Wikipedia:Reference desk/Archives/Science/2017 May 21

= May 21 =

What is the exact number of hormones in human?
What is the exact number of hormones in human? Is there a short answer or it's complicated issue? By searching I found the article here: List of human hormones. Then I read it and I found that it mentions 4 hormones only under the category of amino acid derivatives while there is no mention for norepinephrine and dopamine which are also considered to be amino acid derivatives, at least according to this table and this academical site. for example. In addition, histamine as well should be there, according to the last source since it is made of glutamic acid. Could I relay on this article (wiki) when counting the exact number of hormones in human body? 93.126.88.30 (talk) 02:41, 21 May 2017 (UTC)


 * 50.4.236.254 (talk) 03:39, 21 May 2017 (UTC)
 * Comment: I think Joseph Fermin gives an approximate answer, because science is an ongoing field with tentative facts. 50.4.236.254 (talk) 04:38, 21 May 2017 (UTC)
 * Thank you for you comment. Anyway, I made googling before putting my question here, and I saw this answer as well, but it doesn't answer my question if you look at it well. (what is the exact number which is known for the moment) in addition to some points that are not clear about Catecholamines and histamine in the context of hormone) 93.126.88.30 (talk) 05:22, 21 May 2017 (UTC)
 * The article talk page discusses norepinephrine and dopamine. They were not included because they were considered neurotransmitters not hormones. Definitions of hormone may vary adding to the difficulty. Rmhermen (talk) 12:43, 21 May 2017 (UTC)


 * There isn't really a definite number, because of some ambiguities in definition. For one thing, some hormones can appear in variant forms, and it isn't clear whether they should be counted as one entity or several.  Also, some act only within a limited part of the body, such as the lymph or the hypophyseal portal system. Looie496 (talk) 14:30, 21 May 2017 (UTC)

Heyday of immune system
Approximately when the human immune system was able to fend off the majority of viral pathogens (including influenza) before viruses evolved to avoid and bypass it? That said, when, for example, anatomically modern humans emerged, were viruses already mutated/adapted by that time or it happened much earlier? Brandmeistertalk  07:16, 21 May 2017 (UTC)


 * Never. Count Iblis (talk) 08:33, 21 May 2017 (UTC)


 * The typical person today can fend off viral pathogens for ~70 years, give or take. Any virus that doesn't kill you is one that your body manages to either fend off, or at least fight down until it is controlled.  The immune system is actually quite good at conquering most invaders.  Dragons flight (talk) 10:27, 21 May 2017 (UTC)


 * It's safe to say that the relationship of pathogens and immune system well predates humans, since there are so many comparable diseases of other mammals. A key difference is social - humans now survive in larger communities - and above all technological - jet airplanes moving things around from one continent to another, or on the other hand of course vaccines.  Influenza is quite effective at killing birds, which potentially takes us back before the dinosaur/mammal split, though I won't assert with any confidence that influenza dates back that far, only the relevant vulnerabilities.  Known subtypes of influenza seem to have diverged only thousands of years ago  but I don't know what came before that.  We have an article evolution of influenza but it is missing far more than it contains.  The, uh, "good" news is that some bright technicians have been able to isolate DNA from hundreds of thousands of years ago in soil, so bio-warriors the world over can dig through sediments from hundreds of thousands of years ago and introduce us to whatever unknown predecessors of influenza existed; maybe we'll see how well they fare against modern immune systems.  (Caveat: influenza is RNA, which ought to be much less likely to survive - DNA viruses are certainly better targets for this)  Wnt (talk) 11:47, 21 May 2017 (UTC)


 * The arms race between the adaptive immune system and viruses began when the former first appeared in early vertebrates, about 550-600 million years ago. Ruslik_ Zero 13:13, 21 May 2017 (UTC)


 * Perhaps even much earlier, in a sense, since bacteria have an adaptive immune system of their own -- the CRISPR mechanism. Looie496 (talk) 14:22, 21 May 2017 (UTC)


