk) 06:41, 19 September 2021 (UTC)[reply]

Would a Space probes named after people article be a good idea?

Let me know! 2405:4803:F190:6577:15C3:CAFB:2446:BE2E (talk) 06:41, 18 September 2021 (UTC)[reply]

What about using a category? See also WP:CLN and WP:SAL. --Lambiam 07:08, 18 September 2021 (UTC)[reply]

Maybe only if there was also an article on space probes not named for people. ←Baseball Bugs ^{What's up, Doc?} carrots→ 12:53, 18 September 2021 (UTC)[reply]

More than 70 space probes have been launched, of which only about 10 were named after people. Cassini was named after Giovanni Cassini, Hubble was named after Edwin Hubble, and Kepler was named after Kepler. How many others do you know? --Lambiam 20:58, 18 September 2021 (UTC)[reply]

I count around 25 in List of Solar System probes, though this includes "sub-probes" and some legendary/mythical/fictional "persons." {The poster formerly known as 87.81.230.195} 90.200.67.3 (talk) 05:22, 19 September 2021 (UTC)[reply]

That list contains many spacecraft that are still in the planning phase, such as Lucy. (Burning question: Do skeletons qualify as "people"?) --Lambiam 07:05, 19 September 2021 (UTC)[reply]

At this point, Cassini, Kepler and Hubble are likewise "skeletons". ←Baseball Bugs ^{What's up, Doc?} carrots→ 18:39, 19 September 2021 (UTC)[reply]

Wouldn't a category suffice? Imagine Reason (talk) 22:21, 18 September 2021 (UTC)[reply]

No, we should not create such a category. Per Wikipedia:Categorization "A central concept used in categorizing articles is that of the defining characteristics of a subject of the article. A defining characteristic is one that reliable sources commonly and consistently define[1] the subject as having" This is not a subject that is frequently discussed about space probes. --Jayron 32 16:06, 20 September 2021 (UTC)[reply]

WP:Listcruft says: "In general, a "List of X" stand-alone list article should only be created if X itself is a legitimate encyclopedic topic that already has its own article. "--Shantavira|^{feed me} 08:26, 19 September 2021 (UTC)[reply]

September 19

Ligature in Surgery

Ligature (medicine) has been a stub for more than 15 years, and has only a few lines of content. Is there is corresponding article on surgery which covers this better and so this can be redirected or merged there? Jay ^(Talk) 10:10, 19 September 2021 (UTC)[reply]

External v internal pressure

For simplicity, I'll say my lab works with worms. There is a cell underneath the hypodermis that (we think) becomes deformed by either an increase in hydrostatic pressure in the body cavity or by focal compression by objects passing through the body cavity. My coworker is convinced the deformation experienced by the cell can be recapitulated by just increasing the atmospheric (external) pressure. Intuitively I feel uniform external pressure through a homogeneous medium (air), transduced through first a solid but deformable barrier (worm skin) and then through an inhomogeneous fluid (worm guts) is a totally different regime from that of internal hydrostatic pressure or focal compression. My coworker "concedes" that different materials experience different compressibility, but somehow we diverge when it comes to the conclusion of this thought experiment I made up: Say we have a box floating in space (no gravity) with the "top" half filled with air and the "bottom" with water, with or without a solid barrier at the interface; and there is an over-easy egg on a wall in the air half and another on the floor of the water half; if we were to increase pressure by lowering the ceiling (air half) at epsilon increments, would both egg yolks burst at the same time? This is our last dance (talk) 20:57, 19 September 2021 (UTC)[reply]

The pressure in the air and water would be identical Greglocock (talk) 00:26, 20 September 2021 (UTC)[reply]

Without gravity, why would the water remain in the bottom half? Imagine Reason (talk) 00:54, 20 September 2021 (UTC)[reply]

Imagine Reason, I forgot that I needed the impermeable barrier in there. JoelleJay (talk) 04:27, 20 September 2021 (UTC)[reply]

If the barrier is flexible enough, its presence makes no difference. But if the moving ceiling reaches the top egg, that one may be first to burst. The forces acting on the worm cell may not be the same in all directions if the deformation is due to non-uniform compression forces, whereas with hydrostatic pressure it does not matter where the pressure comes from. --Lambiam 05:17, 20 September 2021 (UTC)[reply]

The egg yolk is an incompressible liquid surrounded by a fragile membrane and in both cases has force applied equally in all directions ; for very much the same reasons that scuba and free divers eyes are unaffected at depth, neither would burst. 2A01:E34:EF5E:4640:DFC:5965:9101:1BA9 (talk) 16:19, 20 September 2021 (UTC)[reply]

Sure. That's why divers don't wear goggles or face masks. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:12, 20 September 2021 (UTC)[reply]

You are out of your depth. Yes 2A01, I agree. Thought for Bugs, what is the air pressure in a Scuba divers mask, compared with the surrounding water? Greglocock (talk) 22:22, 20 September 2021 (UTC)[reply]

I wouldn't know. But I would like to see a reference for the assertion the IP is making. ←Baseball Bugs ^{What's up, Doc?} carrots→ 01:25, 21 September 2021 (UTC)[reply]

Divers wear a dive mask because water has a different refractive index to air, not to compensate for hydrostatic pressure (which of course it doesn't; the pressure within the mask is maintained at ambient to prevent exophthalmos). I'm really not sure what sort of mystical force you think might be operating unequally upon the yolk membrane to cause it to rupture, so I can't give you a reference for its inexistance (although I can cite the notorious non visual impairment of french scuba diver Jacques Cousteau), but you might since rupturing clearly involves one part of the surface moving in a different direction to another, you might like to ponder Newton's laws of motion and ask yourself where those opposing velocities come from in a homogeneous medium with only symmetrical pressure forces in play. 78.245.228.100 (talk) 05:22, 21 September 2021 (UTC)[reply]

September 20

Which reach first at the other end of the cell?

