Wikipedia:Reference desk/Archives/Science/2013 October 1

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.

October 1

Car power

For fun I took the definition of power as energy over time, substituted it into the equation for kinetic energy, and rearranged to give an expression for velocity as a function of time for cars of known weight and power, which should allow me to approximate the 0-100km/h time. Finding reliable numbers for cars' horsepower at the wheels was harder than I originally anticipated, so instead I used known weights and 0-100km/h times to calculate the WHP. The numbers I get back seem far too low to be believable. I realize I'm neglecting air resistance, but I can't see that making the difference. For example, for the 2014 Porsche 911 Turbo S, the curb weight is 3538 pounds (1605kg) and the 0-60mph (26.8m/s) time is 3.1 seconds. Plugging it into the formula gives:
$P={\tfrac {mv^{2}}{2t}}={\tfrac {1605\times 26.8^{2}}{2\times 3.1}}=185.9kW$
The engine power is rated at 552HP (412KW). Is over half the engine's output really used up by the drive train, accessories and whatever effect air resistance has? 202.155.85.18 (talk) 03:29, 1 October 2013 (UTC)

At high speeds, air resistance can eat up more than half of the engine power! 24.23.196.85 (talk) 04:59, 1 October 2013 (UTC)

Can you define high speed, or better, provide a ref that explains in detail the relevant processes? I imagine there would be a major change in the energy lost to air resistance at the boundary from laminar flow to turbulent flow. 202.155.85.18 (talk) 05:17, 1 October 2013 (UTC)

Yes, there is a jump in drag there, although you don't instantly go from fully laminar to fully turbulent flow, but rather the turbulent regions expand as speeds increase. Therefore, drag also increases as speeds increase, more than proportionally to the speed. StuRat (talk) 06:08, 1 October 2013 (UTC)

I believe drag is proportional to the SQUARE of the speed -- except for induced drag, which is proportional to the angle of attack! 24.23.196.85 (talk) 02:00, 2 October 2013 (UTC)

Yes, but that's only a rough approximation, as things like the transition to turbulence, cavitation (for boats), and supersonic speeds (mainly for planes) all will affect drag. StuRat (talk) 16:15, 2 October 2013 (UTC)

Yes, and the efficiency of cars is even lower if you figure that the "payload" (passengers plus cargo) is typically only around 5-10% of the total mass of the vehicle. Compare that to a bicycle. StuRat (talk) 05:06, 1 October 2013 (UTC)

This is a 4speed gearbox car. To acclerate to only 60 mph, you would not use top gear. Most of the time would be spent in 2nd and 3rd gear. The engine will only develop full rated power at a certain RPM, reached only for an intant in each gear. For all except that instant, the engine is forced to develop less than rated power. As the Porsche will be quite highly tuned, it would be reasonable to derate it 25% for this reason, so the mean power at the flywheel during acceration is only 412 x 0.7 ie 309 kW. Car engines are rated "as runnable" ie measured while driving oil pump, cooling fan. But not with an alternator or any other accessories, as the power absorbed is driver dependent. Allow 5 kW for alternator etc, leaving 304 kW. Gesrbox losses are about 2%. Differential losses are quite high, due to the gear teeth in sliding mesh becuse of the hypoid configuration, normally about 4% at steady highway speeds. Diff losses rise with torque, so under hard acceleration, allow about 30%. This leaves power at wheels 304 x 0.98 x 0.7 ie 209 kW. 58.164.224.95 (talk) 05:19, 1 October 2013 (UTC)

Interesting. That comes up pretty close to what the function returned. I can imagine that the remaining 25kW are consumed overcoming air resistance. So, more generally, for any highly tuned sports car doing a 0-60mph test, is it reasonable to assume the rated engine power x 0.98 x 0.7 x 0.75 = average power at the wheels for a first approximation? This would theoretically allow a ballpark approximation of the 0-60mph time from the engine power and weight. 202.155.85.18 (talk) 05:29, 1 October 2013 (UTC)

Yep. For accuracy, you need to allow for tire friction, about 3 to 4%, and aerodynamic losses, probably only about 2% at low speeds for this fairly aerodynamic car. Is the weight you gave the dry weight or the curb weight? Some manufactuers give the dry weight. To this you need to add the oil, fuel, and driver. Coolant too, for cars with radiators. Also add any options, like airconditioning. 58.164.224.95 (talk) 05:45, 1 October 2013 (UTC)

