Science desk
< August 14	<< Jul \| August \| Sep >>	August 16 >

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

August 15

How many hours after aphelion could "the moment of the year you're furthest from the Sun" be?

Without exceeding airliner speed/altitude. What's the approximate path? It seems like it'd involve avoiding the antisolar point from >12 hours before to >12 hours after then flying over the antisolar point (actually you cannot fight the Earth's orbit all the way to the antisolar point since you'll eventually fly too perpendicular to the "Earth-to-Sun" line to overcome Earth's (increasing) inward orbit motion so you're done before actually reaching the point). Sagittarian Milky Way (talk) 00:38, 15 August 2017 (UTC)[reply]

I'm not quite sure how to interpret your Q. I thought you were asking for a particular point on the surface of the Earth, when is it farthest from the Sun. The path would approximately be an epicycloid. Whether the Earth's rotation is more significant than the Earth's distance from the Sun I'm not sure of. So, it could either be the point nearest aphelion where that point on the Earth is rotated farthest from the Sun, or it could be at the exact moment of aphelion (somehow I doubt this, though, unless you are close to a pole).

But then you introduced the possibility of flying in a plane, which confused me. In that case, you would just fly to the point of the Earth furthest from the Sun at aphelion, in advance, and then aphelion would be the moment. StuRat (talk) 01:32, 15 August 2017 (UTC)[reply]

Well a stationary point that's noon when Earth's center is furthest and with the Sun underfoot on the midnights of that day should have one or both the adjacent midnights be the time it's furthest (I can look up how many miles Earth moves in during this time but from dividing how much Earth moves in in the inward orbit half by 365 half days I doubt it isn't under an Earth radius) If the point can also move at Mach 0.85 then you should be able to extend the disparity to over 12 hours with the right path. Sagittarian Milky Way (talk) 04:36, 15 August 2017 (UTC)[reply]

Earth's center is only moving 3.something meters per second away from the Sun 12 hours after aphelion. Sagittarian Milky Way (talk) 05:47, 15 August 2017 (UTC)[reply]

OK, by comparison, the Earth's rotation moves a location on the equator a max of about 296 meters per second toward or away from the Sun. Of course, the minimum movement drops to an instantaneous 0 right at the nearest and furthest points on Earth from the Sun. So, based on that, including the Earth's rotation in the calcs would likely be worth the Earth itself not being directly at aphelion. StuRat (talk) 18:01, 15 August 2017 (UTC)[reply]

There is an instant in time when the earth's center is farthest from the Sun in a given year: this is the instant of aphelion. at this instant, there is a subsolar point where the Sun is at zenith. for a spherical Earth, the antipodal point of this subsolar point is farther from the Sun than any other point on the Earth's surface, and is farther from the Sun than any other point will be during this year. The geoid is not exactly a sphere, and the earth's surface has mountains, etc., but without doing the math I suspect that these will not affect the exact point by much at all. -Arch dude (talk) 23:58, 16 August 2017 (UTC)[reply]

Right but the center only gets 1 Earth diameter closer in the first week. So if your top speed was infinity you could be furthest ~July 11 while Earth's center was furthest ~July 4. (by staying at the subsolar point for ~2 weeks centered on aphelion then teleporting to the antipode). The question is how much more than 12 hours can you make the disparity if your top (air)speed was ~Mach 0.85? Sagittarian Milky Way (talk) 00:41, 17 August 2017 (UTC)[reply]

Electrical Engineering question, Therevin reduction

Two questions. I have an example in my textbook of a step by step Therevin equivalent circuit, from a few series and parallel connections, given the per unit (pu) values. I believe that the rule for parallel circuits is:

