Wikipedia:Reference desk/Archives/Science/2012 May 10

= May 10 =

Why are salad shrimps so cheap?
Jumbo shrimps cost $5 a pound usually even on sale (have seen $4 once), but cooked salad shrimps cost $3 regular price and $2 this week at my favorite supermarket. Why the price difference? 66.108.223.179 (talk) 00:56, 10 May 2012 (UTC)
 * Supply and demand. As with any other seafood, it takes longer for a critter to grow to larger size.  Older shrimp are rarer, because there's more time for them to be eaten by predators.  Thus, it is relatively easy to catch lots of little tiny shrimp, or in the case of Shrimp farming, it takes a lot of resources to grow shrimp to larger sizes.  In either case (wild caught or farmed), it is going to be more expensive per pound to bring larger shrimp to market than smaller, because of the numbers of shrimp or the resources needed to get them that large.  -- Jayron  32  01:01, 10 May 2012 (UTC)
 * Jayron, would you know by chance the cost differential between growing a pound of jumbo shrimp and a pound of baby shrimp? I totally believe you, but it strikes me as counter-intuitive that growing the same amount of meat but on a larger number of animals is actually the more efficient way to go. Someguy1221 (talk) 01:13, 10 May 2012 (UTC)
 * How about this: Economic Update on Gulf Shrimp Fisheries, a recent report from the South East Regional Office of the NOAA Fisheries Service. This report details an economic and ecological model for different-sized shrimps, and the economics of catching them using differently-sized shrimping boats.  "The magnitude of the price decline has varied by shrimp size category, with the under 15 count (“jumbo”) and 68 and over count (“small”) size categories seeing the smallest declines (approximately 23%) and the 31-40 and 41-50 count (“large” and “medium”) size categories seeing the largest declines (approximately 35%). Due to inflation, these price declines are even larger in real terms. ... According to Haby, et al. (2003), increases in shrimp imports have been the primary cause of the recent decline in U.S. shrimp prices. A complete discussion of the factors contributing to the increase in imports can be found in Haby, et al. (2003)."  You can't get much better an answer than that!   "...Relatively lower wage rates have allowed major shrimp exporters (e.g. Thailand) to increase production of more convenient and higher value product forms, such as hand-peeled raw and cooked shrimp. With respect to the second factor, changes in farming technology and species have allowed production of foreign product to shift towards larger, more valuable sizes. As a result of these factors, imports are more directly competing with the product traditionally harvested by the domestic industry, thereby reducing the latter’s historical comparative advantage with respect to these product forms and sizes."
 * (I found this page via the main page links at the Southwest Fisheries Science center, which I regularly read; I navigated to the "Resource Fisheries" section and went straight for the South East region, including Gulf of Mexico, where much commercial shrimping takes place in the United States; though much shrimp in your grocery store comes from the Pacific Northwest or from Mexico, or overseas imports). Nimur (talk) 01:38, 10 May 2012 (UTC)


 * Here is a simple way to think about it. Suppose you have 2 small shrimp and one eats the other.  If it was 100% efficient at converting the shrimp it ate into it's own body, then the total mass would be the same before and after the cannibalism.  However, digestion is nowhere near 100% efficient.  So, your cannibal shrimp ends up maybe 10% larger than it was.  Thus, you went from a weight of 2x for both shrimp to 1.1x for one shrimp.  Even if the shrimp aren't cannibals, similar ratios apply, whatever they eat. StuRat (talk) 05:22, 10 May 2012 (UTC)
 * The problem with you scenario is you appear to have started with 2 magic small shrimp. If these shrimp aren't magic, how did you get them in the first place? If the shrimp needed to eat 10x it's final body weight to reach its final bodyweight, it doesn't matter whether you have 2 shrimp weighing 5g, or 1 shrimp weighing 10g, you still needed to feed 100g. Don't get me wrong, there are plenty of reasons why raising 1 large shrimp may be less efficient then raising 2 small shrimp (e.g. possibly lower food conversion efficiency) but your explaination doesn't really help. You're explaining why it's usually better/more efficient to eat something lower down in the food chain, rather then why eating something smaller is better. Nil Einne (talk) 08:20, 10 May 2012 (UTC)


 * It's not only the amount of food the shrimps eat to grow to mass X, you also need to factor in the time. If we assume shrimp growth is a linear process it takes 1n shrimp time t to grow to mass x while 2n shrimp would only take t/2 to grow to mass x. Thus the output of the shrimp farm would double if they harvest them at "half size". Roger (talk) 08:48, 10 May 2012 (UTC)


 * Jayron said it all: supply and demand. It doesn't actually matter how much it costs to produce something, the margin might be bigger for some product than for other, but manufacturers are not passing the cost to you, they only charge as much as possible, and incidentally, consumers are willing to pay more for bigger shrimps, which is not amazing, since for the same weight, you would get more eatable meat. 95.20.183.251 (talk) 13:26, 10 May 2012 (UTC)


 * 1) Why is the ratio of eatable meat higher in a larger shrimp ?


