Science desk
< March 1	<< Feb \| March \| Apr >>	March 3 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is a transcluded archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

March 2

Colliding EM waves and energy conservation

Please refer to the image.

I create 2 EM waves by the simplest method - moving 2 charged objects back and forth once.

Both E and B fields values are opposite to each other. That is, at any given time, if in wave 1 B=a and E=b, than for wave 2 at the same time B=-a and E=-b.

Problem - At the instance both waves overlap they cancel each other completely.

This cannot be - It breaks conservation of energy.

What is answer to this seemingly contradiction?

I asked this question a while ago, got a horrible (and partially wrong) answer that I didn't think through. The false claim in the answer was that the drawings are irrelevant. This is wrong: at any given moment E and B fields have a distinct value at any point in space, and this is what the drawing depicts.

אילן שמעוני (talk) 05:32, 2 March 2021 (UTC)[reply]

The drawing is not "irrelevant", but it's underspecified. Taken literally, it seems that you're giving the E and B fields at one instant in time, and only on two line segments. In that case, the total energy is zero, and nothing measurable happens.

Probably that's not what you really mean, but I can think of at least two other things you might mean. If you'd be more precise about exactly what you do mean, someone might be able to answer your question, or you might find out you've answered it yourself. --Trovatore (talk) 05:58, 2 March 2021 (UTC)[reply]

I fail to understand what you mean by "...and only on two line segments.". If you refer to that the B and V values are only supplied for the waves, just assume we do this in intergalactical space where the background B and V values are negligible. If you refer to that I only measure values along this single axis and ignore that the waves are in fact 3D - this is true but I can't see why it's relevant.

Please note that the test is conducted after the situation depicted in the image, where both waves advanced to the point they overlap.

I'll rephrase - what data regarding B and V values you seek that is absent from the drawing?אילן שמעוני (talk) 07:27, 2 March 2021 (UTC)[reply]

I'm not sure where V came from; before you were talking about E and B.

The reason it's relevant that you only measure values along a single axis is that Maxwell's equations are three-dimensional. How are you going to compute a curl in one dimension?

So the information I'm missing is, what are the E and B values at all other points in space? Let's say the direction you're taking to be the direction of propagation is the z-axis. What is the dependency on the x- and y-coordinates, and what are the values of E and B at z-coordinates other than those of the two wave packets you show? --Trovatore (talk) 07:44, 2 March 2021 (UTC)[reply]

My bad, it's E, not V.

Mmmmm, I think I understand your question and can not answer it (last time I dealt with cross-products and calculus was 30 years ago), BUT I can't see why this matters to energy conservation.

Energy is supposed to be conserved for any volume without flow of energy in or out.

This true for a volume as small as you wish around the overlapping segment on the z-axis. We can reduce the x and y coordinates around this segment to our liking, and the E and B values will converge to zero.

In the same manner - flow of energy in and out of given volume requires time. Since we talk abot the instance of overlapping, we can again reduce the interval around measurement, and the flow will converge to zero.

I suspect that somehow the cross product that dictates E and B is such that they cannot both be opposites - that if for given time (for wave 1 E values are positive while wave 2 are negative) than (at the same time, B values for both waves must be equal). Buy I lack the knowledge how to check this. I only found how to check the values for moving charged particle, not for EM wave.

אילן שמעוני (talk) 12:14, 2 March 2021 (UTC)[reply]

Link to previous discussion: WP:Reference_desk/Archives/Science/2019_October_31#Destructive_interference_and_conservation_of_energy. You're still assuming you can choose a direction for the E field, choose a direction for the B field, and also independently choose a direction of propagation for the wave. This is not correct. They are related by a cross product as described in Poynting vector. Therefore, the direction of propagation shown in the right-hand half of your drawing is not consistent with the indicated E and B fields. --Amble (talk) 17:29, 2 March 2021 (UTC)[reply]

Ah, thanks, I missed that too. I was trying to figure out how the energy would get in and out of a standing-wave region — I think there's no speed-of-light problem per se, but the energy flow would increase without bound as the region got bigger, which seemed at the very least odd. But it's just the wrong picture. The energy doesn't need to go anywhere; it just sloshes back and forth between electric and magnetic fields. --Trovatore (talk) 20:58, 2 March 2021 (UTC)[reply]

So my suspicion was correct. Thanks. אילן שמעוני (talk) 19:24, 2 March 2021 (UTC)[reply]

Steel bollards on road - higher risk of accidents

Are there any scientific studies to what extent steel bollards on the streets increase the risk of accidents?

