Wikipedia:Reference desk/Archives/Science/2011 October 19

= October 19 =

Does a shipping crate made of plywood pass international ISPM 15 standards?
Hi All,

I have to build a sheathed shipping crate so I can ship some furniture to my sister in Germany (from the US). I want to build the crate out of plywood. I just want to make sure that plywood is OK to use, because there is a the ISPM 15 requirement for lumber used in crates. That page doesn't specifically say that plywood meets the ISPM 15 certification, though I suspect plywood does. What do you think? No other lumber will be used in the crate. I'll make the skids and reinforcing cleats out of plywood too. Thanks. — Preceding unsigned comment added by 98.225.77.201 (talk) 00:38, 19 October 2011 (UTC)
 * This is getting on for legal advice (in that you're looking for something to fulfil legal requirements), so please note the disclaimer that Wikipedia does not offer legal advice . You may be interested in this page, which itself states that it is offered "as a guide only". It states (their caps) "THE EASIEST WAY TO AVOID POSSIBLE PROBLEMS RELATED TO THE ISPM 15...IS TO...PACK YOUR CARGO WITH CARDBOARD, PLYWOOD, VENEER ETC ONLY". Your best bet really is to talk directly to whoever is actually carrying the crate to see what their requirements are, however. WP cannot be held responsible for any advice given on the Reference Desk. Tonywalton Talk 00:52, 19 October 2011 (UTC)

Future ice age
Will another ice age happen in the future? What will be its effects on humanity? --Jigsaqqq (talk) 01:03, 19 October 2011 (UTC)


 * According to Wikipedia: "Glaciologically, ice age implies the presence of extensive ice sheets in the northern and southern hemispheres; by this definition we are still in the ice age that began at the start of the Pleistocene (because the Greenland and Antarctic ice sheets still exist)." Bus stop (talk) 01:23, 19 October 2011 (UTC)


 * The extent of ice ages (actually, glacial periods, since we're technically in an ice age) is typically determined by Milankovic cycles, attenuated by atmospheric gases and solar/volcanic activity. However, one major factor in ice ages, the Earth's orbital eccentricity, is approaching 0 and may postpone an immediate ice age. The Earth's current spate of global warming will likely also affect the onset of the next ice age glacial. ~ AH1 (discuss!) 01:52, 19 October 2011 (UTC)
 * When I was a child in the 1960s and 70s, there were two "We're all gonna die!" predictions put about by otherwise reputable scientists; that the earth's population would exceed any possible means of feeding ourselves by the 1990s, and that the next ice age would be here in the next few decades. I also remember a 1970s newspaper headline; "World's Oil Reserves will be Exhausted by 1984". Alansplodge (talk) 08:44, 19 October 2011 (UTC)
 * One prediction has come to pass: peak oil. [[File:Hubbert_world_2004.png]] Imagine Reason (talk) 12:58, 21 October 2011 (UTC)

how much do sound waves obey the superposition principle?
Models of sound waves as pressure waves in my physics class seem to assume the ideal gas law. But if a gas is a real gas, and furthermore, subject to turbulent flow, how much does this change the assumption that two sound waves in phase with each other will interfere constructively and perfectly ? elle vécut heureuse à jamais  (be free) 01:51, 19 October 2011 (UTC)


 * The superposition principle is also valid for fluids or solids. What will make this break down is the fact that the equations for fluid flow are non-linear. So, the break down can happen e.g. if you have extremely strong sound waves or in the case of turbulence that you mentioned. Count Iblis (talk) 02:04, 19 October 2011 (UTC)


 * Say I installed waterproof speakers in a swimming pool, and calibrated it so I could hear all my favourite songs in the water at roughly the same pitches. How would turbulent flow (discounting the noise of splashing) affect things? Sometimes I notice soft sounds blocking louder ones. In a room with many reflective surfaces, could the sound generate its own turbulence? elle vécut heureuse  à jamais  (be free) 02:25, 19 October 2011 (UTC)

Unless the sound is extremely strong, it won't cause such effects. However, you do have fluid motion inside the water, and that does affect the way sound propagates. 'm not an expert in this, so I wrote down the fundamental equations and tried to derive the equation for sound propagation. We have the Navier-Stokes equation, which is just Newton's Second Law:

$$\rho\frac{\partial \vec{v}}{\partial t} + \rho\vec{v}\cdot\nabla\vec{v}=-\nabla P$$

Where we ignore the viscosity terms.