One effect ignored in the above is the rotational kinetic energy of the engine. In first gear this is approximately 30% of the vehicle's KE, so first gear performance is even worse than you'd hope. Of course this /may/ be released back into the driveline on a gearshift. Greglocock (talk) 23:14, 1 October 2013 (UTC)

30% ? - I don't think so. Even for this very fast car, the acceleration time is given as 3.1 seconds. As we are accelerating it at the fastest rate, the engine RPM must be kept quite high, say in the range 3000 to 5000 RPM. Have you not sat in a car with engine idling, and floored the accellerator? Do it, and the time to get to 5000 RPM is only a fraction of a second in performance motors of this sort. For accuracy, the engine rotational kinetic energy should be taken into account, but it will affect the result by very much less than 30%. And in the shift from 1 to 2 and 2 to 3, the driver does have the option of letting the clutch out and using the car to bring the RPM back, thus utilising the stored K.E., as you realised. Another factor I overlooked above is the time spent changing gears - probably 0.3 second or so in total. That means another 10% or so increase in acceleration time. 58.164.224.95 (talk) 23:48, 1 October 2013 (UTC)

Ah, I thought we were talking about physics and engineering, not opinions. I wrote the first ever straight line performance simulation software for a major car manufacturer in 1979, as I was bored with working it out on graph paper. It was still being used 6 years later. Your call. Greglocock (talk) 23:28, 2 October 2013 (UTC)

On the internet, nobody knows you are a dog. 120.145.10.221 (talk) 01:29, 3 October 2013 (UTC)

Solubility units

Good afternoon.

I noticed recently that solubilites for compounds (I was looking at inorganic salts) are listed in a mix of units:

 - grams / 100 mL
 - grams / L

This can lead to confusion & I was wondering why they are not in a common format (preferably g/L)?

Regards, Wayne Kiely — Preceding unsigned comment added by Wayne Kiely (talk • contribs) 05:39, 1 October 2013 (UTC)

Same reason all sorts of other combinations are used in measurements, like quoting car speeds in km/hour instead of meters per second. In SI, there are preferred units. But for many purposes, multiples are more convenient. For caculation of say salt concentration in water, you'd normally use grammes per litre. But for quoting the typical PSA level in the blood of a human male, about 2 to 4 nanograms per litre, quoting it in grams per litre, ~ 0.000000002 to 0.000000004 - would be silly wouldn't it? Heck, some people don't even use any metric system. Would you quote the speed of a snail in terms of c? 58.164.224.95 (talk) 05:51, 1 October 2013 (UTC)

It varies substantially from species to species, but your typical garden snail moves at a speed of 0.00000000004c. Someguy1221 (talk) 06:01, 1 October 2013 (UTC)

I think the question relates to Wikipedia's chemical infobox where a wide array of different units get used for various things. The OP might want to take this to Wikipedia:Chemistry, though I suspect there's already a style guide somewhere that isn't being applied universally. 202.155.85.18 (talk) 05:56, 1 October 2013 (UTC)

To accurately reflect the source? Solubility data can come from old sources, the accuracy of the data varies, sometimes sources don't agree... If the source gives 2g/100ml, should you interpret that as 2 ±1g /100ml or 2 ±0.5g /100ml? And if you convert it to g/L, are you going to write 2*10¹g/L, since 20g/L suggests 20±1g/L? When the accuracy of the data is uncertain, maybe it's best to quote the source verbatim.

My 1976 edition of the Merck index has 10000 compounds listed, but doesn't show any system either. Solubility units vary a lot: some examples of solids:

• "Sol. in about 250 parts cold water, 1200 parts boiling water"
• "Soly in water: 1.5 gr/100 ml at 25°C"
• "one gram dissolves in 2.8 ml water at 25°C"
• "sparingly soluble in water, freely soluble in alcohol"
• Sol in warm alcohol, ether, benzene, acetone. Practically insol in water and alkalies".

That's why when you google Solubility per 100 ml per liter, most hits you get are exercises on converting between different measurements. Often accompanied by a text like "Sometimes, the solubility is given in grams per 100 mL, rather than ..."

The g/100 ml probably reflects laboratory practice, 100 ml is a good quantity to work with, ideal for the most common size erlenmeyer (250ml).