{\frac {1}{Z_{\mathrm {total} }}}={\frac {1}{Z_{1}}}+{\frac {1}{Z_{2}}}

And the pu for a parallel circuit is $Z_{1}=0.032+j0.958$ and $Z_{2}=0.042+j0.610$ . Their solution was $Z_{\mathrm {total} }=0.021+j0.373$ , but when I put this sucker in my calculator I get $Z_{\mathrm {total} }=0.018+j0.373$ . They are awfully close, and maybe some significant digits were rounded somewhere, but I'm worried I am using the wrong equation or using my calculator wrong or I'm skipping a step. What I'm doing:

Z_{\mathrm {total} }={\frac {1}{({\frac {1}{.032}}+{\frac {1}{.042}})}}+{\frac {1}{({\frac {1}{j0.958}}+{\frac {1}{j0.610}})}}

As you can see, I get the imaginary portion correct. The real portion is close, but off. So am I doing something wrong?

One more question, they have this equation in the text:

I_{\mathrm {FAULT} }={\frac {1}{\left\vert 0.021+j0.623\right\vert }}=1.605\mathrm {pu}

And I have no idea how they did that. What steps are between that? Maybe I don't know what the vertical lines mean, but taking a reciprocal of a complex number in rectangular form somehow can be reduced to a real number? -12.38.236.194 (talk) 04:10, 15 August 2017 (UTC)[reply]

Re the first bit, that's not how I'd do the reciprocal of a complex number. Re the second bit, vertical bars mean the magnitude of the complex vector. Greglocock (talk) 09:10, 15 August 2017 (UTC)[reply]

See reciprocal of a complex number. When

z = a + bi

,

{\frac {1}{z}}={\frac {a}{a^{2}+b^{2}}}-{\frac {b}{a^{2}+b^{2}}}i.

. Blooteuth (talk) 12:32, 15 August 2017 (UTC)[reply]

Yes, for #2, I googled magnitude of complex vector, and came up with Pythagorean theorem, which totally makes sense (convert rectangular to polar, and ignore the angle), and I put that in my calculator and got 1.6042, which is very close to the answer given.

However, for #1, I don't exactly follow, and don't understand where to utilize the reciprocal formula given. I'm using the rule for parallel circuits, which formula I gave above. Is that not the correct formula? Or did I combine the numbers wrong when I went to do the math, or do I have order of operation wrong? Like I said, my method did give me the exact same number for the complex portion, but the real portion is slightly off. So would you mind explaining a little more detail where I went wrong, or show your work (if you agree the book answer is correct, and my value which is .003 lower, is wrong). Thanks for your help so far. -50.95.197.232 (talk) 13:14, 15 August 2017 (UTC)[reply]

here's the answer with a few too many sig figs 0.0205157+0.372802j . Hopefully that will help you debug your method. Greglocock (talk) 18:54, 15 August 2017 (UTC)[reply]

Here is what I was getting from my formula above with a few too many sig figs 0.018162 + j0.372691. I provided the formula above that I am entering into my calculator, but it seems that is the wrong formula. -50.95.197.232 (talk) 02:09, 16 August 2017 (UTC)[reply]

Problem #1:

Apply the reciprocal formula to Z₁ to get 1/Z₁. I used i instead of j and hope that didn't confuse you, here they both mean the same imaginary unit (and not electric current). Do it right and you find 1/Z₁ = .034828 - 1.04268 j. Keep working with 5 decimal places.
Apply the reciprocal formula to Z₂ to get 1/Z₂.
Add 1/Z₁ + 1/Z₂
We have used the textbook formula to get 1/Z_total.
Take the reciprocal of 1/Z_total and bingo, you have Z_total
Round your answer to 3 decimal places and have the correct textbook solution.

