Wikipedia:Reference desk/Archives/Science/2019 November 2

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November 2

Electric current

Does it flow, or does it just exist,? — Preceding unsigned comment added by 213.205.242.221 (talk) 00:35, 2 November 2019 (UTC)

In DC current, the electrons do actually flow in a steady direction, like a river, although far slower than the speed of light. In AC current, they flow forward and backward, so that there is no net movement, similar to the tides coming in, then going out, in a repeating cycle. SinisterLefty (talk) 01:50, 2 November 2019 (UTC)

Kind of? The question sounds simple, but its more complex than posited, because there are different things being asked by the question "Does electricity flow?" There's a YouTube channel called "The Science Asylum", which is fantastic for rather well-explained, intuitive videos on very complex issues like electricity. His videos on electrodynamics are rather good, and directly address the OPs question. Does Electricity REALLY Flow? and Energy doesn't FLOW the way you THINK! are two electrodynamics videos that have some rather non-intuitive facts about electricity. You'll also probably want to watch some of his earlier videos like What is electric charge. --Jayron32 16:25, 4 November 2019 (UTC)

Flux in cores

Does the flux in a transformer core increase as the power transfer increases? — Preceding unsigned comment added by 213.205.242.221 (talk) 00:37, 2 November 2019 (UTC)

Yes, the current will increase, causing more flux. Graeme Bartlett (talk) 12:57, 2 November 2019 (UTC)

• Up to a point. Transformers can saturate their cores, if there's too great a flux.

As an efficient transformer (i.e. for production cost relative to power capacity) will run their flux close to, but not exceeding, this saturation point, it's not a good assumption to expect that transformer power can be increased beyond their design value – even ignoring any heating or insulation problems with the windings.

Deliberate saturation is used in a few devices as a limitation mechanism, such as transductors or even the current-limited isolating transformers for shaver sockets in UK bathrooms. Andy Dingley (talk) 16:09, 2 November 2019 (UTC)

What's the velocity never exceed of an SR-71 and X-15 at sea level?

If they're unclassified by now that is. Sagittarian Milky Way (talk) 02:52, 2 November 2019 (UTC)

At sea level they'd start taking on water. ←Baseball Bugs ^{What's up, Doc?} carrots→ 03:02, 2 November 2019 (UTC)

There's lakes below sea level. Sagittarian Milky Way (talk) 03:33, 2 November 2019 (UTC)

How many SR-71's or X-15's buzzed the Dead Sea? ←Baseball Bugs ^{What's up, Doc?} carrots→ 04:53, 2 November 2019 (UTC)

But Baseball Bugs, the term "at sea level" in relation to air speed doesn't mean actually flying level with the surface of the sea (which as you say would be dangerous), it means flying at "standard sea level air density", at least according to our Equivalent airspeed article. Alansplodge (talk) 17:51, 3 November 2019 (UTC)

This is definitively a military secret. It depends on temperature in theory but these "crafts" are build to "work" securely at a given height far above sea level anyway, so its nonsense to ask. Also they are build to fly long distances for their purpose, so their top-speed is actually not that high (X-15 2000m/s) compared to for example the russian S-500 missile systems that manage 7000m/s but "just" over 600km distance. --Kharon (talk) 05:09, 2 November 2019 (UTC)

You can't really compare speeds of planes and missiles, that's apples and oranges. But yes, they can only reach top speed in thin air, so their speed at sea level would be far less. SinisterLefty (talk) 05:27, 2 November 2019 (UTC)

Going on a tangent; reading our article on the S-500, as well as other sources, indicates that the S-500 is claimed to be capable of intercepting a target moving at 7000m/s, NOT that the missile itself is capable of achieving that speed (intercepting a fast missile with a slower missile isn't all that hard, as long as you don't get caught in a tail chase). The missile itself is likely developed from the missiles used on the S-400, which have speeds up to 3840m/s. WegianWarrior (talk) 06:01, 2 November 2019 (UTC)

According to page 5-9 in the declassified SR-71 flight manual, the Blackbird was restricted to a "mere" 500 knots equivalent airspeed... which at sea level with a standard atmosphere should equal 257.2 m/s (575.4 mph / 926 kmh / 0.75 Mach). Further reading in the manual indicates the limiting factor was the compressor inlet temperature (CIT), which should not exceed 427°C (800.6°F).

According to the declassified flight manual for the X-15, section V, the absolute speed limit at sea level is comparable to the SR-71. However the limiting factor for the X-15 is the max q, which should not exceed 2200PSI (154kg/cm2 / 151.7 bar) to avoid structural damage.

