Wikipedia:Reference desk/Archives/Science/2023 November 4

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November 4

Electric current & electron numbers

In a sinusoidal electric current, how do the electrons behave in a section of cable? Is their number constant or does it follow the voltage, knowing that the intensity follows the voltage to the nearest phasing. I imagine 2 scenarios, the first where the number varies with the voltage at the same time as their speed and the second where the number is constant and only the speed varies, knowing that in both cases the intensity is a number of electrons per second. What about it? Malypaet (talk) 06:21, 4 November 2023 (UTC)

For a metal the number of electrons that move around in response to a current is constant, so the same electrons would move back and forward in a sinusoidal current. for semiconductors or insulators the story will be different, eg in a diode only one direction of current will flow if the voltage is high enough. And for an insulator a high voltage peak might cause an electric arc. Graeme Bartlett (talk) 09:21, 4 November 2023 (UTC)

For an ideal conductor carrying a sinusoidal current, the electrons move back and forth a little bit. Their velocity is proportional to the current. Every real conductor has some capacitance, so the concentration of electrons changes with voltage. Only at high frequencies or in long cables this capacitance is relevant. There may be a phase difference between voltage and current. PiusImpavidus (talk) 12:11, 4 November 2023 (UTC)

100-year old cables and switches that have served in Alternating current power supplies contain when they are finally scrapped mostly the same electrons as when they were manufactured, although it's difficult to label electrons to prove this. Philvoids (talk) 23:32, 4 November 2023 (UTC)

Instead of labelling electrons we could replace the delocalized electrons by muons in a thin slice of the circuit, measure the dispersion of their drift velocity, and extrapolate from there to a one-century period, assuming that the dispersion increases proportional to the square root of time elapsed. A theoretical problem is how to account for muons being about 200 times as massive as electrons. A practical problem is (apart from developing an effective lepton substitution method) that we'd have to be quick, the mean lifetime of the muon being 2.2 microseconds.  --Lambiam 09:55, 5 November 2023 (UTC)

But instead of the fleeting muons, there are proton conductors, sodium ion conductors and even silver ion conductors. These are solids that allow those positive ions to move around through tubes. Graeme Bartlett (talk) 11:09, 5 November 2023 (UTC)

The difficulty that daunted me may yet succumb to Lambiam's demonic muonic mastery. I suggest a minor modification to his drift extrapolation period: after drifting one way for 16+2⁄3 milliseconds the electrons, assumed to be patriotically powering American industries and homes, spend the following 16+2⁄3 milliseconds returning whence they came, with zero net drift after 1/60 second. Or during any 1/60 second thereafter of the century. (For no obvious reason apart from causing a distraction to electric clocks there are nations whose Utility frequency differs so that same zero-sum cycle repeats every 1/50 second.) Philvoids (talk) 11:38, 5 November 2023 (UTC)

The drift is the average velocity of an aggregate of particles. Individual particles may move slower or faster. Their position relative to a cross section moving at the average velocity will, I assume, follow a Wiener process, which is why I used the term "dispersion". The dispersion relative to the moving cross section will, I also assume, not depend on whether the cross section is moving uniformly in one direction or moving back and forth. Eventually, an individual particle will almost surely migrate over any given arbitrarily large distance, the expected time being proportional to the square of the distance.  --Lambiam 10:26, 6 November 2023 (UTC)

You can't really replace the delocalised electrons by delocalised muons. The electrons are delocalised thanks to the Pauli exclusion principle, that prohibits them from taking low energy states. The muons don't care about what quantum states are already taken by electrons and will end up in the low energy states where they are localised. Your metal will become an insulator. PiusImpavidus (talk) 09:34, 6 November 2023 (UTC)

What if we replace all electrons by muons? Then Pauli's prohibition promises us delocalized muons.  --Lambiam 10:37, 6 November 2023 (UTC)

Labelling electrons isn't just difficult; it's fundamentally impossible. PiusImpavidus (talk) 09:34, 6 November 2023 (UTC)

Impossible according to our currently best theoretical understanding. The proton was once thought to be an elementary particle.^[1] How can we be sure electrons are fundamental?  --Lambiam 10:54, 6 November 2023 (UTC)

I don't think it's about whether they're elementary; it's about whether they're identical. You can't really label protons either, as far as I know. --Trovatore (talk) 20:12, 8 November 2023 (UTC)

The medium of current is electron flow, and its analog hole flow. If charge is being moved, that is power, regardless of it being AC or DC, electrons carry quantums of charge. refer to Electric_charge

Current is the charge per unit time moved past a point. An ampere of current over a second is a ton of electrons, and yes they are moving through that wire, even in AC current, since power is being delivered. Power being the product of voltage and current. Dominick ^(TALK) 20:41, 6 November 2023 (UTC)

Actually, by my calculations it's a bit under 6 nanograms of electrons. I don't think you meant "ton" literally, but actually on first reading I did think that, so in case anyone else was confused... --Trovatore (talk) 17:20, 8 November 2023 (UTC)

Tower crane

How tall was the tallest tower crane ever assembled? On a photo of the Statfjord B oil platform under construction https://www.google.com/imgres?imgurl=http://statfjord.industriminne.no/wp-content/uploads/sites/4/2020/05/s%25C3%25B8yler.jpg&tbnid=qwFH7mRRwExJ8M&vet=1&imgrefurl=https://statfjord.industriminne.no/en/category/platforms/&docid=FdJQEzKaYFcWDM&w=2362&h=1776&source=sh/x/im/m1/1&shem=uvafe2 , I once saw several tower cranes which look at least 900 feet tall -- could this be right? 2601:646:9882:46E0:48DF:DBFF:D215:C39B (talk) 10:35, 4 November 2023 (UTC)