 * If anytime, that heyday is now, worldwide. People with better nutrition and general health seem more able to fight off diseases on their own.  However, in the US and a few other places, due to poor diet and lack of exercise, we may now be headed back down.  Also, artificial immunity due to vaccinations, and lack of contact with pathogens due to improved sanitation, may cause our natural immunity to decline.  Note that new diseases like AIDS don't indicate that our immunity has declined.  Had those existed thousands of years ago, they would have been even more deadly. StuRat (talk) 16:08, 21 May 2017 (UTC)

Power consumption of black and white screen
How does a black and white screen power consumption compare to a color consumption? --Hofhof (talk) 15:02, 21 May 2017 (UTC)


 * About the same, these days. The technology used may matter, though.  With an LCD screen the backlight consumes most of the energy, and it's either always on, or varies with the scene brightness, but not with the color.  Old CRT screens might have had more difference. StuRat (talk) 15:27, 21 May 2017 (UTC)


 * Brightness only increases the power consumption when the pixel is the lamp itself. This applies to CRT and LED (also OLED) displays. LCD/TFT use backlight. Projectors use though (TFT) or reflected light (DLP). Backlight should be based on energy efficient LED technology, today. Older models used CCFL. Compared to a light bulb, CCFL needs about 50%, LED 10 to 20%, of the bulb. -- Hans Haase (有问题吗) 17:25, 21 May 2017 (UTC)
 * There are still millions of glass Cathode ray tube (CRT) screens in use on Television sets and Computer monitors. A color CRT contains three electron guns and less than 1/3 of each gun's cathode current contributes to the picture brightness. The remaining 2/3 of current is intercepted by a Shadow mask. In contrast a black and white CRT has no shadowmask, only a single gun and nearly all its cathode current contributes to the picture brightness. The color CRT display also consumes higher power in its Deflection yoke around the larger diameter tube neck and in circuits for Chrominance signal that the monochrome display does not need. Blooteuth (talk) 16:13, 22 May 2017 (UTC)


 * I'm no expert, but I would expect that the power requirements for backlit LCD monitors would also be roughly 3:1 between RGB and monochrome to achieve the same effective brightness. A monochrome LCD panel could theoretically pass 100% of the available backlight (or maybe 50% after accounting for polarization). An RGB panel would have color filters in place that would only pass a about one-third of that polarized white light through each of the red, green, or blue sub-pixels. The result would be that for the same backlight intensity, the RGB panel would only be roughly 1/3 as bright. Alternately, to achieve the same effective brightness, the RGB backlight would have to be 3 times as bright. --  Tom N  talk/contrib 02:35, 23 May 2017 (UTC)


 * I don't follow your argument. For a white scene, all 3 of the filters would be open, so the brightness would be the same.  For a scene which is all red, blue, or green, then only 1/3 of the light would get through (although reflections may allow some of the light blocked by one filter to pass through another).  But, on a black-and-white set, a red, blue, or green scene would also be displayed at reduced brightness.  Also note that CRT screens often were brighter than LCD screens. StuRat (talk) 02:42, 23 May 2017 (UTC)


 * How can the brightness be the same when in one case each individual pixel is blocking ~66% of the light while in the other case each individual pixel is blocking ~0% of the light? --Guy Macon (talk) 03:54, 23 May 2017 (UTC)


 * OK, I see what you're both saying now. StuRat (talk) 03:59, 23 May 2017 (UTC)

Continuity equation for fluids
Can someone explain why $$A_1v_1=A_2v_2$$ is valid because of conservation of mass?

How did we get from conservation of mass to conservation of mass per time? All I know is $$\rho_1A_1L_1=\rho_2A_2L_2$$, but I don't see why to divide by $$\Delta t$$.



