Wikipedia:Reference desk/Archives/Science/2017 July 17

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July 17

Animal identification?

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The first one is some sort of dragonfly; the second is either antelope or deer. Both were taken in Lake County, Oregon. MB298 (talk) 00:00, 17 July 2017 (UTC)

Some wonderful editor(s) have given us List of mammals of Oregon, which lists some ungulates not detailed in other sources. Mule deer and Pronghorn antelope seem like the top contenders, and I'd favor the latter, but you should look at the photos yourself. Wnt (talk) 01:04, 17 July 2017 (UTC)

@Wnt: Thanks for the images, I'm fairly sure they are pronghorn antelope after looking at the pictures. MB298 (talk) 02:43, 17 July 2017 (UTC)

Agree those are pronghorn (although it's sometimes called that, it is not an antelope). I'm not good with dragonfly identifications, but there's this to help out. The angle of your pic is not making it very easy. Matt Deres (talk) 22:33, 17 July 2017 (UTC)

3/8" Diameter Hose

When a connectors fits "3/8" Diameter Hose"[1], is the 3/8" referring to the outer diameter or the inner diameter of the hose? Covfefe beans (talk) 04:47, 17 July 2017 (UTC)

It is more likely that the nominated dimension is descriptive of the internal diameter. Dolphin (t) 07:15, 17 July 2017 (UTC)

Neither inner nor outer Diameter. This is Nominal Pipe Size and you have to look up the exact diameters in Nominal Pipe Size tables.

(Article explanation citation:)The reason for the discrepancy for NPS(Nominal Pipe Size) ⅛ to 12 inches is that these NPS values were originally set to give the same inside diameter (ID) based on wall thicknesses standard at the time. However, as the set of available wall thicknesses evolved, the ID changed and NPS became only indirectly related to ID and OD.

Nevertheless you can usually rely on that parts with the same NPS are designed to fit each other. --Kharon (talk) 09:51, 17 July 2017 (UTC)

Actually NPS is for rigid pipes. This type of fitting looks like what you'd use in a flexible air line. For a flexible line, the 3/8" is the inner diameter while the outer will change depending on what the pressure rating in. However Kharon's point still stands. You buy 3/8" hose to go with your 3/8" fitting.Tobyc75 (talk) 21:12, 17 July 2017 (UTC)

Thanks, guys. I can't actually buy 3/8" hose because I'm not from the US and they sell hoses only by metric sizes here. Hence the question. Covfefe beans (talk) 02:19, 18 July 2017 (UTC)

From what I can find, a 3/8" hose barb is 0.39" (slightly larger than 0.38" ID of the hose, to ensure a good seal). A 9.5mm hose is 0.374" and so should work just fine for you.Tobyc75 (talk) 18:06, 18 July 2017 (UTC)

Eyes Bandaged After Brain Surgery

I have noticed that some people have one or both of their eyes bandaged after they've undergone brain surgery, especially if the surgery involved removing one or more tumors. Is this because the point of incision is at or near the eye, or is it because the eye is rendered (temporarily) unusable because of the surgery? 173.52.236.173 (talk) 06:09, 17 July 2017 (UTC)

This seems because traumatic brain injury or surgery frequently triggers Photosensitivity or Photophobia. --Kharon (talk) 10:12, 17 July 2017 (UTC)

I have noticed blackness below both eyes, resembling Black eyes due to bruising, after head surgery where there was no injury to the face, and wonder what is the cause. Blooteuth (talk) 10:45, 17 July 2017 (UTC)

That implies that some blood escaped the circulatory system and settled there, due to gravity, while the patient was sitting or standing. It should be absorbed in a few days. StuRat (talk) 13:50, 17 July 2017 (UTC)

The eye socket can be a route of entry, most famously with prefrontal lobotomy. Hopefully that's not the brain surgery you saw being done! Wnt (talk) 12:58, 17 July 2017 (UTC)

Through both eyes? No, I think I would have noticed that operation. Blooteuth (talk) 13:16, 17 July 2017 (UTC)

This story makes it sound like something of an expert-level procedure ... even for brain surgeons. But it is done for some tumors. Wnt (talk) 14:54, 17 July 2017 (UTC)

Can a vibrating net produce sound?