Problem #2:

You are correct, your calculator is correct, but your sloppy textbook has wrongly rounded the answer 1.604225319 ! Blooteuth (talk) 22:12, 15 August 2017 (UTC)[reply]

Here is what I got using your method .020515 + j.3728085, but I had a negative sign at some point, so that part is still confusing me. This was a lot of steps, and using the (bad) formula above got me pretty close answers to 3 other questions, but this one was off more than the others. Not sure why this bad formula gets me so close. And I wonder if there is a way to reduce or simplify your method:

Z_{\mathrm {total} }={\frac {1}{({\frac {a_{1}}{a_{1}^{2}+b_{1}^{2}}}-{\frac {b_{1}}{a_{1}^{2}+b_{1}^{2}}}i)+({\frac {a_{2}}{a_{2}^{2}+b_{2}^{2}}}-{\frac {b_{2}}{a_{2}^{2}+b_{2}^{2}}}i)}}.

Doing the reciprocal formula 3 times is a hassle.. But it worked, and I appreciate you giving me step by step! -50.95.197.232 (talk) 02:09, 16 August 2017 (UTC)[reply]

Your result differs a little from Greglocock whom I find to be correct to 6 decimal places, so something is not quite right. The formula for the reciprocal of a complex number definitely introduces a negative sign (and the link shows why). The real and imaginary parts of the reciprocal both have the same denominator so you can simplify a program or minimise calculator keysteps by storing (a²+b²) for the 2nd time it will be needed. Blooteuth (talk) 16:52, 16 August 2017 (UTC)[reply]

The vertical lines represent the modulus = absolute value of the complex number. It is the hypotenuse per Pythagorean theorem, so the modulus of (1+i) = sqrt(2) for example. In general, when you multiply any two complex numbers, you can add their angles and multiply their absolute values to get the product. (See [1] for an explanation) Anyway in this case you get that 1.604, then just invert that real number. Either the author rounded wrong or he figured (almost correctly) that when you have a vector that goes 0.623 in one direction and only 0.021 in another, its length is basically 0.623 because the square of 0.021 is so small, and never bothered doing the Pythagorean calculation at all. Almost, but it's still off. Wnt (talk) 02:04, 16 August 2017 (UTC)[reply]

From genes to organelles, bones, organs, their parts and shapes

In the college biology textbooks that I have read, there is an explanation of how genes are transcribed and translated into proteins but not how one goes from genes to shapes and parts of organelles (during the synthesis of organelles before mitosis), organs, and bones. Exactly how are the structures of organelles and organs encoded in the DNA and expressed during development? VarunSoon (talk) 09:11, 15 August 2017 (UTC)[reply]

This is the realm of the study of ontogeny, especially the subfield known as organogenesis. While the Wikipedia articles seem a bit light on the details, that will provide you some search terms and some initial starting points to begin your research. --Jayron 32 11:07, 15 August 2017 (UTC)[reply]

This field is still a big miracle for todays science. Its known that the Stem cells are the core of this. I believe some are busy for quite some times now to understand and eventually copy the stunning regrow of lost bodypart (Regeneration (biology)) some amphibians are capable of. --Kharon (talk) 11:24, 15 August 2017 (UTC)[reply]

Another aspect which is growing in the field of research here is the role of epigenetics. It turns out that there are aspects of development which cannot be explained directly by DNA, and yet still involve heritable traits. DNA is very important, but it is likely not the only factor in determining growth and development as described above. --Jayron 32 12:46, 15 August 2017 (UTC)[reply]

You may enjoy our page on EvoDevo, and look for textbooks in that area, here [2] is a reading list. SemanticMantis (talk) 16:16, 15 August 2017 (UTC)[reply]

The pages on hox genes and homeotic genes may also be useful. --OuroborosCobra (talk) 19:22, 15 August 2017 (UTC)[reply]

Some organelles are explicable - for example, microtubules form from known protein structure, and small microtubule-based organelles like centrioles can thus be put together. Others can be deduced to some extent from various known factors - it looks like [3] might be worth an interlibrary loan from Sci-Hub. I am certainly not an expert on all these scattered and detailed topics, but I think at this point much is known but it is not so clear how much is unknown. Wnt (talk) 01:48, 16 August 2017 (UTC)[reply]

ketosis

When a person fasts (eats absolutely nothing at all) how long after their last meal does it take for their body to enter ketosis? — Preceding unsigned comment added by 52.8.172.72 (talk) 10:56, 15 August 2017 (UTC)[reply]