יהודה שמחה ולדמן (talk) 15:43, 21 May 2017 (UTC)


 * This is assuming a constant volume flow rate. In the case of pipes of varying cross-sectional area all connected up as shown, that's a good method to determine the velocity.  However, if the volume flow rate is changing, the calcs would need to be changed.  Moreover, with a compressible fluid, you could also get pressure waves moving through the system, even if the overall flow rate is constant.  So, those equations have their place, but shouldn't be used everywhere. StuRat (talk) 15:52, 21 May 2017 (UTC)
 * Yeah, I know we are assuming that $$\rho_1=\rho_2$$, but why are we allowd to divide by $$\Delta t$$ and get $$A_1v_1=A_2v_2$$.
 * Even so, in the case of $$A_2<A_1$$ - how can you prove that the fluid doesn't go faster than $$v_2$$? How? Conservation of momentum/energy? Let me know. יהודה שמחה ולדמן (talk) 18:52, 21 May 2017 (UTC)
 * $$\Delta t$$ is no divisor but a multiplier. Also you can not combine both terms cause they are not equal because $$s_1 != s_2$$. --Kharon (talk) 20:21, 21 May 2017 (UTC)
 * What? What is the last thing?
 * Still, if $$A_2<A_1$$ - how can you prove that the fluid doesn't go faster than $$v_2$$? Conservation of momentum/energy? יהודה שמחה ולדמן (talk) 23:57, 21 May 2017 (UTC)
 * Its average velocity won't be faster than $$v_2$$ but there definitely could exist local areas of higher velocity.--Jasper Deng (talk) 02:01, 22 May 2017 (UTC)

An easy way to derive the continuity equation is to apply the divergence theorem. The mass flux and velocity fields are parallel with constant density (i.e. incompressible flow) so the derivation proceeds similarly for both mass and volume conservation in that case.--Jasper Deng (talk) 02:10, 22 May 2017 (UTC)
 * Assuming v1, v2 are constant in time and the same over their respective cross-sections, consider the volume between A1 and A2. In a time $$\Delta t$$, it gains a mass $$A_1 v_1 \rho_1 \Delta t$$ flowing in at A1 and loses $$A_2 v_2 \rho_2 \Delta t$$ flowing out at A2 (corresponding volumes are in light blue on the figure). With the steady-state assumption, the inner mass must be constant hence those quantities must be equal, and the time cancels out. (This is true for any $$\Delta t$$, thus in particular you can choose a $$\Delta t>0$$ which allows you to divide.) Tigraan Click here to contact me 11:15, 22 May 2017 (UTC)
 * I'm not convinced.
 * You're Assuming $$v_1,v_2$$ are constant. Why should I believe it's true? How do I know the flow through $$A_2v_2$$?
 * Let us assume the fluid is non-viscous and non-compressible. $$\rho A_1L_1=\rho A_2L_2$$ is of course the volume, but who ever said the fluid has to flow at the same rate through both cross-sections of the pipe?
 * Again, what if the volume at $$A_2$$ has more energy/momentum such that $$v_3>v_2$$? יהודה שמחה ולדמן (talk) 12:25, 22 May 2017 (UTC)
 * Because the continuity equation does not apply in the absence of the steady-state assumption. Also (with the steady-state assumption), if the fluid does not flow at the same rate through both cross-sections then there is a net sink or net source in the segment of the pipe, which is not allowed.--Jasper Deng (talk) 15:18, 22 May 2017 (UTC)
 * Well, with the additional assumption of an incompressible flow, it does apply to nonconstant v1 and v2. Same argument, the mass balance becomes $$\int A_1 v_1 \rho dt = \int A_2 v_2 \rho dt$$, valid for any integration time bounds (but only because the constant trapped volume means there is a constant mass as well, i.e. no sink or source), hence you can drop the integration sign (assuming some mathematical hypotheses on the continuity which should be verified in a real physical problem). Tigraan Click here to contact me 15:52, 22 May 2017 (UTC)
 * This is part of Bernoulli's equation in fluid dynamics which is probably part of any higher technical education. Its best to learn it from ground up. Some parts may be boring at start but starting somewhere in the middle seem not working well. So there are masses of books and webpages, probably even videos, you can use to learn how Bernoulli developed it and how its applied to calculate the flow, temperature, pressure, volume, exit velocities etc. of contained fluids and gasses. --Kharon (talk) 16:45, 22 May 2017 (UTC)


 * The OP may also be interested in my question from last year where the stream of water was forcibly narrowed by the force of gravity.--Jasper Deng (talk) 05:33, 23 May 2017 (UTC)