Wikipedia:Reference desk/Archives/Science/2017 May 11

= May 11 =

Psychology study about the perception of achievement
I know that there is a specific psychology study about scholastic achievement. Basically, there are two groups of students. One group of students is assumed to do very well. The other, no response. Well, the result is that the teacher's expectations can influence the achievement of the students. What's the name of this? 50.4.236.254 (talk) 03:54, 11 May 2017 (UTC)


 * This paper has an overview of many such studies. -- Jayron 32 10:01, 11 May 2017 (UTC)


 * It's the Pygmalion effect or Rosenthal effect. Loraof (talk) 14:52, 11 May 2017 (UTC)


 * Also note that if the teachers expect more, they may interpret the same subjective answers on a test more positively. This is a good reason to exclusively use objective tests, wherever possible, and grade tests blindly (no student names) when grading subjective tests, like essay questions.  See confirmation bias.  StuRat (talk) 13:50, 13 May 2017 (UTC)

Convection coefficient for Heat sink Calculation
we know about heat dissipation equation derived for a fins.see.kindly tell me where can i get a data book that specifies convection coefficient values for (aluminium -air) contact at 540^m3/h.If no such kind of data book exists,where/how can we find/calculate h(convection coefficient) value. Thanks in Advance — Preceding unsigned comment added by Sameerdubey.sbp (talk • contribs) 10:13, 11 May 2017 (UTC)
 * Convection is a local phenomenon that depends mostly on the fluid properties and its speed (outside the boundary layer) relative to the solid. The convection coefficient itself is relatively independent on the solid (but see Biot number for the competition of convection vs. solid conduction effects). Your "540^m3/h" is not a speed, but probably a flow rate which you will need to divide by the cross-section.
 * A priori evaluation of the convection coefficient is deceptively hard. (For instance, this paper about a very rough approximation for it in a very specific case has been cited more than a thousand times.)
 * gives a correlation between the convection coefficient and air speed: $$h [W/m/K] = 10.45 - v [m/s] + 10 \sqrt{v}$$ (for speeds between 2 and 20 m/s). But I have no idea where it comes from and what are the conditions for validity, so... Tigraan Click here to contact me 11:38, 11 May 2017 (UTC)


 * Perry's Chemical Engineers' Handbook, 7th edition, section 5-12 suggests using the Nusselt number N_Nu = h*k/L to get h. It then notes that N_Nu = a*(N_Gr*N_Pr)^m where N_Gr is the Grashof number and N_Pr is the Prandtl number and gives values of a and m for a few geometries.--Wikimedes (talk) 20:14, 11 May 2017 (UTC)

Elephants in a balanced ecosystem
What kept historically the population of elephants limited? What were their natural predators? --Hofhof (talk) 12:09, 11 May 2017 (UTC)
 * Elephant mentions several predators (primarily taking calves). Also note their slow reproduction. HenryFlower 12:34, 11 May 2017 (UTC)


 * Predators don't typically limit a prey population much - either the prey can sustain the predators even in lean years, or else the predators would die. (The exception is if the predators are sustained by something else and the prey is really bad at surviving - I mean, if you could let loose a flock of dodos in the woods the foxes and coyotes would figure out something to do about them; then they might be restricted, say, to a few local back yards where the predators are chased off by the owners' mutts.  Or in this case, poachers who spend most of the time sticking people up on the road or something but who aren't averse to going in after an elephant if they happen to see one)


 * The usual problem is food availability: see .  The ecologists talking about culling elephants (!) are of course trying to prevent nature from taking its course by converting the ecosystem to the point where very few can survive; in former times perhaps bands of elephants roamed around looking for fresh forests to destroy.  The implication from that source seems to be that the overall ecology of Africa (forest vs. grassland) has been impacted by the reduction in elephants! Wnt (talk) 13:24, 11 May 2017 (UTC)
 * "Predators don't typically limit a prey population much" - careful. Trophic cascades go both ways. Some systems are under top-down control. Prey switching is the real and common phenomenon where predators seek alternative prey when primary prey is scarce. What is true is that most predators are not highly specialized, and can switch among many prey species. One famous example where single-predator control of single prey species is obvious is the lynx and hare system.
 * Anyway, one class natural controls on elephant populations that is often overlooked is parasites e.g. and disease (e.g. ). SemanticMantis (talk) 15:36, 11 May 2017 (UTC)
 * Let's not forget that elephants are considered keystone species - a species that has a disproportionately large effect on its environment relative to its abundance. DrChrissy (talk) 19:12, 11 May 2017 (UTC)
 * What you say is all true, but ... if you look at that figure you linked, the number of hares fluctuates over a ten-fold range or so. This is important, but in terms of understanding whether hares are common or rare, it's not all that important.  The other thing I have to note is that the data doesn't really show a rigid cause and effect relationship -- in 1907, the lynx harvest drops to nothing, but the hares take years more to recover; in 1886, both lynx and hares reach an all time peak and then all decline together - yet just a few years before that, the lynxes were at near peak levels relative to other cycles, and yet, the hares were about to see their population size explode.  I don't deny there is an effect, overall, and that statistically significant results can be obtained to prove it, but I doubt that it is really very much compared to the kind of population differences we see between sparrows and eagles.  I think mostly, the number of animals is determined by the amount of their food source. Wnt (talk) 01:24, 12 May 2017 (UTC)
 * In Britain, deer used to be culled. They were recently made a protected species.   Now the population is higher than it's been for a thousand years. 62.30.204.247 (talk) 13:00, 12 May 2017 (UTC)
 * That's interesting, but an artificial situation. On one hand, some people deliberately feed deer; on the other, some hunt them with weapons and tactics they never evolved to contend with.  This provides far more potential for the population to vary in either direction than a natural environment and ecosystem. Wnt (talk) 16:16, 12 May 2017 (UTC)


