Wikipedia:Reference desk/Archives/Science/2019 February 21

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February 21

Carbon 14 beta decay

Why does ¹⁴
₆C
undergo beta decay to ¹⁴
₇N
? I estimated the binding energies of both nuclei using the semi-empirical mass formula, and found that, as expected, ¹⁴
₇N
's advantage in the 4^th term (asymmetry) overshadowed ¹⁴
₆C
's advantage in the 5^th term (spin coupling). But since the effect of the 3^rd term (electrostatic repulsion) also significantly advantages ¹⁴
₆C
, the ¹⁴
₇N
system overall is predicted to have less binding energy than ¹⁴
₆C
. I suspected that this may be due to inaccuracies in the semi-empirical model, so I checked by calculating the overall binding energies from the mass defect values in the US Department of Energy Nuclear Wallet Cards. Based on those data, I also found that the binding energy of ¹⁴
₇N
is lower than the ¹⁴
₆C
nucleus. So why would ¹⁴
₆C
undergo beta emission if it causes it to lose binding energy? The overall difference in energy of the two systems shows that ¹⁴
₇N
is actually lower in mass-energy than ¹⁴
₆C
by 0.157MeV/c². This difference is the sum of the difference in binding energies as calculated from the DoE data, -0.625MeV, and the difference in rest mass between a proton and a neutron, 0.782MeV/c². So it seems to be suggesting that the beta decay of ¹⁴
₇N
is driven by the lower energy of a proton than a neutron, as with free neutron decay. But if I use the difference in binding energies from the semi-empirical formula, about -3MeV, the difference in neutron and proton rest masses cannot overcome it. Therefor, the prediction is that this decay would not occur. Obviously, this decay type does actually happen, so what's the defect in the semi-empirical model that leads it to make the wrong prediction. 202.155.85.18 (talk) 04:22, 21 February 2019 (UTC)

@202.155.85.18: You've gotten it the wrong way around: since nitrogen-14 has less mass-energy than carbon-14, the decay of the latter to the former comes with a net release of (mass-)energy from the system and is thus energetically favored. A decay can only occur with such a net loss of mass-energy since any particle given off (be it an alpha or beta particle) will possess kinetic energy as it flies out.--Jasper Deng (talk) 04:51, 21 February 2019 (UTC)

Perhaps the way I worded it was confusing at points. I understand that less mass-energy means more stability. The ¹⁴
₆C
has more binding energy though, which actually means a reduction in its mass-energy (because binding energy is the source of the mass defect). Overall, the ¹⁴
₆C
still has more mass though, which seems to be a consequence of the difference in mass-energy between a neutron and a proton. 202.155.85.18 (talk) 06:31, 21 February 2019 (UTC)

I don't know nuclear physics, but I think I should try to break down the question as I'm curious myself. We know that a lighter nucleus will not decay to a heavier one (unless you have a star or a hydrogen bomb or something to provide extra material). So for C-14 to turn to N-14 is the only way this reaction can go, and you've said why, namely the proton mass is less. In other words, the neutron in C-14 is fairly far out there (in our figure there are six proton balls and eight neutron balls) and if it should turn into a proton, there's a place for it. A free neutron would do such a thing very quickly. But, had your semi-empirical formula, plus the weights of the nucleons, come to the conclusion that the overall nuclear weight would be increased, presumably that means that the proton that would be created would not be able to exist (Pauli exclusion principle) in the state where it would be made, so it can't come into existence? What I don't understand is why this process of beta decay is so much slower here than for a free neutron. Our article describes several mechanisms of beta decay, their effect on overall nuclear spin, but I'm not sure what to look for. In any case, using thermodynamics will generally tell you which way a reaction goes, but not the kinetics of how fast it occurs. Wnt (talk) 15:19, 21 February 2019 (UTC)