We have the continuity equation, which is just conservation of mass:

$$\frac{\partial\rho}{\partial t} +\nabla\cdot\left(\rho\vec{v}\right)=0$$

And then we need the equation of state. We can assume to first approximation that while the fluid flow is incompressible, the sound waves are not, but they are isentropic. If we take the isentropic compressibility to be some constant then then a generally valid relation allows you to write:

$$\frac{d\rho}{dP}= \frac{\rho\beta}{\gamma}$$

Here $$\beta$$ is the isothermal compressibility of water and $$\gamma = \frac{c_{p}}{c_{v}}$$ is the heat capacity ratio for water.

Then what you do is you assume that there exists some (turbulent) velocity field v0(x,t) that satisfies the above equation, but which is incompressible, so div v0 = 0 and the contininuity equation is satisfied with some constant density rho_0. Then you add to this constant density a small perturbation so that the total density is is rho_0 + rho_1, where rho_1 represents the density change due to the sound wave. This is then related to a perturbation in the pressure via the above equation. And the velocity field is modified by adding to V0(x,t) a small perturbation. If you then work to first order in the perturbation and solve the equations, you find:

$$\frac{\partial^{2}\rho}{\partial t^2} + \vec{v_{0}}\cdot\nabla\left(\frac{\partial\rho}{\partial t}\right) + 2 \frac{\partial\vec{v_{0}}}{\partial t}\cdot\nabla\rho= c^{2}\nabla^{2}\rho$$

where $$c=\sqrt{\frac{\gamma}{\rho\beta}}$$

is the speed of sound in water.

So, you find the familiar wave equation but with the second derivative w.r.t. time replaced by a convective derivative. This means that locally the sound propagates in the frame co-moving with the flow according to the usual wave equation. The sound waves will thus be distorted by any turbulence. But because the equation for the density is stil linear, the superposition principle still holds. Count Iblis (talk) 03:51, 19 October 2011 (UTC)


 * Thanks so much! elle vécut heureuse  à jamais  (be free) 07:31, 19 October 2011 (UTC)


 * I will add: a "splash" is actually a very complicated thing! There's a P-wave (a typical acoustic sound wave) which you will hear, even if you are underwater; but there is also a surface wave (a shear wave, which moves at a different speed); and there is a dynamic instability at the surface that involves separated flow ("droplets" of water - you know, the "splash" part!)  Both the surface wave and the fluid separation will propagate, and will create new P-wave according to Huygen's principle.  The result is messy.  The field of wave coupling is actively researched; it's a great case for numerical wave modeling.  In water, we often simply ignore the shear component, because it's quickly attenuated; but the definition of "quick" depends on your scale - i.e., the distances, time scales, and frequencies you care about.  Nimur (talk) 21:52, 19 October 2011 (UTC)

Dead zone (satellite) and GIS talking bus system
Because on public bus system in Orange County, now public buses in Orange County have automate announciator systems which announces by GIS with women's voice intersections and points onf interests. one of my bus routes goes through places like Ladera Ranch, Rancho Santa Margarita, it makes less than 1/2 of the gross announcements with GIS programming system. Is satellite still able to track remote sensing in these dead zones, or limit number of announcement has more to do with the way the computer system is programmed/set-up? Does mountains and trees or dead zones block satellite signals and GPS Systems from tracking informations. Does satellites signals still work actively even through dead zones with alot of mountains and trees in the way. Does it matter which electric signal, if it is weak or strong. Because the computer by my bus drivers desk is monochromatic green screen. Does monochromatic green screen vs. digital computer screen matter how GPS work?--69.226.34.145 (talk) 02:57, 19 October 2011 (UTC)
 * Trees and mountains will block the GPS signal from satellites, but if there are some satellites visible in the sky left, then the bus can get the signals from those and estimate its position. Dead zones for telephones do not have to match that for GPS.  Tunnels will wipe out GPS all together.  A smart system will also consider the movement of the bus and know how far the bus has moved. GPS is normally close to the limit of not working, so if something weakens the signal it will drop out.  But a high priced system could use multiple antennas to counteract that. Green or coloured digital screen will have nothing to do with how GPS works. Graeme Bartlett (talk) 09:41, 19 October 2011 (UTC)
 * Usable signals from at least three GPS satellites must be received simultaneously by a GPS receiver to calculate a valid position. Roger (talk) 20:10, 22 October 2011 (UTC)