According to Ketosis#Diet, it takes the brain about 48 hours before the process of using ketone bodies for energy begins. --Jayron 32 11:05, 15 August 2017 (UTC)[reply]

The liver starts producing ketone bodies for other organs long before the brain starts consuming them, though. I'll check my biochem textbook at uni tomorrow. Adrian J. Hunter^{(talk•contribs)} 09:55, 16 August 2017 (UTC)[reply]

Two textbooks had nothing, but PMID 21983804 (not free, sorry) shows a graph of plasma ketone levels during starvation. It shows nearly constant low levels over the first day (looks like about 0.15 mM), slowly rising throughout the second day to around 0.4 mM, then continuing to rise to around 4.5 mM by the end of the eighth day where the graph ends. Adrian J. Hunter^{(talk•contribs)} 11:33, 17 August 2017 (UTC)[reply]

I think the question cannot be answered because information is missing. The human body converts glucose to glycogen in the liver. It is the first source of energy used. After it is used up then the body will burn fat producing ketones. So, it depends upon how much glycogen has been stored and the size of the liver. There is variability to liver size which would certainly affect the time to onset of ketosis. The liver's mean weight, 1288 g (range, 775-2395 g) Am J Forensic Med Pathol. 2015 Sep;36(3):182-7. NCBI Andrew124C41 (talk) 02:22, 18 August 2017 (UTC)[reply]

power supply

I have a 12v DC power source at 20 AMPS. I want to adjust the voltage from 1V to 12V whenever I want. What is the best way to achieve this? How would I build a voltage adjuster? Thanks for your time. Okiaoa1 (talk) 11:01, 15 August 2017 (UTC)[reply]

The best way is to use a thyristor chopper (I can't tell you how to make one, though, because I forgot). 2601:646:8E01:7E0B:1C44:225C:6039:189C (talk) 11:16, 15 August 2017 (UTC)[reply]

Buy a new, better power source. --Kharon (talk) 11:33, 15 August 2017 (UTC)[reply]

That's just about what I was going to say. Dmcq (talk) 11:43, 15 August 2017 (UTC)[reply]

Yes, and I believe the logic is that the new adjustable voltage PS may cost less than the equipment to make your existing PS adjustable, plus you will then have 2 PS's, which is presumably better than 1. Also, if you can describe the reason you need it, then other solutions may apply as well, such as a device which rapidly opens and closes the circuit, providing the average voltage you want. This approach is good, for example, to power a dimmable light bulb and has the advantages of not wasting electricity (particularly important if running off a battery) and generating heat. StuRat (talk) 12:29, 15 August 2017 (UTC)[reply]

The article Voltage regulator covers a wide range of technologies. There is no simple "best" choice because it depends on the load. This tutorial is helpful. Begin by establishing the maximum output current you need. Review Voltage_regulator#Comparing_linear_versus_switching_regulators since this choice will dictate the complexity and power dissipation (i.e. heat production) of your solution. If you need only very low currents (a few mA = amps/1000) and not exact regulation, a single Potentiometer can serve as a voltage reduction device. A component to consider is a wirewound 100 ohm (linear taper) potentiometer rated at 3W or more. Another alternative, not necessarily a recommendation, is you can add a variable transformer or "Variac" at the mains input to your power supply; however this is an expensive component and is only workable if your supply is mains powered and a linear, not a switching type. Blooteuth (talk) 12:24, 15 August 2017 (UTC)[reply]

20A? 240W? Buy one ready made from someone who knows how to make 20A power control circuits. Andy Dingley (talk) 12:31, 15 August 2017 (UTC)[reply]