I was wondering about the structure of an electromagnetic speakers. I know they produce sounds waves with a vibrating surface. But can that surface be perforated and still produce sound waves to act as a speaker? I talking about a about something as fine and perforated as a mosquito net for example? Is it even hypothetically possible? 95.28.24.124 (talk) 12:07, 17 July 2017 (UTC)

A single vibrating string produces sound. A mesh of vibrating strings can produce sound. Bass drums have had drum heads with holes in the past. I expect that some musicians still put holes in their bass. A cargo net used in aircraft makes noise when it vibrates. I don't see the counter-argument. Where did you see a net that was not capable of producing sound? I think it is necessary to start with that case. 209.149.113.5 (talk) 12:21, 17 July 2017 (UTC)

Cut-away view of a dynamic loudspeaker

Every year thousands of cheap Transistor radios are thrown away so this experiment is easy to make. While the speaker plays music, cut holes in the loudspeaker's paper cone diaphragm with a Scalpel. Observe that the sound weakens, this is because the Voice coil is mechanically coupled to a diminished area of air. Observe also that bass notes disappear and the sound becomes both weak and distorted, this is because the rigidity of the cone is lost. There will also be rattles if the damage to the cone moves the voice coil off center so it rubs against the pole pieces. You will find that a substitute cone made from ordinary writing paper sounds reasonably well but Mosquito net material is practically useless here. Could you even tell whether a person is speaking directly to you or from behind a net? Blooteuth (talk) 13:11, 17 July 2017 (UTC)

And the reason mosquito net won't work is because it's soft enough to absorb any vibrations and turn them into heat. Some type of a metal mesh, like a chain-link fence, could work, and produce some sound, but still far less than a device designed to do so. StuRat (talk) 13:46, 17 July 2017 (UTC)

Vibrating meshes are used in nebulizers. I wonder if they make a sound? --jpgordon^{𝄢𝄆 𝄐𝄇} 15:17, 17 July 2017 (UTC)

They do but Ultrasound is sound waves with frequencies too high for humans to hear. See Nebulizer#Ultrasonic wave nebulizer. Blooteuth (talk) 16:11, 18 July 2017 (UTC)

That's one reason they use a mesh, so that they don't. A viscous fluid, like a liquid, sees the mesh as solid, but air sees it as acoustically transparent. Andy Dingley (talk) 18:50, 17 July 2017 (UTC)

• I've never seen an electrostatic speaker that didn't use a lightweight, but solid, film. However that's sandwiched loosely between two perforated meshes which don't move but which form the pole pieces. Are you confusing the solid membrane with the fixed meshes? Andy Dingley (talk) 18:50, 17 July 2017 (UTC)

Can you ladle water with a strainer? No. A net can not function as a loudspeaker, unless you would construct and use it like a Plasma speaker. --Kharon (talk) 19:51, 17 July 2017 (UTC)

A vibrating mesh (or a speaker with lots of holes) will make a sound. But it won't make a loud sound, or more specifically, it will be very inefficient in terms of sound pressure level for a given input power. The main issue is that the pressure wave from the back of the element is out of phase with the pressure wave from the front of the element. The spaces between the wires in your mesh or the holes in your perforated speaker allow the front and back waves to meet each other and cancel out. You'll still get some sound, but it will mostly be at high frequencies where the wavelength is small enough to reduce the phase cancellation. Managing the interaction between the out-of-phase front and back pressure waves is one of the (if not the) fundamental problems in Loudspeaker enclosure design.