Do you mean a free-standing tower crane? As used in the construction of a tall building a tower crane will typically be attached to the inside or outside of the building and will increase in height along with the building. In other words it's a tall as the building plus a bit extra for clearance. Shantavira|^{feed me} 14:55, 4 November 2023 (UTC)

That may often be the case, but by no means always applies. I personally have seen free-standing tower cranes hundreds of feet tall on numerous occasions over decades of casual observation in London and other British and European cities. {The poster formerly known as 87.81.230.195} 94.2.5.208 (talk) 17:00, 4 November 2023 (UTC)

I'm on wikibreak so am unlikely to reply further but I think Shantavira's point is that if you aren't limiting to free-standing tower cranes, the fact that some free-standing tower cranes are quite tall is irrelevant if they are still only ~300 metres [2]. A crane on top of the roof of Burj Khalifa when it was only half of its final roof height would still probably be taller [3] and a crane on top of the roofs (i.e. not counting spires) of I think 30 or more of the tallest buildings in the world would also be taller. So if you aren't limiting to free-standing tower cranes, you'd want to look at tower cranes for one of these buildings and probably Burj Khalifa. Nil Einne (talk) 07:43, 5 November 2023 (UTC)

Just wanted to add that you could make a definition of crane height/'tallness' that precludes the under construction structure it's constructed on. Although I also suspect this is more difficult than it seems when you consider the wide diversity of cranes and structures under construction out there albeit those complexities probably won't change the answer. I strongly suspect any reasonable definition which excludes the structure the tower crane is on effectively also precludes any crane that is isn't free-standing so the net result is probably the same. Nil Einne (talk) 11:43, 5 November 2023 (UTC)

Tower crane #2

Have there ever been notable construction accidents where a tower crane already present on site for construction work was used in the rescue effort (e.g. to remove the debris and give rescue workers access to trapped/buried victims), either instead of or in conjunction with the mobile cranes normally used for this purpose? 2601:646:9882:46E0:48DF:DBFF:D215:C39B (talk) 10:39, 4 November 2023 (UTC)

As a former construction worker I can assure you that a tower crane would be of little use in most such circumstances. One would need mainly bulldozers and diggers. Shantavira|^{feed me} 14:50, 4 November 2023 (UTC)

Mobile cranes have definitely been used to lift heavy slabs of concrete from collapsed buildings.^{[4][5][6][7][8]} So why couldn't a tower crane fortuitously present at a disaster site be used equally well (if it did not collapse from the quake^[9][10])?  --Lambiam 09:00, 5 November 2023 (UTC)

A mobile crane is much more versatile and the operator is relatively close to the action and can see exactly what's going on and respond immediately with incremental movements in any direction. The operator of a tower crane often cannot see what is happening on the ground, or only from a great height. They are usually directed by radio or handsignals from a banksman and will normally only raise, slew, and lower a load, which itself needs to be equipped with the appropriate fittings required for attachment. Shantavira|^{feed me} 09:52, 5 November 2023 (UTC)

What kind of accident are we dealing with? Suppose a construction worker works on the 74th floor (under construction), falls down to the 73rd floor and breaks a leg. Now you have to get a stretcher up to the 73rd floor, tie the worker to the stretcher and get him down to the waiting ambulance. The stretcher may not fit horizontally in the temporary lift, a mobile crane can't reach to the 73rd floor (and it would take several hours to order one), the tower crane is already there to move building materials. PiusImpavidus (talk) 09:53, 6 November 2023 (UTC)

I know it's a little late, but I've just come across a short clip which answers the question, and shows that User:PiusImpavidus is right! https://www.youtube.com/shorts/lUg_Zp_1TpE 2601:646:9882:46E0:F81B:1D07:2C0A:DEA6 (talk) 23:45, 23 November 2023 (UTC)

Antarctic weather

Scott Base in January

This picture shows New Zealand's Scott Base in January some years ago. Since the climate section says that the average high is below zero, even in summer, I wondered if the climate is particularly dry for some reason, like the McMurdo Dry Valleys or some of the dry areas of northern Greenland. However, a relative arrived at Scott Base yesterday as part of a research expedition, and so far, all the pictures he's sent are heavily snowy. If it can be snowy in November, and it doesn't typically get above zero in January, how could it be snow-free in January? Do I have to assume that the picture was taken after several exceptionally warm days, or is sublimation (phase transition) somehow responsible for snow disappearing, or something else? Nyttend (talk) 21:29, 4 November 2023 (UTC)

Can't find a full copy of "Snow and Climatic Characterization Influencing Snowmelt at McMurdo Station", but maybe "Runoff Characterization and Variations at McMurdo Station, Antarctica" could help? Surface temperature is not the temperature of the surface, but the air temperature above the surface, so solar radiation can cause snowmelt even when the temperature is recorded as above freezing. There are nearby perennial snowfields, but probably lack of precipitation, topography, polar days, and considerable snow removal efforts (which i assume would happen at Scott also) play a part. fiveby(zero) 22:59, 4 November 2023 (UTC)

The climate data in our article are from 1981-2010. With the current pace of climate change, you can add about 2°C, so I expect afternoon temperatures above freezing, even up to 4°C, are normal in December and January by now. At 184 mm/year, the place is pretty wet for a place that cold, so I don't think sublimation is to blame. It's still dry enough not to get a very thick layer of snow. You only need a few days above freezing to melt decimetres of it. A bit of sunshine helps too, combined with the low albedo of the rocks once patches of those appear. PiusImpavidus (talk) 10:24, 6 November 2023 (UTC)