I couldn't find a specific wikipedia policy on the subject (Wikipedia talk:Chemical infobox). There has been some discussion about changing g/100ml water to g/100g water, but that would have caused errors for things like brine apparently. The fact is that in chemistry, data isn't completely standardized yet. Ssscienccce (talk) 15:08, 1 October 2013 (UTC)

Common ancestor in the theory of evolution

Hello,

According to evolution, do all animals in a given group descended from an individual from a previous species, or from many individuals from a previous species? For example, do all mammals descended from a specific individual of ancient fish, or from many individuals of fish from a specific ancient species? I get the impression that the first possibility is the right one, but I can not understand why all animals in a group should descended from a specific individual. Thanks! 94.159.218.148 (talk) 06:34, 1 October 2013 (UTC)

Going back in time, you have more grandparents than parents, more greatgrandparents than that, etc. Looking back far enough, you reach the point where it is likely that every individual within a species can claim one past individual as an ancestor --- the last common ancestor. This is true for the descendants of that species even if they split into many species later on.

That said, you can picture circumstances in which the individual, species, and gene are separable - namely, lineage sorting. For example, suppose two species of butterflies hybridize, one of which has red wings and one of which has blue wings. The population might briefly enjoy success as a hybrid species with a red-and-blue pattern, then the fickle finger of fashion moves and they start to do better as red and blue again. If two species eventually sort out, one with red wings and one with blue wings again, they might have random assortments of all the other genes that went into the hybrid species, with one form or another becoming genetically fixed in the descendant species. You could look at this confusing situation and see that the red gene and the blue gene had no common ancestor since some time many millions of years earlier, even though the red and blue species had a common ancestor species (the hybrid species). One or more single individuals of the hybrid species might or might not have become common ancestors of all the members of the hybrid species before it split apart again. And there might or might not be common ancestors in the original red and blue species from before the hybridization. So it's sort of tricky - you have to think carefully about exactly what you're trying to measure or answer or model in each case. Wnt (talk) 07:34, 1 October 2013 (UTC)

Note that, in the case of animals with sexual reproduction, the absolute minimum population size from which the species can originate is 2. And, in the case of asexual reproduction, yes, every species, strain, etc., can be traced back to one individual.

The Neanderthals might be of interest here. They predated modern humans, but modern humans appear to have interbred with them, so many of us have Neanderthal DNA (some apparently more than others). So, this makes me think that a species which evolves solely from one breeding pair is a rare case. Also, this extreme inbreeding would cause the species to lack the genetic variability needed to survive changes in the environment. StuRat (talk) 13:29, 1 October 2013 (UTC)

What is the current theory on what the first sustained life forms were? Presumably microbes of some kind or perhaps of many kinds. ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:38, 1 October 2013 (UTC)

Look at LUCA. Dauto (talk) 15:11, 1 October 2013 (UTC)

Excellent. That should be the answer to the OP's question. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:00, 1 October 2013 (UTC)

As I recall from The Origin of Species, often new species develop when some members of a species become geographically isolated from the rest of the species. I would think that in such a case, after enough generations all members of the first generation (if the isolating event was cleanly defined enough that we can specify a particular "first generation") are ancestors of everybody. Then there are many members of the earlier species that are ancestors of the new species. So in this non-hybridization scenario, the answer to the OP's question would be no, there is not a single ancestor (or male-female pair of ancestors) in the previous species that gave rise to the new species. (Of course if you go back far enough in the previous species you find an original common male, and female, ancestor, but I don't think that's what the OP was asking about.) Make sense? Duoduoduo (talk) 15:13, 1 October 2013 (UTC)

The general rule is that all animals in a given group descend from many individuals from a previous species. This is evidenced by the existence of so-called trans-species polymorphisms. A gene is said to be polymorphic if it has two or more variants. The most polymorphic region of the human genome is the major histocompatibility complex. For many of its alleles, similar variants are found in humans and apes. In other words, the split between two different human alleles can be more ancient than the split between humans and other apes. This indicates that groups of individuals diverged into separate species, and that the polymorphisms that were present in the founder population, have been maintained in each of the species. What is more uncertain, is how large the groups have been. Here (PMID 2183415), Jan Klein, estimates the minimum size of possible bottlenecks since humans diverged from other apes, to a minimum of ten individuals, and more likely closer to 10,000. Here's a more recent abstract documenting trans-species polymorphisms between humans and other apes, Here's one for whales and another for rodents. Another gene locus where humans and apes share similar polymorphisms is the ABO blood group genes. It has been controversial whether this is a result of convergent evolution or whether it is another example of true trans-species polymorphism. Here's evidence indicating that it is the latter. --NorwegianBlue ^talk 20:15, 1 October 2013 (UTC)

NorwegianBlue's got the correct answer. For example, chimps have the ABO blood system. They wouldn't have that in common with us if both our species had only one ancestor. What do trace to a more recent single ancestor are the non-recombinant Y chromosomes and mitochondria. μηδείς (talk) 23:46, 1 October 2013 (UTC)