Seismic travel time graphs
How does one read a graph like this? --130.216.46.30 (talk) 05:24, 19 October 2011 (UTC)
 * This is showing that the earthquake motions came as several signals delayed differently in time. The horzontal axis is how many degrees on the great circle the geophone is from the earthquake. The first wave to come is that P wave. If you are at a known angle follow the line upwards to see what waves are expected.  So at 30 degrees you will expect a p, PP PcP(I expect reflected off core boundary) S (shear wave), PcS, SS SSS and then any surface wave, then ScS, ScS2.  The directions of these will differ.  P will move to and back from the earthquake, S will move from side to side, and Surface will move up and down. Graeme Bartlett (talk) 09:31, 19 October 2011 (UTC)
 * The way I read that graph is, "this graph is too complicated; the empirical observations of wave-speed are evidently insufficient to cleanly and succinctly summarize all the possible seismic wave couplings and consequent wave travel times. Avoid using simplified estimates of wave velocity; instead rely on full numerical inversion of a teleseismic wave with a high-resolution Earth model."  Here is a paper that an acquaintance of mine had published in the Journal of Geophysical Research that exemplifies the technique: ...''Multiparameter inversion of teleseismic waves..." (JGR, 2001).  Nimur (talk) 21:30, 19 October 2011 (UTC)
 * But seriously: this is a chart that shows the delay time that different earthquake wave modes will have, in order to travel to other parts of the earth (as measured in "angular separation," visualizing the Earth as a giant 2-D circular slice. The reason I say the chart is "too complicated" and "not very useful" is because for almost every X,Y position on that graph (in other words, for any position and any delay after the earthquake), there is some wave mode present; in other words, there is no useful predictive power.  Any location (0 to 180 degrees around the Earth) may receive energy from an earthquake; and the energy may arrive with any delay ranging from 0 to 60 minutes; and if such energy is measured, we can almost identify the probable wave mode, but with a lot of ambiguity, because so many different waves have similar arrival-times.  Nimur (talk) 21:36, 19 October 2011 (UTC)

constants
many constantsn such as heat capacity, fluid resistancesn etc. are measured empirically, even for pure substances. now my question is that is it possible (in theory or practice) to measure these constants by analyzing the microscopic behavior of matter? like a bottom-up aproach or something? is it already done for all such constants or some? if so, can anyone give me some examples?--Irrational number (talk) 06:23, 19 October 2011 (UTC)
 * I think the Boltzmann constant would meet your requirments; it is a way to connect individual particle properties (energy) with the macroscopic bulk properties of a material (temperature and entropy). You may be more familiar with the Boltzmann constant's other form known as the Gas constant.  Another fundemental constant which exists at the particle level is the Planck constant, which relates the energy of a particle with the associated wavelength of that particle (either light wavelength in terms of photons, or the DeBroglie wavelength in terms of massive particles).  -- Jayron  32  12:51, 19 October 2011 (UTC)


 * I'm not sure either of those are good examples of what the original poster is asking for. Boltzmann's constant is essentially a conversion between energy in Joules and temperature in Kelvin, and so is pretty fundamentally an empirical constant (i.e. dependent on our arbitrary definitions of the units involved).  Similarly, I can't think of any way of getting Planck's constant aside from some sort of measurement.  Dragons flight (talk) 16:16, 19 October 2011 (UTC)