 * 2) Cost is not solely a question of how much people are willing to pay for an item, the cost to produce it must also be considered. Indeed, when competition works, the price should be pushed down until only slightly above the production cost.


 * 3) An advantage I've found with larger shrimp is that they are easier and quicker to clean, since you have fewer shrimp to clean. StuRat (talk) 15:22, 10 May 2012 (UTC)

Measuring human tasked energy consumption with calorimetry
How might you extend the concept of the calorimeter to measure the energy consumption of a person doing a particular task, such as riding a bicycle? --Accountness (talk) 01:18, 10 May 2012 (UTC)


 * I put your huge title into the body and gave it a short title. StuRat (talk) 05:16, 10 May 2012 (UTC)


 * Stu, that one word title makes it just about useless if someone later on tries searching for it in archived questions. May I suggest a new title "Measuring human tasked energy consumption with calorimetry"?  Wickwack60.230.217.112 (talk) 05:47, 10 May 2012 (UTC)


 * OK, changed it. However, there aren't going to be all that many calorimeter questions, and we can search through the contents, as well as the title, to narrow it down further. StuRat (talk) 05:53, 10 May 2012 (UTC)


 * AFAIK human energy consumption in such situations is usually determined by measuring respiration - oxygen intake versus carbon dioxide output. One could put the subject inside a very well insulated room and measure the rise in air temperature but it's much easier just to measure the respiration rate. Roger (talk) 08:38, 10 May 2012 (UTC)


 * Easier except for the subject. Doing a VO2 max with a mask on is pretty unpleasant, from my experience. --BozMo talk 20:04, 10 May 2012 (UTC)


 * This sounds like a another homework assignment question. It would help if you (the original questioner) expanded a little on what you want.
 * The energy consumed over a period of time by a human under sustainable conditions may be approximately considered as:-
 * ....ETOTCON = EFOOD + EBODYMASS + ETHERM
 * where ETOTCON is the total energy consumed;
 * ..........EFOOD is the energy contained chemically within the food and drink eaten;
 * ..........EBODYMASS is the energy consumed (or replaced) by the body consuming its' own mass (as in fat stores being converted back to energy, and muscle mass being converted back to energy)
 * and ...ETHERM is the energy contained as heat in the food and drink consumed by virtue of these being at a certain temperature.
 * Therefore, to properly understand and measure the energy consumed by a human, whether at some task or not, you must understand and measure each of these sources.
 * EFOOD can be measured by burning samples of the food in a Bomb Calorimeter (see http://en.wikipedia.org/wiki/Bomb_calorimeter#Bomb_calorimeters). The term bomb here does not meaning blowing anything up, it means that the combustion is contained wholely within a measurement container.  There is a considerable volume of literature giving the energy content (in kJ or calories) of almost all types of food and drink.  Energy content is normally given on food packaging, at least in Australia and the USA, but the accuracy is not stated.  So, if you want to measure it for yourself, you need a bomb calorimeter.  NOTE:  the energy value obtained for any given food type does vary significantly depending on the actual measurement technique used.  Bomb calorimetry is not by any means an exact science, particularly with food.  There is a definitional issue as well:  Food is a fuel - as with any fuel, you can consider the chemical energy as a Low Heat Value or as a High Heat Value.  When fuel is chemically consumed, the products ultimately are CO2 and H20 and perhaps "ash" (residual stuff that cannot be burnt).  If the H2O is emitted/excreted as vapor (as it is in sweating) it still contains heat of vaporisation ( http://en.wikipedia.org/wiki/Heat_of_vaporization ), so the energy extracted is the Low Heat Value, otherwise it is the High Heat Value.
 * The energy ETHERM supplied in food and drink may be calculated with sufficient accuracy (given everything else to do with human energy is not at all accurate) by weighing the food, measuring its temperature, and taking the heat contained (in kJ) as though it is water (~4 kJ kg-1 K-1)
 * The energy EBODYMASS contained in any body mass consumed or replaced could be estimated by weighing the human before and after the task, correcting for sweat and any urine excreted. The best way would be to rehearse the test so you get the human to maintain his/her weight the same throughout the test, and make the test long enough in duration (at least several hours a day for a week or more) so a small change can be detected.
 * To make any useful sense of energy consumed, you also need to measure energy expended (ie in mechanical work, sweating, etc), and compare energy consumed and energy expended. The total consumed will of course be equal to that expended. Mechanical work done will range from zero to about 25% of ETOTCON depending on the task, the human, and how comfortable you keep him/her.   As previous poster Roger has said, you can use oxygen consumption and/or CO2 expiration to measure energy chemical conversion, but humans are not terribly efficient not consistent in converting chemical energy as measured this way into mechanical work.  You can also use an instrumented room as Roger said - this is the use of a Calvet Calorimeter.  However, this on its own is very inaccurate for this purpose and has serious limitations re comfort of the human.  I may make a second post on accurately measuring energy expended later.
 * Ratbone58.164.236.109 (talk) 03:59, 11 May 2012 (UTC)