(For reference, the wavelength at 41 Hz, the lowest note on a string bass or a 4-string bass guitar, is about 8 meters. The wavelength at 15 kHz, roughly the upper limit of human hearing for adults is about 2 cm.) Shock Brigade Harvester Boris (talk) 00:33, 18 July 2017 (UTC)

So far 209.'s answer remains most persuasive to me. This is a question about what is hypothetically possible, and we cannot deny that a guitar string (that article references musical strings in general) can make a loud sound. We can only ask whether it is possible to create such a vibrating strand in a "mosquito net" type speaker. We know that diameter of real music strings is practically important for generating low frequencies; however, this question is hypothetical. Can a very thin piano string generate a loud bass tone if it is sufficiently long? That's a question; I honestly don't claim much musical knowledge. Also, what happens if you simulate a long string by putting actuators on either end of a really short string - can you generate a sound effectively from a string that is much shorter than its wavelength? There are a lot of tinny walkmen that would seem to disagree, but I don't know how persuasive they are. I should look up earphones - what if you tie a few dozen of those into a mosquito net? Of course, they produce less sound, though in some public settings it doesn't seem like that much less. A parallel situation is detailed at extremely low frequency - it is hard to make antennae that broadcast enormous wavelengths. Nonetheless, they show a little facility in the woods that the article says somehow uses electrical length manipulation (not to be confused with antenna effective length, according to that article...) to get a signal out that can be heard around the world (several times even). If you were expecting an actual answer from me you'll be disappointed; mostly I'm wondering now what kind of sound you can get out of a single rigid piezoelectric circular loop. Wnt (talk) 14:32, 18 July 2017 (UTC)

Can a very thin piano string generate a loud bass tone if it is sufficiently long?

Mersenne's laws says you can make a bass sound with any length of string, you need just adjust tension and linear mass, in this regard, the bass sound is in the "easy" range, as it is easy to reduce tension and increase mass (whereas the requirement to high pitch sound, high tension and light string, make string more prone to break).

can you generate a sound effectively from a string that is much shorter than its wavelength? wavelength: "The wavelengths of sound frequencies audible to the human ear (20 Hz–20 kHz) are thus between approximately 17 m and 17 mm, respectively". Obviously bass sound are produced from much shorter string than 17m...

Gem fr (talk) 15:42, 18 July 2017 (UTC)

The strings of instruments such as guitar, violin, etc. make "loud" sound only when their vibrations are coupled via a bridge to a sound board or detected by an electromagnetic pickup and electric amplifier. Despite impressions that electric guitars don't need cables their strings alone are barely audible. (The video predates widespread use of radio microphones by performers.) Blooteuth (talk) 16:05, 18 July 2017 (UTC)

Indeed. sound power is a tricky thing, as evidenced for example by a stethoscope or a megaphone (the unpowered kind), both making sound "louder" without any sound amplification. Gem fr (talk) 16:42, 18 July 2017 (UTC)

...by focusing it in a particular direction, which makes it louder there and quieter elsewhere. StuRat (talk) 17:07, 18 July 2017 (UTC)

A problem is that "just increase mass" goes against the idea of "very thin" - that's where my uncertainty comes in. Wnt (talk) 17:05, 18 July 2017 (UTC)

A guitar string doesn't make "a loud sound". It makes a very quiet sound. If it's a solid body electric guitar, there's a magnetic pickup and an amplifier - without it, the string alone is hardly loud enough to hear when practising. An acoustic guitar has the vibration of the string coupled to the soundboard of the guitar, which then acts as a diaphragm.

The problem is the impedance of the movement, the relationship between force and distance in its movement. The string may involve quite some energy when vibrating, but in a stiff, solid wooden body. So it's a high force, low amplitude vibration. Air, in contrast, needs more amplitude (i.e. displacement distance) to make a loud sound. Violin and acoustic guitar bodies are mechanical transformers: they transform the impedance, just as an electrical transformer does (ratio between voltage and current). Short vibrations from the bridge can deflect the stiff wooden bridge, making the large area soundboard move. This then acts upon the elastic and easily displaced air, with a lower pressure, but greater volume of air moved, thus louder. Andy Dingley (talk) 19:13, 18 July 2017 (UTC)

Bowel epithelium regeneration

Hi. The bowel epithelium is said to completely regenerate every few days. Yet people with inflammatory bowel disease get deep, long-standing ulcers. What is the answer to this apparent paradox?--90.69.12.160 (talk) 13:32, 17 July 2017 (UTC)

Perhaps the ulcers prevent regeneration of normal tissue, much as scars on the skin do. StuRat (talk) 13:48, 17 July 2017 (UTC)