 * Yes, the fields of statistical mechanics and solid state physics do routinely derive estimates of quantities like heat capacity, density, sound speed, etc. It is actually quite easy to make crude estimates (e.g. plus or minus a factor of 3), for many substances given knowledge of a few basic assumptions about the substance.  For example, all gases are roughly ideal gases, and in solid / fluid states the distance between atoms is roughly an angstrom.  There is a very large literature on techniques for improving such estimates as you take into account more details of the atoms' electronic configuration.  Such literature gets very complicated pretty quickly and at the high end tends to use supercomputers to model electronic configurations.  In many cases, you can get within a few percent of the right answer given enough effort and the techniques we now know.  However, trying to reach that precision is labor intensive, and will often take more effort than simply doing the empirical measurements.  As a result, there is often no incentive to do a first principles computation.  However, there are still many people working in chemistry, metallurgy, and biology that use computational models to try and understand the properties of compounds that we haven't yet been able to synthesize in bulk.  Dragons flight (talk) 16:16, 19 October 2011 (UTC)

homology between yeast and human alcohol dehydrogenases
Okay, people in the past have kept emphasising to me in class that yeast and humans share many conserved genes, but when I do PDB alignments, I have difficulty finding the evolutionary trees connecting the homologies of yeast alcohol dehydrogenases with human/mammalian ones. From a PDB search, Yeast ADH1 / ADH2 are only at max 43% similar or identical to human equivalents. This is okay I guess -- but there are a lot of equivalents -- including dehydrogenases that don't react with ethanol/acetaldehyde/acetate (this is to be expected). However I'm drafting a lab report and I need to get a 10,000-mile-perspective homology map in a hurry. Can anyone clue me in? elle vécut heureuse à jamais  (be free) 07:42, 19 October 2011 (UTC)
 * Can you restrict human sequences to those expressed in particular tissue types, in this case liver cells? Humans have more than 90 tissue types with different expressions (or 270 by a different count, or more depending on how much physiological differentiation you want to distinguish) while yeast are single cells. The process of stem cell differentiation may cause different subsets of sequences to be expressed. I realize that doesn't help much, but it's the best I can do. Dualus (talk) 14:03, 19 October 2011 (UTC)
 * Definitely not a time to reinvent the wheel - just search the literature. To start with  ADH1 and ADH2 are believed to have diverged from a single ancestor Adh(A), perhaps in the Cretaceous.  Past that, various motifs have been identified by authors - I have to admire these people  in 1976 who figured out some with good old fashioned peptide mapping - no RNA editing missed there!  Past that there are a lot of interesting-looking papers, e.g., but it all depends on just what exactly you're interested in. Wnt (talk) 14:44, 19 October 2011 (UTC)
 * Big topic. You should consider the difference between a homologous gene and an orthologous gene (see Homology_(biology)).  Many genes have "homology" to similar genes across organisms, but this amino acid sequence conservation might be limited to the active site of the enzyme, for example, whereas the remainder of the coding region might be quite divergent.  These homologous genes might encode proteins with similar generic functions but quite different specific roles or modes of regulation.  On the other hand, the orthologous gene usually refers to a gene that has a high degree of identity and also carries out the same function. --- Medical geneticist (talk) 23:01, 19 October 2011 (UTC)
 * Well, if I read correctly yeast had one gene as of the Cretaceous, so there should be no gene in other species more orthologous to Adh(A) than any other. (Well, at least provided the last common ancestor of yeast and humans or whatever didn't have two copies...) Wnt (talk) 00:15, 20 October 2011 (UTC)