The type of bollards I'm talking about: https://www.bz-berlin.de/data/uploads/2019/05/radweg_1558535427-1024x576.jpg --88.78.14.45 (talk) 15:51, 2 March 2021 (UTC)[reply]

Probably decrease the risk of accidents at the expense of average vehicle speed. Abductive (reasoning) 16:02, 2 March 2021 (UTC)[reply]

It's worth considering what type of accident we are talking about. As positioned in the photo, it would appear that they are designed to prevent accidents involving a automotive vehicle hitting a bicycle or hitting a pedestrian. A moving chunk of metal vehicle, even at a gentle 40 km/h, hitting a pedestrian has a pretty large chance of killing said pedestrian. A vehicle moving at 40 km/h and hitting one of those bollards is unlikely to kill the occupants, especially if they are wearing seat-belts. On a risk/benefit analysis, I'd say that even if they somewhat increase the frequency of accidents as cars hit the things (cost), the benefit in eliminating a large portion of fatal accidents and replacing them with non-fatal accidents is high. --OuroborosCobra (talk) 17:14, 2 March 2021 (UTC)[reply]

Different types of barriers have certainly been studied. I don't know if you'll be able to find a paper on exactly the kind of barrier in that picture. They might not have been common enough long enough.

Here are some good starting points, though

An insight into the performance of road barriers − redistribution of barrier-relevant crashes You'll need journal access for this one. Or you'll need to fork out $41.95
Performance Assessment of Road Barriers in Indiana This one is free.

I really expected the NTSB to have something, but I could find anything. ApLundell (talk) 04:49, 3 March 2021 (UTC)[reply]

As an aside, does anyone know the American English word for "bollard"? I'm quite sure that word was not in my active vocabulary, but I'm familiar with objects of that sort and I don't know what I would have called them. Maybe just "barriers". I don't see anything at Wiktionary. --Trovatore (talk) 01:09, 6 March 2021 (UTC)[reply]

"Pylon". Hayttom (talk) 17:01, 9 March 2021 (UTC)[reply]

Pylon suggests it refers to traffic cone, which in the UK at least is not the same as bollard. Bazza (talk) 17:22, 9 March 2021 (UTC)[reply]

physical properties of octane isomers

I have searched for the following physical properties of octane isomers, "molar volume, total surface area and octanol-water partition coefficient", on internet but could not find anywhere, can any body help me to find it. — Preceding unsigned comment added by 182.187.18.182 (talk) 19:26, 2 March 2021 (UTC)[reply]

Before we run into an XY problem-type situation, can you elaborate the purpose for this information? If we know what larger problem you need this information for, perhaps we can better help you find the information that will help you. --Jayron 32 19:30, 2 March 2021 (UTC)[reply]

This may be helpful for the octanol-water partition coefficient... I found n-octane and 2,2,4, trimethyl pentane. There are a lot of C8H18 compounds though. There's n-octane, 3 different methylheptanes, 2 different ethylhexanes, 6 different dimethylhexanes, and a bunch of pentane derivatives I don't feel like counting, plus your 2,2,4,4 tetraethylbutane. --Jayron 32 19:42, 2 March 2021 (UTC)[reply]

The octanol-water partition coefficient article is too technical, in my opinion, and duplicates much of the information at partition coefficient (a much better article overall). The concept is very important in drug and pesticide design and chemists doing such work normally use the LogP as a vital parameter when comparing compounds within a given analogue series. Very many of Wikipedia's articles on commercial products of these types includes the LogP in the Chembox and there are online collections at repositories such as "The Pesticide Properties DataBase".. I have no idea why the OP wants the value for octane isomers but such values can be pretty reliably calculated when they have not been experimentally determined (see cLogP in the partition coeffficient article). Mike Turnbull (talk) 10:24, 4 March 2021 (UTC)[reply]

I have made a proposal to merge the articles and added the appropriate templates. Please comment at Talk:Partition coefficient#Merger proposal: Merge Octanol-water partition coefficient into this article if you wish. Mike Turnbull (talk) 10:37, 4 March 2021 (UTC)[reply]