So my guess is, that the reason the process of beta decay of ¹⁴
₆C
is so much slower than for free neutron decay, is that the energy difference between the states of free neutron vs proton is 0.782MeV, whereas for ¹⁴
₆C
vs ¹⁴
₇N
it's only 0.157MeV. Being less energetically favorable limits the pathways to beta decay. On the other extreme, there are halo nuclei like ¹¹
₃Li
where the outer
n
is loosely bound and spontaneously undergoes beta decay faster than a free neutron. The only way I can explain that, is if you think of the loosely bound neutron as a free neutron near a nucleus, the presence of a nearby nucleus gives the free neutron an option to transition not just to a free proton, but to a lower energy state again: a bound proton, which makes this more energetically favorable than simple free neutron beta decay. 202.155.85.18 (talk) 01:15, 22 February 2019 (UTC)

An interesting response, but I think you may be on the wrong track. The net energy difference is, once again, thermodynamics. For kinetics at the very least we would need to figure out the energy difference to the transition state. The catch is I know nothing at all about nuclear transition states. Wnt (talk) 16:33, 23 February 2019 (UTC)

You also need to consider other barriers to decay. The net energy change is not the whole story; the energy barrier involved (very roughly analogous to activation energy) is also important. A "taller" barrier will be harder to tunnel through. Consider the fact that many decays that would naively be considered energetically favorable are not observed. Tantalum-180m is a pretty good example of how energy differences are not the whole story.--Jasper Deng (talk) 18:53, 23 February 2019 (UTC)

"Mamankanois"

When John White first described the species he called "Mamankanois" (Papilio glaucus) after Francis Drake's Virginia expedition, is it known whether he commented about it being bigger than any butterfly species he had ever seen in Europe? (No pictures of this critter please -- unless the picture is very small!) 2601:646:8A00:A0B3:1924:14C4:1666:AB35 (talk) 07:32, 21 February 2019 (UTC)

The description that goes with the original woodcut is "The first Day-Butterfly being the greatest of all, for the most part all yellowish, those places and parts excepted which are here blacked in ink. Moreover, the roundles of the inner wings are sky-colour, insomuch that you would think that they were set with Saphire stones; the eyes are like the Chysolite: the bignesse and form is so exactly set forth in the figure, that there need no more be said of it" - this was a translation that appeared in 1658 of the original latin text that appeared in Thomas Moffet's Theatre of insects in 1634. See here, note that the link does contain pictures of the butterfly. Mikenorton (talk) 17:27, 21 February 2019 (UTC)

Thanks! So he did say it was very big -- which makes sense, given that the biggest European butterfly, P. machaon, is "only" 3.5 inches at most, whereas P. glaucus can grow up to 5.5 inches! 2601:646:8A00:A0B3:1924:14C4:1666:AB35 (talk) 02:16, 22 February 2019 (UTC)

P. glaucus has two subspecies P. glaucus. glaucus (the northern one) and P .glaucus. maynardi (the southern one), which have different sizes. In North Carolina, White will have seen the northern subspecies, which is significantly smaller (see here), although on average still bigger than P. machaon. Mikenorton (talk) 09:13, 22 February 2019 (UTC)

Thanks! You actually answered two questions now: my other question was, on this map (which does have pictures of tiger swallowtails, but small enough that they don't scare me at this size), Florida is shown as the range of a mysterious species "Papilio maynardi", but I could not find any info whatsoever about it and was intending to ask about it (but you confirmed what I suspected, that this is a subspecies of P. glaucus). Now, would you mind answering a third question: What's the maximum size of the northern subspecies of P. glaucus, and what's the geographic range of both subspecies? (As before, no pictures please unless they're smaller than 2 inches in size!) 2601:646:8A00:A0B3:1924:14C4:1666:AB35 (talk) 10:43, 22 February 2019 (UTC)

This map shows the overlap areas of the two subspecies in Florida and Georgia and the overlap with P. canadensis to the north. I haven't been able to find anything about maximum sizes. Mikenorton (talk) 11:23, 22 February 2019 (UTC)

Thanks, this is useful info (and quite a relief that the bigger and scarier P. glaucus maynardi subspecies is only found in Florida and south Georgia -- although the ones I've seen in Tennessee are plenty big enough to make me run for cover!) 2601:646:8A00:A0B3:1924:14C4:1666:AB35 (talk) 07:49, 23 February 2019 (UTC)