The epithelium regenerates in a specific way normally. There are intestinal crypts with anchored stem cells, which produce various derivatives, most of which move rapidly upward and are systematically eliminated when they reach the end of the intestinal villi. But if the entire crypt is disrupted or lost, that pattern no longer makes sense. And an ulcer is invariably something larger than a crypt. Wnt (talk) 14:57, 17 July 2017 (UTC)

The epithelium does regenerate even in the setting of IBD but the regeneration process is disrupted as a result of continues inflammation. On the other hand if IBD goes in a remission the epithelium often regenerates successfully. I also want to note that in Crohn disease the inflammation is not limited to the epithelium. Also see, this. Ruslik_Zero 18:37, 17 July 2017 (UTC)

Inappropriate usage re "Before Adam"

In the lede of the article on Jack London's novel Before Adam (1906) is the unreferenced sentence:

"It is the story of a man who dreams he lives the life of an early hominid Australopithecine."

As far as I can tell (though I could be mistaken), the term "australopithecine" could only have been coined in/after 1925 when Raymond Dart announced his discovery of the Taung skull, coining the new taxon Australopithecus africanus. London does not of course use the word in the novel, and given the state of knowledge in 1906 cannot himself have had such a precise comparison in mind. Is this not WP:Synthesis? (Alternatively, does anyone know of a suitable RS that could be cited to support the use of it?) (Being about a literary work, this could have gone in Humanities, but I thought questions concerning the early use of scientific terms would be better addressed here.) {The poster formerly known as 87.81.230.195} 94.12.78.91 (talk) 17:53, 17 July 2017 (UTC)

In the meantime I've removed the word from the article, since it's at best unnecessary. Henry^Flower 18:21, 17 July 2017 (UTC)

If all else fails, check the book and see what term (if any) the author uses. ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:56, 17 July 2017 (UTC)

I had of course already done that. The protagonist describes his "racial memories" of an ancestor in terms of a college-educated man of 1906 (or perhaps a little earlier), but says only "He lived . . . in that period that we call the Mid-Pleistocene." My point is that London could never have intended the deduction of "Australopithecus" because any such knowledge, let alone the actual term, lay some 20 years in the future. {The poster formerly known as 87.81.230.195} 90.202.209.109 (talk) 11:53, 18 July 2017 (UTC)

Shockingly, no "Australopithecine" is to be found. [2] But one might try to infer the phylogeny from this (he also describes his "mother" but not as precisely):

He was not an extremely prepossessing father, as fathers go. He seemed half man, and half ape, and yet not ape, and not yet man. I fail to describe him. There is nothing like him to-day on the earth, under the earth, nor in the earth. He was a large man in his day, and he must have weighed all of a hundred and thirty pounds. His face was broad and flat, and the eyebrows over-hung the eyes. The eyes themselves were small, deep-set, and close together. He had practically no nose at all. It was squat and broad, apparently with-out any bridge, while the nostrils were like two holes in the face, opening outward instead of down.

The forehead slanted back from the eyes, and the hair began right at the eyes and ran up over the head. The head itself was preposterously small and was supported on an equally preposterous, thick, short neck.

There was an elemental economy about his body—as was there about all our bodies. The chest was deep, it is true, cavernously deep; but there were no full-swelling muscles, no wide-spreading shoulders, no clean-limbed straightness, no generous symmetry of outline. It represented strength, that body of my father’s, strength without beauty; ferocious, primordial strength, made to clutch and gripe and rend and destroy.

His hips were thin; and the legs, lean and hairy, were crooked and stringy-muscled. In fact, my father’s legs were more like arms. They were twisted and gnarly, and with scarcely the semblance of the full meaty calf such as graces your leg and mine. I remember he could not walk on the flat of his foot. This was because it was a prehensile foot, more like a hand than a foot. The great toe, instead of being in line with the other toes, opposed them, like a thumb, and its opposition to the other toes was what enabled him to get a grip with his foot. This was why he could not walk on the flat of his foot.