You appear to want to vary the DC output. Purchase a PWM dimmer or "motor speed controller". These are inexpensive (less than $20.00 on Amazon.) You cannot use a Variac: these are transformers and only work for AC, not DC. You can in theory use a potentiometer, but these work by consuming the power your circuit is not using, so they are wasteful and they get hot. A PWM works by turning the 12V on and off very, very quickly, then using a capacitor to smooth out the output. This presents a lower average amperage to the 12V supply while presenting the selected voltage to the load. Since the PWM controlelr is on the output side of the 12Vdc power supply, it is indifferent to the AC input of the 12V power supply. -Arch dude (talk) 23:46, 16 August 2017 (UTC)[reply]

@Arch dude Please review the underlined words in what I wrote and say if you have an objection. "Another alternative, not necessarily a recommendation, is you can add a variable transformer or "Variac" at the mains input to your power supply;...". Blooteuth (talk) 14:58, 17 August 2017 (UTC)[reply]

Did the OP say he had a 12V 20 Amp supply powered from the mains? No. It's possible he has a 12V heavy duty gel cell battery or car battery and wants to limit the current drawn to 20 Amp because of an inline fuse or for some other reason which may involve solar charging during daylight hours. This is another case of an annoying OP who asks a wide open question and walks away, neither confirming nor expanding on what he meant. If his power supply is mains operated, then a PWM dimmer is absolutely unsuitable. Sure, it will vary the DC output under no load or light load, but if the current drawn increases, the regulation will be very poor and the voltage will drop. That's not what I call a regulated power supply, which should maintain a constant DC voltage under widely varying load currents. The OP has not provided enough info. Akld guy (talk) 21:40, 17 August 2017 (UTC)[reply]

@Akid guy If you are responding to me regarding the Variac, please review what I further wrote: "this...is only workable if your supply is mains powered and a linear, not a switching type.". I agree the OP's question doesn't say that is the case, which leaves many possibilities open to speculation. Blooteuth (talk) 17:37, 18 August 2017 (UTC)[reply]

Depending on the application make Your choice to solve this. A pulse width modulation (PWM) will only work on heating resistors and light bulbs, but not recommended due inrush on cold filaments. A buck converter is your choice when getting is inexpensive. A buck converter PCB up to 3 amps output, You can get for a dollar. With some engineering, the buck converter device can be extended with transistors and further chokes. If the PSU is a SMPS, reverse engineer the controller circuit. By modifying the voltage divider from output to ground, between the resistors the voltage relation for the output is being sensed by the feedback input. Such voltage dividers are used at the linear regulators LM317 or switching regulators like LM2596. Note: when using potentiometers, aware to rise the voltage when the potentiometer glider fails, causing a rise of the output voltage. This can be prevented by using a parallel resistor from sense to output, causing a voltage drop instead of a rise. Electric hazard Warning: SMPSs use large capacitors on the grid side, buffering hazardous rectified voltage from grid. Have discharge these capacitors before opening the device. Theres minimum knowledge necessary to access electric devices inner. --Hans Haase (有问题吗) 23:17, 18 August 2017 (UTC)[reply]

Does "Absolute Colorimetric" intent do any good under uncontrolled light?

In color management, when even the proofs of a printed item are to be viewed under uncalibrated lamps, and/or natural light filtered through uncalibrated window glass (and where the weather at viewing time may or may not be predictable at printing time), is there any benefit to using the "Absolute Colorimetric" intent versus the "Relative Colorimetric" one? Neon Merlin 12:29, 15 August 2017 (UTC)[reply]

I believe it comes down to a question of whether the image will be viewed on a light-emitting device such as a monitor or on a passive medium such as paper. On a monitor the "absolute" intent will (if properly done) produce defined RGB outputs, which you might or might not desire. On paper it isn't even possible to render "absolutely" under undefined lighting conditions. Etc. Looie496 (talk) 14:04, 15 August 2017 (UTC)[reply]

Are there any receptors that detect fat?