Nutrients for human brain tissue culture
What nutrients are required or beneficial for cultures of human brain tissue in vitro? Neon Merlin  05:10, 10 May 2012 (UTC)


 * Is this some sort of medical school homework question? I should think that if you are in a position to legally obtain human brain tissue, you would have the qualifications or team backup to already know the answer to this question.  Ref Desk policy is not to give homework/assignment help unless you provide evidence you've worked on it and got stuck.  Wickwack60.230.217.112 (talk) 05:53, 10 May 2012 (UTC)


 * Whole point of Wikipedia is, people have a right to know things. The model that knowledge is something to be doled out only to Authorized Personnel according to occupation, assignment, need to know and security clearance ... not our model.  And this is not written like "homework", just legitimate curiosity. Wnt (talk) 12:55, 10 May 2012 (UTC)
 * Yes, people do have a right to know. Not only that, spreading the knowlege around enables creativity and benefits mankind. In no way was I seeking to restrict knowledge to those authorisied.  Rather, logic tells me that this is not likely to be a genuine need (as a person in actually thinking of doing this would already have the answer, in order to satisfy authorisation requirements), therefore it may be a homework or assignment question.  The OP's talk page shows he IS a student.  Pure curiosity is perfectly fine, and we love to help with that.  But the OP should still have made his own research effort before expecting others to do so.  If the OP confirms it is not homework, and/or he indicates he has given it some thought, he is likley to get good answers.  He hasn't done so yet.  Wickwack124.178.180.170 (talk) 00:31, 11 May 2012 (UTC)
 * I don't follow your distinctions and have little interest in your restrictions. It isn't obviously a homework question to me; that's enough (indeed, if that rule is going to get questioners this kind of reaction we'd be better off without it). Wnt (talk) 12:40, 11 May 2012 (UTC)
 * Maybe so. But I didn't suggest or impose any restrictions on answering.  I merely pointed out that if the OP provided some evidence that he's made some effort himself, he may get more and better quality answers.  But you've answered, you probably enjoyed doing so, and that's fine.  Wickwack124.178.33.132 (talk) 01:20, 12 May 2012 (UTC)


 * It apparently depends a lot on what kind of brain tissue you are growing. Someguy1221 (talk) 06:14, 10 May 2012 (UTC)


 * Well, fish is alleged to be "brain food". That could be worth a try. ←Baseball Bugs What's up, Doc? carrots→ 12:35, 10 May 2012 (UTC)


 * To clarify, are you asking solely about the actual nutrients like glucose, or also about regulatory signals meant to mimic the body environment such as nerve growth factor or the ubiquitous (but extremely uncontrolled) fetal bovine serum Wnt (talk) 12:57, 10 May 2012 (UTC)


 * Anyway, to answer the question, see for some general nerve growth protocols;  is a rather disturbing description of short-term maintenance;  (not readily accessible) describes culture of neurospheres;  (not readily accessible) describes organotypic culture of slices.  But this one is perhaps closest to what you're looking for.  There are many more I could cite - it depends on your desired cell type, length/extent of replication wanted, form of culture, etc. Wnt (talk) 12:40, 11 May 2012 (UTC)

Two diagonal rings, one touching the other at just one point and spinning
Is there a name to that? Is that possible, maybe with magnetic rings? I saw that in a science fiction film, but want to know if there's something like that. It would be a kind of toy like the Newton's cradle. — Preceding unsigned comment added by 95.20.183.251 (talk) 13:09, 10 May 2012 (UTC)
 * If you mean this  from  Superman II, it was simply two metal rings welded together and attached to a turntable.  The centre part of the turntable was stationary, so the actors could stand and have the rings appear to spin around them.--TrogWoolley (talk) 14:02, 10 May 2012 (UTC)