The conventional direction of flow of current is from the positive terminal of the cell to the negative terminal of the cell through the external circuit. But Electrons flow from the negative terminal to the positive through the circuit. So which reach faster at other end of the cell: free electrons or current? Rizosome (talk) 06:32, 20 September 2021 (UTC)[reply]

Conventional current is not a real physical flow. It is a mathematical trick traditionally used in circuit analysis that works because things like Kirchhoff's circuit laws work when thought of as electron flow or when we pretend something flows in the other direction. Nothing flows from the positive to the negative. 85.76.72.46 (talk) 07:26, 20 September 2021 (UTC)[reply]

That's pretty much the same. Be aware the electric current flow in a circuit is not like a fountain, where water appears and disappears as pumps start and stop. It is rather like piping system of a heating circuit - metallic conductors, making a circuit, are literally full of free electrons ('electricity particles'), same as water pipes are full of water molecules. When you close a circuit (remove an isolating gap) a directed movement of electrons appears - and that is current. Similarly, when you remove an obstacle in a water circuit by opening a valve, water starts to run. Then, of course, there is some time needed for the electrons from one end of a cell to arrive at the other end, just like some time is needed for water molecules pushed from one end of a water pump to arrive at the other end of the pump. However the flow (electric or water current) appears almost immediately in the whole circuit. Additionally, electrons are not distinguishable, so you can not tell when 'the same electrons' come at some point which left some other point some time ago. They have some average velocity which can be determined, but some may move slower and other move faster than that, racing each other. So the question when 'they' (meaning 'precisely the same') reach the other terminal of a cell can not be answered precisely. However the 'mean' flow of electrons reaches the other terminal precisely at the same moment as the current, because that is current. --CiaPan (talk) 10:04, 20 September 2021 (UTC)[reply]

There is a flow of traffic from New York to San Francisco, formed by moving cars. Which reaches faster at the other end of the road: the moving cars or the traffic flow? The question is meaningless. --Lambiam 23:18, 20 September 2021 (UTC)[reply]

September 21

Bouncing upwards from walls

In various Super Mario Maker levels I have watched on YouTube, Mario can jump seemingly arbitrarily long distances upwards if he's inside a canyon between two walls, simply by hitting each wall in turn and bouncing off it. Surely this can't be possible in real life, even discounting any damage hitting a wall does to Mario's body? JIP | Talk 02:12, 21 September 2021 (UTC)[reply]

Wall jumping is a well established video game tradition. It appears in many games.

But can it be done in real life? Well, obviously nobody can jump four times their own height like Mario does, but could it be done with normal human-scale jumps?

A single wall-jump is a pretty common parkour move, but there are a few YouTube videos that purport to show repeated wall-jumps up a narrow gap. (example) Whether these are real or not is another question. It doesn't seem impossible, but I sure couldn't do it. ApLundell (talk) 03:06, 21 September 2021 (UTC)[reply]

@@ Line 123: / Line 123: @@
 ::::You are out of your depth. Yes 2A01, I agree. Thought for Bugs, what is the air pressure in a Scuba divers mask, compared with the surrounding water? [[User:Greglocock|Greglocock]] ([[User talk:Greglocock|talk]]) 22:22, 20 September 2021 (UTC)
 :::::I wouldn't know. But I would like to see a reference for the assertion the IP is making. ←[[User:Baseball Bugs|Baseball Bugs]] <sup>''[[User talk:Baseball Bugs|What's up, Doc?]]''</sup> [[Special:Contributions/Baseball_Bugs|carrots]]→ 01:25, 21 September 2021 (UTC)
+::::::Divers wear a [[dive mask]] because water has a different [[refractive index]] to air, not to compensate for [[hydrostatic pressure]] (which of course it doesn't; the pressure within the mask is maintained at ambient to prevent [[exophthalmos]]). I'm really not sure what sort of mystical [[force]] you think might be operating unequally upon the yolk membrane to cause it to rupture, so I can't give you a reference for its inexistance (although I can cite the notorious non [[visual impairment]] of [[french]] [[scuba]] diver [[Jacques Cousteau]]), but you might since rupturing clearly involves one part of the surface moving in a different direction to another, you might like to ponder [[Newton's laws of motion]] and ask yourself where those opposing velocities come from in a homogeneous medium with only symmetrical [[pressure]] forces in play. [[Special:Contributions/78.245.228.100|78.245.228.100]] ([[User talk:78.245.228.100|talk]]) 05:22, 21 September 2021 (UTC)
 = September 20 =