Glutamate receptors detect umami. Glutamate is also an amino acid. So, what detects fat? For example, one reason why the avocado is tasty is that it is mostly fat, giving a creamy flavor. 140.254.70.33 (talk) 16:46, 15 August 2017 (UTC)[reply]

For some time, most science in this field held that fat was primarily identified by mouthfeel rather than taste, specifically. However, in just the past 2-3 years, research has turned up a "fat taste" [4] [5] called by some "oleogustus". See Taste#Fattiness_.28oleogustus.29, which discusses the involvement of a possible fatty taste receptor. --Jayron 32 16:50, 15 August 2017 (UTC)[reply]

You can actually do a test of this at home. Get some beef and pork, and carefully separate the fat from the lean. With a meat grinder, make some ground beef with pork fat and some ground pork with beef fat. Cook up a couple of patties. You will find that they taste like the meat the fat came from. Or, as one source[6] puts it, "What gives meat its flavor is the fat". --Guy Macon (talk) 20:48, 15 August 2017 (UTC)[reply]

It's not that simple, since many sensations, including mouthfeel and smell, get mixed up with taste. Actually figuring out which portions of the eating experience are sensu stricto taste and which are arriving via other senses such as touch or smell is way more complex than just "try it out yourself". The OP asked about specific taste receptors. They are unlikely to discover those eating burgers on their own.--Jayron 32 21:02, 15 August 2017 (UTC)[reply]

indeed, "this is not that simple", as it only proves that some important flavors molecules are lipophilic, which is quite well known. Gem fr (talk) 10:24, 17 August 2017 (UTC)[reply]

That only means they are flavors which are transported via fat, not that the fat itself is the flavor. Culinarily, flavors (and their associated smells) are often classified as a) water soluble b) fat soluble or c) alcohol soluble.[7][8][9]. Other than the possible "oleogustus" flavor specific to fat, MOST of what we associate with the flavor of fats is the flavors/smells being transported by the fats. As those flavors reach our tongue/olfactory apparatus, we detect those flavors that we wouldn't otherwise detect had the fat not been present. The adage "fat equals flavor" exists not because of the taste of the fat itself, but because of the role of the fat in helping us taste fat-soluble flavor compounds. Also, the fats themselves are not pure fat; unrendered pork fat contains flavor/scent compounds peculiar to pork, which sensu stricto are not fat, just stuff in the fat. --Jayron 32 19:18, 17 August 2017 (UTC)[reply]

"fat" is not as specific as sugar, salt, or glutamate. It comes in a wild variety of 3D shape and chemical formula, so i don't understand how we could have a chemical receptor for generic fat. You can have a chemical receptors for some specific kind of fat, and for sure we have flavor detectors for lipophilic and fat-related substances (rancid taste, for instance). Gem fr (talk) 10:24, 17 August 2017 (UTC)[reply]

Rancidity, strictly speaking, is a smell not a taste. It's the odor of things like butyric acid. This makes sense from an evolutionary point of view as well; you'd want to be able to identify rotten foods before they reached your lips. Taste is actually quite limited. Strictly speaking, when we say we only have 5 or 6 or 7 tastes, we really can only detect those specific tastes. Sour things all taste identical if you completely cut off the sense of smell (vinegar, lemon juice, citric acid). The difference is in the smells, which you get through your nose. This article draws an interesting distinction between taste (detected on the tongue) smells (detected in the nose) and flavor (the interplay of taste and smell). --Jayron 32 19:27, 17 August 2017 (UTC)[reply]

Indeed. However, the OP mentioned "receptors" and mouthfeel of avocado, so it was not limited to the tongue receptors. Gem fr (talk) 07:59, 18 August 2017 (UTC)[reply]

In quantum mechanics an electric current can move in opposite directions simultaneously. Where can I find the Wikipedia article?

In quantum mechanics an electric current can move in opposite directions simultaneously. Where can I find the Wikipedia article? Antonquery (talk) 23:15, 15 August 2017 (UTC)[reply]

Flux qubit. Count Iblis (talk) 23:26, 15 August 2017 (UTC)[reply]

Thanks!