 * Yes, I meant that, but what if you put the two rings into a transparent tube? Would a construction with two independent rings be possible? OsmanRF34 (talk) 17:33, 10 May 2012 (UTC)


 * Intriguing. There's no doubt that a ring or coin can, for a short time, spin around in a narrow circle, perhaps even in place?, supported by only one spot on the edge at any given time.  And if a second ring/coin could be positioned precisely, so that it spun exactly the right way, then the top one would never know it was on anything but a flat surface.  Doing that, of course, makes the usual acrobat and a tower of chairs seem like child's play, and we start to think of cheating with glass tubes or magnets or superconductors or gyroscopes.  But what if we cheated with some kind of computerized mechanism, that applied only small corrections?  That would seem closer to achieving the feat virtuously. Wnt (talk) 17:58, 10 May 2012 (UTC)

Master of Public Health
What are the different jobs that a Master of Public Health prepares one for? I don't have a previous medical background. Thanks. Rodney Boyd (talk) 21:39, 10 May 2012 (UTC)


 * This page provides some suggestions. --Mr.98 (talk) 22:03, 10 May 2012 (UTC)
 * Thanks. I saw that page. I was hoping for something more detailed. For example, what jobs would be included under International Health? Rodney Boyd (talk) 00:27, 11 May 2012 (UTC)


 * Working at NGOs, mainly. Some government agencies and think tanks, too. See International health. --Mr.98 (talk) 01:12, 11 May 2012 (UTC)

Lava tornado
What would happen if a tornado passed over a lava field would it create a lava tornado,  cause it to lose strength and die out  would  it strengthen. — Preceding unsigned comment added by 86.41.95.141 (talk) 22:33, 10 May 2012 (UTC)
 * Yes. ←Baseball Bugs What's up, Doc? carrots→ 00:11, 11 May 2012 (UTC)
 * Lava is very sticky and dense, the strongest tornado wouldn't be able to lift much if any of it. Lava is just liquid rock, after all, and tornadoes don't pick up boulders; they pick up things that catch the wind well. Occasionally you will find in the most violent tornadoes that some of the ground, or even pavement, has been stripped, but that is more from debris scouring than anything else. The stuff you see lofted high by tornadoes are lighter objects like dust, paper, insulation, etc., or heavier debris that has a large cross-sectional area to catch the wind, like large trucks and roofs of houses.
 * As to whether it would intensify, it's tough to say. Tornadoes run mostly off low-level instability, caused by warmer, lighter air below cooler, denser air, so in theory a bunch of added heat at the surface would intensify the tornado. But tornadoes also need moisture, so increasing temperatures without adding any water vapor would actually lower the humidity, which is not good. Then there is the consideration of what a lava field might do to the overall structure of the parent supercell; I think more than likely, it would disrupt the storm.
 * However, brief tornado-like circulations can be caused by fires (as seen in this video). Indeed, the only fatal "tornado" ever recorded in California was from an enormous fire caused when lightning struck an oil storage facility. These firewhirls have different formative mechanisms than classical tornadoes, however, and die quickly if they move away from the source of heat. - Running On Brains (talk) 01:00, 11 May 2012 (UTC)
 * That would be a rather unusual situation! Lava could not be picked up at the base of the twister unless the twister was very strong and/or the lava was splashing, if, however, it was picked up, it would not stay as lava long; it would soon turn into lava bombs and eventually rocks, so no lava tornado, just a tornado carrying some lava. As for the last part, I don't think it would effect the tornado much. Lava would mean heat (obviously), meaning rising air, which is what gets storms started in the first place. So it might strengthen the storm as a hole, and indirectly the tornado, but by a very small factor. --  T H F S W  (T · C · E) 01:05, 11 May 2012 (UTC)


 * It would quickly die if lifting something so heavy. Even waterspouts, which lift water, quickly lose their strength. StuRat (talk) 04:51, 11 May 2012 (UTC)
 * There's no time limit on waterspouts, they can last as long as sufficient instability exists. It's not that they're lifting too much water, it's that evolution of the storm creates cool downdrafts which cut off the warm, unstable inflow. And as I stated above, I don't think a tornado would lift the lava at all, but it could feed off the heat produced. - Running On Brains (talk) 05:03, 11 May 2012 (UTC)