The tortoise and the hare

Which travels further in a lifetime, a tortoise or a hare? almost-instinct 13:09, 1 October 2013 (UTC)

In what sense? Distance from birthplace? Total miles walked? Average miles per year? (Tortoises can live several rabbit lifetimes.) ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:26, 1 October 2013 (UTC)

Total miles walked. almost-instinct 13:33, 1 October 2013 (UTC)

ps I'm expecting that relative difference between Potential Length of Life and Actual Length of an Average Life is greater for the hare than the tortoise. almost-instinct 13:36, 1 October 2013 (UTC)

Based on the mean distance recorded for Galapagos tortoises (http://www.migratoryconnectivityproject.org/study-shows-galapagos-tortoises-are-long-distance-migrants/): 200 to 300 meter, and for European hares in the winter: 3.91 km (http://rcin.org.pl/Content/12314), combined with the average life span in the wild (100+ years vs 4 years), the tortoise would win with 9125 km versus 5708 km. But that is based on hare displacements between 50*50m grids in the winter on the one hand, and a migration of tortoises to a place 6 km away on the other hand, so the real figures would likely be higher for the hare and lower for the tortoise. Ssscienccce (talk) 15:54, 1 October 2013 (UTC)

My money is on Lonesome George. Gandalf61 (talk) 16:03, 1 October 2013 (UTC)

George doesn't do a lot of walking these days. Richard Avery (talk) 06:40, 2 October 2013 (UTC)

Limits of a dynamo

What limits the output of a dynamo? If I just keep spinning it faster and faster would it eventually reach a high enough voltage to blow the bulb? What if instead of a bulb there was a more robust load? If the dynamo is rated at 6 watts, does that mean it cannot physically transduce a greater power or that it will begin to fail? Does it make a difference if the current is rectified? --2.97.26.56 (talk) 17:54, 1 October 2013 (UTC)

The limits depend on the how the particular dynamo was designed. If the speed is great enough, it could break into pieces and fly apart. If the voltage becomes too great, the electrical insulation between windings will break down and internal arcs will occur. If the current is too great over a long enough period of time, the windings will heat until the heat causes catastrophic failure of the dynamo. This might take the form of electrical insulation failure, bearing failure, or a part expanding until it hits another part. The heat might also damage some nearby material, causing an indirect failure. Jc3s5h (talk) 18:09, 1 October 2013 (UTC)

If the output is rectified and smoothed with a capacitor, the usable output is less, as the RMS (heating value) of the dynamo current is increased with respect to the DC output. This is because the dynamo current becomes a series of short pulses instead of a smoothly changing sine wave. As well as the issues mentioned above, the internal heating in a dynamo rises with RPM faster than what the electrical output does, becuase the cummutation frequency increases, increasing eddy current and hysteresis losses in the iron parts. 58.164.224.95 (talk) 23:57, 1 October 2013 (UTC)

Bad eggs?

I noticed that my carton of chicken eggs has today's date stamped on it. But it doesn't say 'sell by', 'use by' or 'expiration'. It got me thinking... how can one tell if a raw egg has gone bad? (I just remember the putrid sulfur smell of un-discovered hard boiled Easter Eggs in May.) These refrigerated eggs look and smell normal. --208.185.21.102 (talk) 18:24, 1 October 2013 (UTC)

Drop it in a pot of water. Fresh eggs should sink. Rotten eggs should float. Whoop whoop pull up ^{Bitching Betty | Averted crashes} 19:18, 1 October 2013 (UTC)

My Google search for egg test found many results, including ones confirming the previous reply.

—Wavelength (talk) 19:54, 1 October 2013 (UTC)

These test do not generally test for whether the egg has gone bad (e.g. is infected with salmonella), but rather simply how long it is since it was laid. Over time, an egg will lose water through evaporation through the shell, which will reduce its weight and eventually make it float. Old eggs may be unpleasant, but need not be "bad" in the sense that they have started to decay or will be smelly. Nevertheless, test is probably perfectly good for your purposes. — Quondum 20:42, 1 October 2013 (UTC)

• Let me note that raw eggs are actually remarkably resistant to spoilage -- if undamaged they will usually remain good for weeks even without refrigeration. That happens because egg white has very potent antibacterial activity. Cooked eggs actually spoil much more quickly than raw eggs, because cooking denatures the proteins that protect them. Looie496 (talk) 23:00, 1 October 2013 (UTC)