It is between fall and follow
The sentence is, The patient is on fall restriction or fluid restriction diet, or it should be The patient is on follow up restriction or fluid restriction diet, What is the right way of representing the things ? aniketnik 09:23, 19 October 2011 (UTC) — Preceding unsigned comment added by Aniketnik (talk • contribs)
 * The patient's diet should be restricted to prevent fluid retention until injury followup indicates otherwise? Dualus (talk) 13:58, 19 October 2011 (UTC)
 * Completely wrong Dualus. Thank God for the RD not allowing medical advise. This diet is designed to prevent fluid retention in the body. 88.8.75.87 (talk) 14:42, 19 October 2011 (UTC)
 * Thank you. I've put s/restricted to fluids/restricted to prevent fluid retention/ in italics. I tried to indicate my uncertainty with a question mark. Dualus (talk) 19:07, 19 October 2011 (UTC)
 * Perhaps you are thinking of fall precautions? This is a typical order given when the patient might be unstable or susceptible to falling down when they get out of the hospital bed. --- Medical geneticist (talk) 22:52, 19 October 2011 (UTC)

It is a typo. It is either a fluid restriction diet (fluids, no solids) or a FULL restriction diet, i.e., nothing at all. μηδείς (talk) 17:36, 20 October 2011 (UTC)

How long will a 9 volt battery power an LED?
How long would a device like this last, and what is the lump thing on the wire?


 * It looks like a battery with a LED. Do you mean last = running without recharging the battery? Or last = useful life-span? In both cases the answer is it depends, but long. LEDs have a low consumption and high life-span. The lump is a resistor, including in the design tto protect the LED. 88.8.75.87 (talk) 15:20, 19 October 2011 (UTC)


 * The duration isn't even well-defined, because unlike incandescent bulbs, LEDs don't abruptly switch off when the current drops too low, they just gradually get dimmer. The lifetime would have to be measured as a half-life, and even then it would depend entirely on the resistance of the resistor in the circuit.  There are lots of LED headlamps with that basic design that last over a hundred hours before fading appreciably. Looie496 (talk) 18:54, 19 October 2011 (UTC)


 * Incandescent bulbs don't abruptly switch off either, but I agree that their output falls to a frequency below that of visible light.   D b f i r s   19:05, 19 October 2011 (UTC)


 * My electronics knowledge is a bit rusty, but I seem to recall the a diode (including the light emitting variety), does shut off abruptly when the voltage drops below a specific voltage (Vd). Mitch Ames (talk) 13:34, 20 October 2011 (UTC)


 * Are the resistor colors red-blue-black-gold? Or orange-green-brown-silver? Dualus (talk) 19:06, 19 October 2011 (UTC)


 * Unlikely red-blue-black.... Assuming a typical 1.8V LED, it'd have 277+mA (!!!) going through it.  Orange-green-brown would still be 20.5+mA.  To me, it looks like red-purple-something.  If it was red-purple-red, then it'd be 2.67+mA, which is more in the realm of a typical LED, as well as being a common resistor value of 2.7k.  I calculated based on 9V, but it's probably closer to 9.6V.  Also, half-life would also not work well, because current draw from batteries isn't very linear at all.  The first half-life could be like 60 hours, and the second half-life 15 hours.  In otherwords, it depends on way too many factors.  --Wirbelwind ヴィルヴェルヴィント  (talk) 21:28, 19 October 2011 (UTC)
 * See the Duracell datasheet, for example. The top right graph in particular.  --Wirbelwind ヴィルヴェルヴィント  (talk) 21:38, 19 October 2011 (UTC)


 * So it sounds like - given a fresh battery - 60 hours is not unreasonable and 100 hours would be a far reach; am I reading this right? -- Ludwigs 2  13:57, 20 October 2011 (UTC)


 * What's a 'typical 1.8V LED'? 1.8V seems rather low for a white LED (or blue which is basically the same thing), 3V+ is far more likely Nil Einne (talk) 00:35, 21 October 2011 (UTC)


 * You're right. I didn't look up any of the values, and just took them from my head at the time.  But yes, white through-hole LEDs seem to be commonly 3-4V, 20-30mA.  A resistor around 300 ohms would probably be the one in use (and still could be 270 ohms).  If it had been a fresh battery, and a softer colored LED, the 60-100 hour value is a good estimate.  But the non-linear battery draw still applies. --Wirbelwind ヴィルヴェルヴィント  (talk) 16:49, 21 October 2011 (UTC)