 * Sabertooth cats preyed on the smaller and older/sicker individuals (as do lions still). Elephants also die from toothwear and parasite load. μηδείς (talk) 22:05, 12 May 2017 (UTC)

Why are combustion motors noisy?
Could a combustion motor be silent? Why are fires (kind of) silent, but combustion in a motor noisy? --Clipname (talk) 17:28, 11 May 2017 (UTC)


 * Thermoelectric generators are silent. Internal combustion engines are noisy because they have rapidly moving parts moving under relatively high pressure of the combustion products, and because the combustion itself is rapid and periodic rather than continuous. Dr Dima (talk) 17:41, 11 May 2017 (UTC). NB we also have a good article on Aircraft noise, you may want to read that one as well. Dr Dima (talk) 17:47, 11 May 2017 (UTC)
 * Most rely on explosions to provide energy (except for gas turbines, jet engines and rockets which have different sounds). See Internal combustion engine.  D b f i r s   17:52, 11 May 2017 (UTC)
 * Would an imaginary frictionless combustion engine make no or little noise? Clipname (talk) 18:37, 11 May 2017 (UTC)
 * There would still be the sound of the explosions (or turbulent flow).   D b f i r s   18:41, 11 May 2017 (UTC)
 * Technically deflagrations but by that definition gunpowder also doesn't explode. Sagittarian Milky Way (talk) 18:55, 11 May 2017 (UTC)
 * True, but see Explosion. It's just a matter of speed.    D b f i r s   19:15, 11 May 2017 (UTC)
 * Fundamentally the engines work on sudden, repeated changes of pressure as the fuel burns, pushes down the piston, and the waste flows out the exhaust.
 * Those pressure changes are almost certainly going to result in noise unless something is done to stop it from happening. Mufflers help, but the vibrations from all those pressure changes is going to be hard to eliminate entirely. ApLundell (talk) 19:03, 11 May 2017 (UTC)


 * Internal combustion engines extract work from a hot fluid, usually heated by a rapid chemical reaction - the oxidation of fuel - constrained inside a combustion chamber and compelled to do pressure-volume work against a piston. There are a few variations on the theme, and some definitions that we could stretch, but the general idea is a hot fluid inside a metal can.  So, why is it noisy?
 * Work is wasted by the working fluid for at least a few reasons:
 * The fluid collides inelastically with the walls of the combustion chamber: this transduces pressure and heat into sound-vibration of the chamber itself, and the wave propagates as sound in the material and thence to the ambient air.
 * At the exhaust valve, the fluid is not in perfect equilibrium with the ambient pressure and temperature. This transduces pressure and heat into sound-vibration of the parts (and creates a pressure-front that directly impinges upon the ambient air), and thence propagates as a convective flow and as a supplemental sound wave into the ambient air.
 * The moving parts - valves, cams, drive-shafts, have metal-to-metal contact friction. These movements transduce kinetic energy into sound-vibration of the material - microscopic and usually-reversible metal deformation - and these sound-waves thence propagate into the ambient air.
 * Tackling the first: how do you make a perfectly elastic collision? Well, you can build the engine's combustion chamber out of an infinitely-stiff material!  (That's impossible in the real world).  You can come close by building the engine out of very heavy metal.
 * Tackling the second: you could perfectly match the exhaust velocity (and temperature and pressure) to ambient air. This is what rocket engine nozzles try to do: optimally expand the working fluid.  This is very hard - it is, actually, rocket science - to design a mechanical contraption that causes fluid to lose pressure and heat at the perfectly-calibrated rate so that it exits the tube at an exactly-desirable thermodynamic condition.  It is not commonly seen in conventional reciprocating engines, although - in aviation - you will sometimes find exhaust expanders, and salesmen who tell you that they allow the engine to extract a few extra horsepower out of the fuel.  For example, here are some after-market mods you can perform to upgrade your airplane.  Those are not the standard tubes, for any readers who are unfamiliar with the normal tubes that stick out of an airplane engine, those tubes are specially-crafted to be more thermodynamically efficient.  Supposedly, it'll give you a little extra kick - maybe a few percentage points faster.  Would you buy that physics (for a few thousand dollars)?
 * Tackling the third - well, nobody in engine-land wants metal-to-metal contact. There's supposed to be a film of lubricant preventing contact, essentially turning every mechanical activity into a perfectly-elastic collision.  Review point number one for more on this detail.  If you could have perfect, frictionless, interactions - and if your reciprocating parts never deformed even a millionth-of-an-inch when subjected to very high dynamic and kinematic forces, you'd lose zero energy transducing metal-on-metal contact deformation into acoustic noise that propagates into the engine (and thence, as a wave, into the ambient air).
 * In case it's not obvious, all of these improvements depend on "magic materials" with perfect properties. In the real world, we do the best we can - trading off against real, available material properties; and balancing agains other constraints like cost, weight, reliability, and so on.
 * Are there other places in the engine where kinetic energy is transduced into sound? Sure!  Imperfect thermal expansion, imperfect kinematic impedance-matching, imperfect manufacturing tolerances, ... if you were the kind of person who used physics and math to design and analyze engines ... (that would be an engineer)..., you could make a lucrative career out of budgeting these energy losses, tracking them down to actionable design-changes, and trying to solve them.  The most awesome engines are the ones that don't need sound-insulation or muffling.  It's one thing to reduce the noise by suppressing it - it's a lot more amazing if you can prevent the sound from being generated in the first place!
 * Nimur (talk) 19:52, 11 May 2017 (UTC)