In the US egg cartons can have the "pack date" printed on them. In the EU the laying date may be used, in which case each egg must be imprinted with the date. If you bought them today and live in the US, it could be the pack date. Isn't there any other text on the carton, something like "for expiration/pack date, see printing on top"? Ssscienccce (talk) 13:01, 2 October 2013 (UTC)

Isn't the expiration date the date stamped on each egg? Whoop whoop pull up ^{Bitching Betty | Averted crashes} 09:28, 3 October 2013 (UTC)

Normally, the USDA website is a great resource for guidelines on food storage, identifying spoilage, proper cooking and so on. Unfortunately, the site is down because of the government shutdown. Katie R (talk) 13:29, 2 October 2013 (UTC)

This reminds me of my father's chicken farm in the 1950's. It was a smallish farm, delivering eggs to shops and high-consumption customers (eg a local orphanage, some hotels) every day. Then a socialist (Labor Party) State Government got elected. They passed a law establishing a Egg Marketing Board, intended to stabilise supply and prices. They made it illegal for farmers to sell to anyone except the Egg Marketing Board. Otherwise nobody would. And the Board set prices and required farmers to deliver at farmer's cost to the Board's city warehouse. Now, producing eggs was always a low margin business, and these clowns forced the price down further. My father, along with other similar farmers, could not afford the cost of having eggs trucked to the capital city every day. So what he and other producers did, was store the eggs until there were enough to fill a very large truck - about 4 week's production. This was more cost effective. You might expect that the eggs waiting to be shipped would be refrigerated. Well no, as the capital and power costs would have bankrupted him. There was no electricity supply in the district anyway. What he did was construct an underground storage chamber, so at least the eggs were stored at between 12 and 25 C depending on season, instead of up to 40 C in summer. Over time there was a drift to larger and larger farms though, so production was after some years sufficient to ship every week. But still definitley no fresh eggs in the shops. 120.145.188.233 (talk) 04:15, 3 October 2013 (UTC)

semiconductors called alloys

I was looking through some III-V semiconductor articles the arsenic article especially and there these compounds like Gallium arsenide are grouped in the alloys section or named alloys. For me this are normal inorganic compounds with a distinct atomic (molar) composition. Is this only for me a little bit strange or is this OK for the people working with semiconductors? --Stone (talk) 20:00, 1 October 2013 (UTC)

Are you talking about AlGaAs? Because, that beckons the only mention of the word alloy on the page. Plasmic Physics (talk) 21:22, 1 October 2013 (UTC)

A whole section in the arsenic article is called alloys and in this section only the III-V semiconductor are located.--Stone (talk) 14:19, 2 October 2013 (UTC)

The definition of an alloy is a metallic substance comprising of one or more elements dissolved into another. That's exactly what all the III-V semiconductors are - A Group III and a Group V element interdisolved in each other. (Without counting the dopant). 58.164.224.95 (talk) 00:03, 2 October 2013 (UTC)

Alloys don't have stoichiometric ratios, compounds (usually) do. But despite the stochiometric ratio, (I believe that) GaAs structure has more to do with the size of the atoms than with chemical bonds between them, so maybe alloy is a better term? See also Intermetallic. Ssscienccce (talk) 13:31, 2 October 2013 (UTC)

Thanks!! The Intermetallic article does the trick!! --Stone (talk) 14:19, 2 October 2013 (UTC)

yDNA results

Can someone tell me how closely the last two entries on these pages are and how many generations back they were related?SuzanneB2013 (talk) 20:36, 1 October 2013 (UTC)

Which two pages, where? AndyTheGrump (talk) 20:39, 1 October 2013 (UTC)

The last two entries in the table at YDNA#Chronological_development_of_haplogroups_in_Europe maybe? Ssscienccce (talk) 12:29, 2 October 2013 (UTC)

Identify species of grasshopper

File:Unknown grasshopper in Norman, Oklahoma.jpg

Unknown grasshopper.

What kind of grasshopper is in the picture on the right? The photo is of a grasshopper that was on my vehicle's dashboard in Norman, Oklahoma on August 15 of this year. Thanks in advance, Ks0stm ^{(T•C•G•E)} 22:12, 1 October 2013 (UTC)

Here you go: http://bugguide.net/node/view/704945 196.214.78.114 (talk) 09:11, 2 October 2013 (UTC)

Hmmm. Very similar but closer to your location. You decide. http://bugguide.net/node/view/153012 196.214.78.114 (talk) 09:26, 2 October 2013 (UTC)

I've never seen anything like this! Perched on the side of a car park, this creature must be at least 60 feet long! Myles325a (talk) 12:47, 5 October 2013 (UTC)