However, note that the very issue our article Australopithecus africanus raises applies here -- the author was expecting bipedalism somehow to represent an intellectual achievement, rather than a predecessor to human-like intelligence. As a result, his description will not closely match any known animal. Wnt (talk) 21:06, 17 July 2017 (UTC)

Using an air conditioner after a power outage

If there is a power outage and your air conditioner suddenly goes off, when the power comes on again a minute later, should the air conditioner be left off for a while? I ask this because, when turning it off normally, it stays on and does something for a minute before shutting down completely.

I know that when you buy a fridge, you should let it sit for a while before plugging it in. Is that related?

Cheers. Anna Frodesiak (talk) 20:43, 17 July 2017 (UTC)

It's a good idea to turn off all heavy loads when the power goes off - otherwise they will all go back on together. That produces a surge, which may cause something to fail again.

A/C controls are a bit of a convoluted law unto themselves. The more sophisticated the control is, the more likely it is to find a reason to not turn on immediately. Dehumidifiers in particular may wait a while, to allow themselves to defrost.

The thing with fridges (and portable A/C) isn't about power, it's that the compressors are usually oil lubricated in a sump at the base, with the freon [sic] refrigerant filling the space above this. If the fridge is tilted during shipping, the oil gets mixed up with the freon. Turning the compressor on then could blow oil into the very long, thin tubing of the refrigeration circuit, potentially blocking the expansion valve. After tilting or shaking a fridge, leaving it for an hour allows the oil to settle back out again. Andy Dingley (talk) 22:09, 17 July 2017 (UTC)

Ah, okay, I see. Thank you Andy. And I always wondered about that fridge thing. Cheers. :) Anna Frodesiak (talk) 22:14, 17 July 2017 (UTC)

While I appreciate your answer, and believe everything you say, I wonder if anyone can find a reference on the topic? I have looked briefly and all I can find are not-that-reliable sources like this [3]. It says to wait 30 minutes for internal breakers to reset, but I don't know widely that applies, or if it's different for window units vs. centralized systems, etc. SemanticMantis (talk) 22:53, 17 July 2017 (UTC)

I believe there's also an overheating concern. If there's a power problem where the power keeps turning on and off, and the A/C does the same, it may not get the fan cooling it normally gets, after you turn it off. I believe most new A/C's have either a heat sensor or timer and won't turn on the compressor until the fan has run for a bit. However, I did have an old A/C unit which apparently lacked this protection, and was destroyed by this issue. StuRat (talk) 00:15, 18 July 2017 (UTC)

• I've been through two three-day blackouts, one in Manhatttan and one in NJ. On both occasions, it was asked that people turn off all non-essential appliances, especially air-conditioners, until the power was fully restored. In NYC, people not following this advice led to the power drain when power was restored (my father says the appliances act as innumerable uncharged capacitors) to cause a short "brown-out" that quickly reverted to new outages as stressed wires tried to carry the abnormally high load. Since this material was communicated by radio and from a retired engineer, I don't have any direct sources for it. Here is a google search that addresses the issue both from the consumer's and the utilities viewpoint. μηδείς (talk) 02:28, 18 July 2017 (UTC)

• The capacitor link isn't really appropriate for AC. The problem is caused by the extra current drawn as an AC motor starts up, but the effect is similar to charging capacitors from a DC supply. Dbfirs 05:35, 18 July 2017 (UTC)

• I could have been more explicit, I did not mean the capacitor of one single appliance needing to recharge when it alone has been off. My father explained that basically the entire neighbourhood (see Anna below) has discharged, and items like fans, cell-phone charges and computers all coming on at once acts as a capacitor that draws a lot more current than would normally be the case when the power is on and the draw is at a normal rate, because most things are already "powered up". My experience in NYC during the 3-day heatwave blackout was that when the power did come on, the lights would come on at full strength, but that within a few seconds there would be noticeable dimming, progressing from a brown-out to a black-out within a minute or two.

Here is a link to a NYT article about the heatwave blackout. I didn't read the entire article, but it speaks of power being out in "Upper Manhattan" for 19 hours. But full power was not restored in certain neighbourhoods (or blocks, or buildings-including mine) for 60 hours. μηδείς (talk) 17:54, 18 July 2017 (UTC)

I am so terribly sorry. I'm talking about when the neighbourhood power goes out. I wasn't clear. I was wondering if the fluid inside all gets stuck somewhere in the wrong pipes or something and when the power is restored and you turn on the AC, is this a problem.