 * An internal combustion engine produces a high pressure inside the cylinder, then opens the exhaust valve(s) and allows these gases to escape. If the valves open whilst there is still a high internal pressure, this exhaust is noisy. If it opens at a low pressure, it is quiet(er).
 * In the 1920s, Daimler were fitting their prestigious luxury cars with a version of the Daimler-Knight engine, which is a sleeve valve engine. As a sleeve valve can have much greater area than a conventional poppet valve, it can open later (after the pressure has dropped) and still provide adequate scavenging. Sleeve valves also have a reputation for high oil consumption and difficult lubrication. Daimlers gained a reputation for departing quietly in a blue haze of oil smoke, but this low noise was considered important enough to persist with the sleeve valve, despite the oil burning. Andy Dingley (talk) 23:48, 11 May 2017 (UTC)


 * In a loose sense, a combustion motor does not need to be noisy at all -- see fuel cell vehicle. It is possible to take ethanol, react it in a fuel cell, extract electricity, and use the electricity to turn a quiet motor.  That may be too far from the intent of the question, but I see no theoretical reason why there should not be something which is halfway between an engine and a fuel cell, i.e. taking fuel and reacting it somehow on a small to microscopic scale to harvest energy with little if any detectable noise, then combining that energy somehow to turn a shaft, or otherwise provide propulsion.  There ought to be thousands of independent designs within those broad specifications waiting to be invented. Wnt (talk) 16:26, 12 May 2017 (UTC)
 * Fish do it. Blooteuth (talk) 19:06, 13 May 2017 (UTC)

Could the US or Latin America have lots of wild elephants if someone had released breeding pairs?
Or would that have caused so much ecological and/or agricultural and/or ivory-related trouble that they would've been hunted to near-extinction like wolves? Sagittarian Milky Way (talk) 23:30, 11 May 2017 (UTC)
 * Maybe not elephants, but you should check out United States Camel Corps... -- Jayron 32 00:43, 12 May 2017 (UTC)
 * One breeding pair would not be enough (inbreeding would set in almost immediately).  (I can't remember what article to link to for about how many you need.)  When did you want to let them loose?  If you did it now they'd be hit my a car almost immediately (i.e. killed by humans) but they might have had a chance earlier in history. RJFJR (talk) 00:46, 12 May 2017 (UTC)
 * If President Lincoln didn't decline the Thai king's offer to release a number of male elephants and a number of female elephants, wait for multiplication, then have a free supply of beasts of burden to catch. Sagittarian Milky Way (talk) 01:10, 12 May 2017 (UTC)
 * The multiplication and catching them later was part of the offer, not implied. Sagittarian Milky Way (talk) 12:21, 12 May 2017 (UTC)
 * Found that article: Minimum viable population. RJFJR (talk) 00:48, 12 May 2017 (UTC)
 * And for the record, the U.S. and Latin America used to have elephants, or at least close cousins. See Mastodon, Columbian mammoth, etc.  -- Jayron 32 00:56, 12 May 2017 (UTC)
 * Apparently, Germany has acquired a population of Nandus from just three breeding pairs escaping from a farm in 2000. The population is s now in the 100s, and farmers are calling for controlling measures, as they eat their crops. --Stephan Schulz (talk) 06:07, 12 May 2017 (UTC)

Elephants are mentioned in the Book of Mormon in connection with the Jaredites. They were noted as being among the most useful animals. The Jaredites are estimated [by whom?] to have arrived in the New World between 2600 and 2100 BC. ''And they also had horses, and asses, and there were elephants and cureloms and cumoms; all of which were useful unto man, and more especially the elephants and cureloms and cumoms. (Ether 9:19)''. Whether Evidence for the survival of the elephant can be found in Native American myths depends on whom you believe. Blooteuth (talk) 08:47, 12 May 2017 (UTC)
 * See rewilding. μηδείς (talk) 22:02, 12 May 2017 (UTC)