To answer the OP's original question, many compounds in Pubchem have recorded data for their physical properties, so the the logP (octanol-water partition coefficient) is 5.18 [1], although our octane article says 4.783 without giving a specific source: it is probably a predicted value, since Chemspider agrees with Pubchem (the ultimate experimental source is MM Miller). No doubt Pubchem will have other isomers. Mike Turnbull (talk) 12:34, 4 March 2021 (UTC)[reply]

For molar volume, you can calculate it from the more easily accessible density and molar mass (which will be about 12×8+18=114 g) ie 114/ρ cc. Graeme Bartlett (talk) 07:31, 6 March 2021 (UTC)[reply]

For molecular total surface area, if you care about it, we probably need an article. For polar surface area, your answer will be 0Å². Graeme Bartlett (talk) 07:37, 6 March 2021 (UTC)[reply]

On the article front I have discovered Accessible surface area, Relative accessible surface area and Van der Waals surface. Are any of these describing the measurement you want 182? Graeme Bartlett (talk) 10:18, 6 March 2021 (UTC)[reply]

Higher taxa named after people

What higher taxa (from genus up to a family, order or higher, like Smithatris after Rosemary M. Smith) are named after people? Preferably full list, if available somewhere. 212.180.235.46 (talk) 23:58, 2 March 2021 (UTC)[reply]

Here's a query on WikiData: [2] that is as complete as the information on WikiData. It returns every taxon that's not at a low level (species, subspecies, variety, strain, or cultivar) and is listed as being named for something (not necessarily a person). There are quite a few genuses and some families named for people. There's a class arachnida named for the mythological person Arachne, and a phylum lokiarchaeota named for Loki's castle, so you can decide whether those count. This query doesn't return a fungal subdivision pucciniomycotina, which seems to be named for a Tommaso Puccini [3], although I'm not sure this is the right Puccini. --Amble (talk) 02:14, 3 March 2021 (UTC)[reply]

Ultimately, within taxonomic nomenclature, Pucciniomycotina gets its name from the genus Puccinia, which was established in 1729 by Micheli, named "in honor of Puccini, a Florentine professor".^[4] Its eponym cannot have been the art historian (not "professor of anatomy") Tommaso Puccini (1749–1811). Merriam–Webster states: "from Tommaso Puccini †1735 Italian anatomist".^[5] The bust shown in our article Tommaso Puccini is dated as being from 1718, and its sculptor died in 1725, so it also cannot depict the future art historian. --Lambiam 09:17, 3 March 2021 (UTC)[reply]

Other sources give 1666–1726 for the anatomist, who reportedly studied with Lorenzo Bellini and was connected to the Hospital of Santa Maria Nuova.^[6]^[7] --Lambiam 09:32, 3 March 2021 (UTC)[reply]

Well done. Do you think the anatomist is notable enough for an article? —Amble (talk) 15:16, 3 March 2021 (UTC)[reply]

He obviously was notable in his days, not only having a fungus named after himself (what was the connection there?) but also having a bust in marble by a famous sculptor as well as a medal with his effigy. The bust is in a museum in Pistoia and the inscription patrit(ivs) pistorien(sis) on the medal also connects him to Pistoia, which makes it likely he was a relative – perhaps the grandfather – of the art historian, a native of Pistoia. I also saw that the anatomist was included in the circle of friends of the painter Pietro Dandini.^[8] Nevertheless, altogether I did not spot enough material for an article. --Lambiam 23:54, 3 March 2021 (UTC)[reply]

One more snippet: Villone Puccini aka "Villa Puccini" in Pistoia was built by the anatomist.^[9] --Lambiam 00:26, 4 March 2021 (UTC)[reply]

Our page list of fungal orders lists kickxellomycota as a division; but kickxellomycotina is a subdivision and that page doesn’t list a division (“incertae sedis”). Whether division or subdivision, it’s named for Jean Kickx. I wonder whether this could be the highest example of taxonomic “promotion through the ranks,” where originally a genus was named after him, and the genus name eventually filtered up to (maybe) a division... —Amble (talk) 16:08, 3 March 2021 (UTC)[reply]