Also, modern ACs in China don't come on automatically after a power failure. You have to turn them on. Oh, and this is true in India too. I know that because some hotels turn the power off and on again at 3am to turn off everyone's AC. This is a trick to save money. Sneaky. :) Anna Frodesiak (talk) 07:28, 18 July 2017 (UTC)

Perhaps some of these references might help: [4][5][6][7][8][9] Here is what I was taught in my advanced refrigeration design class:

It is harder to spin the shaft of a positive displacement compressor after it starts running and there is a head of pressure between the hot and cold sides. Low-cost small capacitor-run single phase induction motors have a lower start torque than maximum torque while at speed. This means that some compressors can't restart until the pressure drops. If the compressor is turned on before the pressure has equalized, the compressor stalls. In the case of small, cheap refrigerators and air conditioners, this causes the overheated motor thermostat to cut power to the motor. By the time the motor cools down, the pressure has equalized and the compressor starts normally. --Guy Macon (talk) 08:52, 18 July 2017 (UTC)

Air conditioners use a fan, so when one "does something for a minute" after being switched off that will be the fan slowing down and coming to rest. All electric fans exhibit this behaviour. 2A00:23C0:7F02:C01:9DCF:5631:446B:F686 (talk) 09:43, 18 July 2017 (UTC)

I don't think it's just spinning down, in most cases, but rather it has a timer to run the fan a bit after the compressor turns off, to cool the device down. If it was just spinning down, a small fan like that would stop in seconds. StuRat (talk) 15:01, 18 July 2017 (UTC)

As for A/C units staying off when the power comes back on, I have 2 window units, one with an electronic keypad, which stays off, and another with just rotary switches, which comes back on. I expect that to be a common pattern. Note that having A/C units that come back on automatically could be very important, in some situations. For example, if you have pets at home and a power failure turns the A/C off right after you leave for work, on an extremely hot day, they could die from the heat by the time you return home, if the A/C doesn't restart. StuRat (talk) 15:06, 18 July 2017 (UTC)

Hi folks. Thank you all for the thoughtful replies. This is all quite above my mental pay grade. This is how my brain hears it:

"...while at speed, capacitor-run pressure between maximum torque and single phase positive displacement motors start with most lower start rotary switch phase induction shafts..."'

So, when the power comes on again, like one minute after the neighbourhood power outage, should I let the AC remain off for while, or is it okay to turn in on right away? Sorry to be so dumb about all this. (I bake. That, I understand. When the AC is on the fritz, I call the man, who comes and grumbles and terrifies us all by standing outside on a ledge without a safety net. Then he leaves us with an empty green flask that is "garbage" but looks like it should be refilled. ) Anna Frodesiak (talk) 18:50, 18 July 2017 (UTC)

Leave it off for a five minutes. If the power seems fine during that time, go ahead and turn it back on. StuRat (talk) 20:24, 18 July 2017 (UTC)

BTW, if only your power comes back on, and not the entire neighborhood, that sounds like a automatic generator kicked in (do you hear an internal combustion motor turn on ?). If so, it may have limited capacity, and running A/C might be too much. I'd ask your landlord if this is the setup and, if so, if you can run the A/C on the generator. StuRat (talk) 20:28, 18 July 2017 (UTC)

Hi StuRat. Thanks for the good advice. No, no generator. When it goes, it's the neighbourhood and all is dark. Cheers! :) Anna Frodesiak (talk) 21:02, 18 July 2017 (UTC)

Another anecdote: our home A/C compressor has some kind of "shut-out", possibly the thing StuRat mentioned above. (This is Southern California, with a whole-home HVAC system.) If the A/C is shut off for whatever reason, and then you turn it back on immediately, the thermostat displays "Wait", and it takes 5-10 minutes before the A/C switches on again. As StuRat stated, presumably this is to protect against overheating or some other damage from repeatedly cycling it on and off; this is a fancy new HVAC system, only about five years old, with computerized everything. (So, Anna, our replies sound like this? ) --47.138.161.183 (talk) 21:23, 18 July 2017 (UTC)