To summarise; both the SR-71 and X-15 was built to reach high speed at high altitude. Near or at sea level they would have to fly at roughly the same speed as a modern airliner's cruise speed to avoid damage (engine and/or structure). WegianWarrior (talk) 05:50, 2 November 2019 (UTC)

Bob and Ray once proposed a satellite that would orbit 6 feet off the ground, so advertising could be posted on it. The notion of flying such high-speed aircraft at sea level is a similarly silly idea. ←Baseball Bugs ^{What's up, Doc?} carrots→ 06:28, 2 November 2019 (UTC)

A quick back of the envelope calculation gives an orbital speed for such a satellite of 7905m/s, which is clearly too fast to be able to read any advertisements posted on it... WegianWarrior (talk) 14:13, 2 November 2019 (UTC)

Bob and Ray were satirists. :) And I say again, if those high-speed planes were flying at sea level anywhere except places like Death Valley and the Dead Sea, they would soon become ladened with water. I'm still trying to comprehend what the OP is trying to get at. ←Baseball Bugs ^{What's up, Doc?} carrots→ 14:26, 2 November 2019 (UTC)

In practice sea level V_NE speed shouldn't be much lower than buzzing the ocean in good weather. They were optimized for high altitude but I was just wondering if the fastest manned plane and plane* could fly really fast by low altitude standards too. But now I know that the X-15 was a structurally weak rocketplane that could only break speed records in extremely thin air and that the unique or nearly unique air compression heat surviving Blackbird features I've heard about do not include "engine that isn't destroyed by ~9/11 airliner speeds". Apparently it's so good at collecting air that it can exceed 800°F before the "turbocharger". Which can compress air to 8.8 times inlet pressure.Sagittarian Milky Way (talk) 16:45, 2 November 2019 (UTC)

Keep in mind that both the experimental X-15 and the reconnaissance Lockheed SR-71 Blackbird are in the past tense. ←Baseball Bugs ^{What's up, Doc?} carrots→ 18:34, 2 November 2019 (UTC)

Wiki-markup lacks tags for marking tounge-in-cheek humour and intentionally missing the point - there are a couple of issues with a 6' orbit apart from going to fast to read... :P

Both the X-15 and SR-71 was built to be good at one thing, and that thing was not going very fast low down. I'm actually surprised to see just how low the load factor of the SR-71 was at full speed (-0.1 to 1.5G, compared to the F-16's -3 to 9G). WegianWarrior (talk) 20:18, 2 November 2019 (UTC)

FWIW (i.e. not much), you do not need to search online very long to find claims of varying reliability that the true top speed of the Blackbird was higher than publicized - according to some claims, as high as Mach 4. In instances like this, there are competing agendas where the government obviously doesn't want to publicize their actual mission limits, while pilots and people who were involved in the Skunk works are obviously proud of this incredible machine. There's no way to know for sure at this point. Matt Deres (talk) 00:23, 3 November 2019 (UTC)

A few weeks back, Brian Shul gave a presentation at the Museum of Flight. As he writes in his book Sled Driver, the top speed (and V_NE, for that matter), was really fast.

As an aside, I have frequently pointed out to many of my friends and colleagues that the fastest, highest, and most secret airplane ever built did not ever carry munitions: the SR-71 was a giant carrying-case for a very fast telephoto camera. More on the topic: ...Overhead Reconnaissance (1992). That book cites some speed and altitude numbers for the aircraft, but it also describes some different speed and altitude considerations for its payload...

Nimur (talk) 16:28, 6 November 2019 (UTC)

What is the rate of Rapid Eye Movement?

In what speed do eyes move in the REM sleep phase? One source I found (and I couldn't find another) stated an average rate of 15 ’cycles’ a minute. That's a cycle in 4 seconds, pretty much the speed you'd move your eyes if someone asked you to ’move your eyes slowly from side to side’. If that source is correct, why keep calling it REM and not SEM? Gil_mo (talk) 18:05, 2 November 2019 (UTC)

I think by 'cycle' your source may have meant distinct 'clusters' of movements, with successive clusters separated by an average around 4 seconds. In films of REM I've seen, the eyes when moving made a few movements in short 'bursts' of a couple of seconds or less each, separated by a few seconds of stillness. {The poster formerly known as 87.81.230.195} 2.122.179.237 (talk) 20:05, 2 November 2019 (UTC)

Could you please place some links to those videos? Thanks Gil_mo (talk) 08:58, 3 November 2019 (UTC)

Videos of interest: Baby REM sleep. Stages of sleep: [1], [2], See REM waves at 3:10, Sleep basics, See REM at 7:35. Normal sleep EEG: [3], [4]. REM sleep disorder RBD: [5], [6], [7]. DroneB (talk) 15:23, 3 November 2019 (UTC)

Thanks, will definitely look at those!! Gil_mo (talk) 05:43, 4 November 2019 (UTC)

As you can see in the first (cute) video, the eyes are moving at a fairly slow rate. None of the other videos/talks are mentioning the actual eye movement, but rather the brain waves connected with the eyes. Gil_mo (talk) 09:01, 6 November 2019 (UTC)