Stopping the bleeding using absorbent dressing

In an emergency situation, why are the dressings for stopping the bleeding absorbent (among other properties)? Wouldn't an absorbing material increase bleeding? What would happen if we covered an injure with some plastic+pressure? --Doroletho (talk) 13:12, 21 February 2019 (UTC)

Pressure is always helpful (per every first aid course I've taken). You seem to be worried that the absorbent material is kind of drawing out the blood and I don't see how that could be the case. What you're trying to do (in most cases) is induce coagulation so that a blood clot forms, reducing blood loss (hemostasis. An absorbent bandage encourages this by slowing down the blood flow, allowing the cells time to coagulate. Matt Deres (talk) 16:17, 21 February 2019 (UTC)

(ec) Allowing a small amount of drainage lets fresh blood clean the wound of infection and form a protective clot. See History of wound care especially Modern wound care. An absorbant bandage also serves as a cushion barrier against external injury to, or infection of the healing process. Sealing a wound would lock in any inflammatory debris which risks delaying healing and creating a chronic wound. DroneB (talk) 16:54, 21 February 2019 (UTC)

Frequency of the sound of water falling

I've noticed that the apparent frequency of the noise of filling a vessel with water seems roughly inversely proportional to the remaining unfilled volume of said vessel. But obviously that cannot hold / isn't the dominant effect for bodies of water like lakes or rivers, or waterfalls would be in infrasound. What determines the apparent frequency of such noises (like waterfalls)? 78.0.242.164 (talk) 13:38, 21 February 2019 (UTC)

The amount of water and the height it's falling make a major difference in the sound. ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:45, 21 February 2019 (UTC)

So you mean that the speed of water hitting the standing water matters? I'm not sure if the actual amount matters (except for loudness obviously) once you get past the turbulent flow threshold. Admittedly my knowledge is only based on experiments in the kitchen sink. 78.0.242.164 (talk) 13:57, 21 February 2019 (UTC)

The vessel has resonance within it, whereas the waterfall has no resonator cavity. I can attest that a certain poorly done park dam I know of has a giant cavity between walls of concrete between which water spills and therefore produces a lot of "infra"sound, though I should note that as far as I'm concerned it seems audible enough to be a degradation to the park. Wnt (talk) 15:23, 21 February 2019 (UTC)

Hm, you're probably hearing upper harmonics. I forgot completely about those. So the main noise could in theory be a resonant sound after all, at least e.g. in a waterfall surrounded by rocks on three sides. I wonder though what components make non-resonant water turbulence noise, e.g. someone pouring a bucket in the middle of the ocean, since it's pretty similar, but not quite the same as white noise. 78.0.242.164 (talk) 16:38, 21 February 2019 (UTC)

Water hitting water will generate sound across a broad range of frequencies. The sound you perceive though will be dominated by resonance characteristics as the vessel - which change as it is filled. — Preceding unsigned comment added by 2A01:E34:EF5E:4640:7415:A981:11C6:BC23 (talk) 07:06, 22 February 2019 (UTC)

See our aricle on organ pipes. --Guy Macon (talk) 17:33, 22 February 2019 (UTC)

As a lavatory cistern fills the pitch of the sound goes up. I've not looked at how the pitch might change if the lid of the cistern is removed. Children sometimes use the phenomenon to fill a line of tumblers with increasing levels of water - thus making a rudimentary xylophone on which they can play tunes. 2A02:C7F:BE2D:9E00:7485:64D1:8F09:C01A (talk) 19:17, 23 February 2019 (UTC)

Whereas Benjamin Franklin made the Glass Armonica. It takes great genius to hold a glass of water sideways. :) Wnt (talk) 22:49, 23 February 2019 (UTC)

IIRC the pitch goes up by an octave (frequency is doubled) if you open the lid. But I already wrote about that stuff in the original post. I want to know what determines the frequency distribution in absence of resonance (like in the middle of the ocean). (78.0) 93.139.62.14 (talk) 13:25, 24 February 2019 (UTC)

Pattern in digital clock deviations

For the past few years I've had a battery-powered digital clock in my bedroom. It's not very good at showing time, but I've kept it around because it also has a fairly accurate thermometer. So far the clock has always been going fast, and pretty regularly, every 6-12 months or so I've had to set it 5 minutes back. However, for a long while in the last year the clock was steadily 4 minutes late and started to fall behind real time enough that it's now 1 minute behind real time (it's moved 5 minutes in the other direction).