Interesting. Thanks for the input, 47.138.161.183. Sorry I cannot view Youtube. It is blocked where I am. Best, Anna Frodesiak (talk) 21:29, 18 July 2017 (UTC)

Make sure to look at Turboencabulator then . —PaleoNeonate - 21:39, 18 July 2017 (UTC)

I just bought a Turboencabulator from Amazon. The instructions are in Swahili, it seems to plug into itself, and I don't know how to turn it off. Anna Frodesiak (talk) 21:44, 18 July 2017 (UTC)

Use a Laxian Key. Andy Dingley (talk) 22:17, 18 July 2017 (UTC)

@Anna Frodesiak: Basically Guy Macon has answered your concern "if the fluid inside all gets stuck somewhere in the wrong pipes or something" ab2ove in a technically correct reference to the build up of a head of pressure if the coolant ("fluid") phases have not come into equilibrium. There are really two separate issues, so let me address them.

First, the air conditioner itself has a resevoir of liquid coolant. This coolant is vented into coils under low pressure, which makes it expand, becoming a gas, and losing heat (see Ideal Gas Law and considered how spray bottles like condensed air get cold when you vent their liquid contents). This coil becomes cold, and air from the house is blown over it, cooling the inside of the house. That, now heated gas is pumped outside under pressure into a condensor coil, where excess heat is vented into the air outside the house. This is all in a closed circuit, and the coolant in the condensor coil will return to the liquid reservoir, and the cycle start over.

If you turn off the AC, and then restart it before the hot vapor in the condensor coil has had a chance to cool, it will overpressure the system, as Guy mentions.

In a neighbourhood blackout, your AC will come to equilibrium after a short period. Once that has happened, there is no real problem with turning on your conditioner, except for the problem mentioned above of the total neighbourhood drain on the electric grid when everyone's appliances all try to power up at once. This is a separate issue, but in each case the advice is not to immediately restart an AC you have just turned off, and to turn off an AC (unplugging is best) during a blackout so as to help the utility company restore power at a safe and moderate rate.

μηδείς (talk) 21:56, 18 July 2017 (UTC)

Hi μηδείς. Now I understand!!!! You should write instruction manuals. You really should. Thank you ever so much! :) Anna Frodesiak (talk) 00:05, 20 July 2017 (UTC)

Better version/correction to Bekenstein–Hawking entropy formula?

Please see Commons:Commons:Deletion requests/File:Bekenstein–Hawking entropy formula.png. The question is whether this image which is supposed to represent the Bekenstein–Hawking entropy formula, was a proper replacement for this image, which is also supposed to represent the Bekenstein–Hawking entropy formula.--Fuhghettaboutit (talk) 21:46, 17 July 2017 (UTC)

since $\hbar ={{h} \over {2\pi }}$, both formula are the same so i don't bother and don't see why we should.

Maybe an image of both formula would suit even better?

Gem fr (talk) 14:27, 18 July 2017 (UTC)

No, YES it is the substitution Planck length $\ell _{\mathrm {P} }={\sqrt {G\hbar /c^{3}}}$ into $S_{\text{BH}}={\frac {kA}{4\ell _{\mathrm {P} }^{2}}}$

which makes it equivalent to this image. The conjectured relation is explained at Black_hole_thermodynamics#Overview. People concerned with making a decorative image of the equation are discussing its handwritten (goofy-looking?) or typeset appearance and its scaleability. In any case the bar on $\hbar$ that signifies the reduced Planck constant $\hbar ={\frac {h}{2\pi }}$ should not be overlooked can be omitted as done in this image. Corrected with thanks to Gem fr. Blooteuth (talk) 16:32, 18 July 2017 (UTC)

why "no"? whatever it came from (and whether it is correct or not!), the formula is just the same in both images, just written using hbar or h/2Pi respectively --anyway the same quantity-- . And the bar in hbar is NOT overlooked in the later image you mention, it is just replace by /2Pi as is fit. Gem fr (talk) 15:57, 18 July 2017 (UTC)

Thanks for the replies. Got it. I'm going to change my !vote to "Keep"--Fuhghettaboutit (talk) 21:19, 18 July 2017 (UTC)