What's the reason behind this slowdown pattern? Is it all just a fluke and normal quartz clock variance? (It's some random cheap Japanese clock and doesn't say it's quartz but what else could it be nowadays.) If this is caused by the battery draining, why is the slowdown so tiny (on the order of 10^-5)? 78.0.242.164 (talk) 13:52, 21 February 2019 (UTC)

Put a fresh battery in and see what happens. ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:58, 21 February 2019 (UTC)

What he said. The frequency of cheap quartz oscillators can change with voltage. You can reduce the effect by choosing a different battery chemistry: http://www.boat-project.com/tutorials/vro.gif

Cheap quartz oscillators can also change with age or temperature. The age-related drift tends to level out eventually, but it could take months or years. Standard quartz oscillators usually drift less than 15 seconds per month. --Guy Macon (talk) 17:23, 21 February 2019 (UTC)

• It's probably an instability in the underlying quartz crystal oscillator. The rest of the circuit is a counter and tends to either work accurately, or so badly as to be useless. In contrast, mains-powered clocks are still often driven from the 50 Hz or 60 Hz mains voltage frequency, which is accurately maintained, but some clocks have a habit of running slightly fast, as they're also sensitive to line noise.

Quartz crystals are mostly sensitive to temperature variations. In one infamous UK case, the Sinclair Black Watch, an early LED digital watch, was so temperature sensitive that it ran grossly slow overnight (losing up to an hour) if taken off, but if worn when sleeping, the batteries went flat instead. High accuracy crystals are often operated in heated temperature-controlled ovens, as it's easier to heat them to a stable temperature than to cool them. However your clock might then run consistently fast instead, which would be of little use. You might (if you were sufficiently interested) replace the internal crystal with a better quality one - clocks mostly use a standard 32,768 Hz frequency, as this can be divided easily (2¹⁵, convenient powers of two) to give a seconds clock. Andy Dingley (talk) 17:28, 21 February 2019 (UTC)

Do mains-powered clocks still typically get their time from the mains frequency? I was under the impression that essentially all powered clocks today use a crystal oscillator because they're so cheap due to being mass-produced as chips. I could certainly be wrong, and would welcome anyone more familiar with the subject chiming in (heh). --47.146.63.87 (talk) 06:48, 22 February 2019 (UTC)

There are still plenty of line powered clocks that consist of a synchronous motor, a gear train, and some hands. --Guy Macon (talk) 11:32, 22 February 2019 (UTC)

• Yes, there are still plenty of purely electronic AC mains-powered clocks that derive their timing from the mains frequency. Surprising to me too. One give-away for them is that any such clock with a battery backup (or a low-voltage DC 'wall wart' supply) will have quartz for that, but if there's no battery then a surprising proportion are still line frequency. This is particularly common for Nixie clocks, again surprising to me, but in this case it's hard to supply the power needed for Nixies from batteries (although they could still keep time from quartz and battery). Andy Dingley (talk) 11:43, 22 February 2019 (UTC)

Thanks Andy, that sounds like it could be the reason. We've had a hot summer (30°C+ room temp.) and a warm winter, so we've been heating to higher temps and I've also moved the clock a couple of months ago accidentally putting it so that it basks in the afternoon sun. A few days ago on a particularly sunny day I walked into the room and the clock's thermometer was showing 37°C while the real room temp. was more like 17°C. I never expected temperature could make such a difference, but I found Crystal oscillator#Temperature effects and it looks reasonably close to my clock's deviations. (78.0) 93.139.17.30 (talk) 14:19, 22 February 2019 (UTC)

Fun fact: Have you ever wondered why so many plastic digital wristwatches have metal backs? Your skin keeps a more constant temperature that the room you are in,[1] so they design the watch so that the part that contacts your skin conducts heat well while the side that faces the room is more of an insulator. This makes the watch more accurate if you wear it a lot. --Guy Macon (talk) 19:29, 22 February 2019 (UTC)

Megachile pluto - Giant Bee

Does Megachile Pluto sting and if so does it use a traditional sting or its large pincers? Does it hurt? Does it make honey? Is the honey edible? Are there any medical benefits or uses? Thanks — Preceding unsigned comment added by 81.131.40.58 (talk) 16:15, 21 February 2019 (UTC)

This article seems to answer some of your questions. --Jayron32 16:18, 21 February 2019 (UTC)

Thanks. Very funny: "...Dr. Robson believes they are capable of stinging, though he wasn’t in a position to provide evidence. 'We were all keen to get stung to see how bad it was,” he said, “but because we only found the one, we treated it very carefully'..." So does it produce honey? Anyone? — Preceding unsigned comment added by 81.131.40.58 (talk) 16:29, 21 February 2019 (UTC)

Well, if he only had the one to go on, it may have been as hard to assess its honey-making capabilities as it would be to assess its stinging. --Jayron32 16:56, 21 February 2019 (UTC)

As a general rule, solitary bees don't produce honey. I'm not sure, but I rather assume this is a solitary bee. Dragons flight (talk) 17:55, 21 February 2019 (UTC)

The article and the first two of its sources that I checked say that the species makes communal nests. I encountered no mention in any of the 8 sources of whether or not it makes honey, but I didn't take out a (free) subscription to that of the article's [4], so didn't read that entire paper. {The poster formerly known as 87.81.230.195} 2.122.1.40 (talk) 01:08, 22 February 2019 (UTC)

I'm by no means an expert, but I believe it tends to only be eusocial bees which produce honey. Bees with communal nests are often considered together with solitary bees, as with our article. I don't know how reliable this source is, but it discuss resin bees in general and the behaviour described doesn't sound conducive to honey production [2] Nil Einne (talk) 07:56, 22 February 2019 (UTC)

The Japanese giant hornet can sting too. even has the most lethal venom/handles the biggest dose of all Bees (til now) which requires hospitalization. So it was probably very wise to handle the giant new one extra carefully. --Kharon (talk) 18:42, 21 February 2019 (UTC)

Agreeing with Dragons Flight... I encountered large bees in Malaysia. I asked about them at the hospital. I was told that they are solitary. They don't make a large nest with a queen and drones. They are like birds. One female makes a nest. A male comes by as necessary. No honey or beeswax is produced. The pollen is collected and used right away. They do have a stinger, but it is not barbed. They rarely sting. The nurses had no memory of anyone ever being stung. Because they don't move around in groups, are very large, and very slow, they don't pose a threat to humans and there's no reason for a human to pose a threat to them. I personally feel that until alcohol is added to the situation, you're not going to find someone getting stung. 209.149.113.5 (talk) 14:57, 22 February 2019 (UTC)

Yes, those giant bees can get very argumentative when they're drunk. Martinevans123 (talk) 15:14, 22 February 2019 (UTC)

Megachile pluto (holotype here: [3]) is also known as Chalicodoma pluto. It's in the Chalicodoma subgenus of Megachile. As far as I know, Chalicodoma bees usually build their nests out of sand and clay, solitary or in groups. They do collect pollen and nectar, but they don't build honeycomb and I don't think they make honey at all. Their closest relatives are leafcutter bees, also in genus Megachile. Leafcutter bees do sting, but weakly and reluctantly. Dr Dima (talk) 00:00, 23 February 2019 (UTC)

As solitary insects. I guess they don't do much dancing? Do they even use any pheromones? Martinevans123 (talk) 00:07, 23 February 2019 (UTC)

Even solitary bees need to attract mates. So, yes. 71.12.10.227 (talk) 03:07, 23 February 2019 (UTC)