Wikipedia:Reference desk/Archives/Science/2012 March 20

= March 20 =

Valerian attracts or repels rats?
Our article Valerian claims that valerian attracts rats, and gives a reference. On the other hand, Eno Raud, Mohaszakállék újabb kalandjai (original title Naksitrallid) shows that the scent of fresh Valerian roots repels rats. Now of course this is a fiction novel so the part about Valerian repelling rats could be just a convenient plot device so that the heroes can keep rats away, but this still seems a strange contradiction. So which one is true? Does Valerian attract or repel rats? Can you find more references? Or maybe I misremember and the novel mentions a different herb used to repel rats? &#x2013; b_jonas 07:49, 20 March 2012 (UTC)


 * Asking for clarification: you've got cats in the title and rats in the question. Which is it? I can confirm that valerian attracts some cats, as you can see in cat toys like this one. --TammyMoet (talk) 08:17, 20 March 2012 (UTC)
 * It also attracts rats, according to the article. Some say it can be planted at the edge of your property to keep them away from the house. 84.197.178.75 (talk) 10:12, 20 March 2012 (UTC)
 * Sorry, I mean rats. I've fixed the question title.  &#x2013; b_jonas 12:06, 20 March 2012 (UTC)


 * There is no idea too old or too obvious to patent. The bright side of this is that you can use patent searches to learn about how things were done in ancient times.  I present . Wnt (talk) 12:51, 20 March 2012 (UTC)


 * Why use Valerian. For anyone that has kept rats as pets will know, that they are very cultured creatures. All that a trap needs is a loop tape playing a little of Handel's Water Music to capture their interest, followed by some of Claude Debussy's suite Bergamasque (the |Clair de lune bit). That will relax them so much that one can then 'humanly' pick them up by their tail and drop the into the garbage disposal unit.--Aspro (talk) 22:21, 20 March 2012 (UTC)
 * Or, if you're into visuals, the blender or the microwave. ←Baseball Bugs What's up, Doc? carrots→ 23:49, 20 March 2012 (UTC)


 * What turn from green to red at the touch of a button?--Aspro (talk) 01:40, 21 March 2012 (UTC)


 * Aspro: if valerian attracted both cats and rats, it would be an all in one solution: the arriving cats would eat the gathered rats. There's be no need for an adhesive, nor for dumping the rats to the garbage can.  You'd only need to change the valerian frequently.  (Or is it the other way?  Maybe valerian isn't effective as a rat bait at all, no matter how sophisticated traps you use with it, because the cats scare the rats away?)  &#x2013; b_jonas 09:28, 21 March 2012 (UTC)

TNO: major problem with "orbital characteristics"-data

 * 90377 Sedna German Wikipedia
 * (90377) Sedna Spanish Wikipedia
 * (90377) Sedna Italian Wikipedia
 * (90377) Sedna French Wikipedia

09:49, 20 March 2012 (UTC): Hello! Having a look at the values of the major orbital characteristics of Sedna given by various language-specific Wikipedias reveals a rather less than awe- and confidence-inspiring chaos of different values. I realize that these stem in part from osculating orbits, different epochs, etc., etc. Correctly interpreting the JPL-Horizon's output isn't easy, either (a guideline for that might be useful, by the way). Anyway, the current state of things does not inspire trust in Wikipedia's reliability. Is there no way to consistently choose a set of values people can rely on and reasonably reflect reality (no one will use Wikipedia-data to calculate actual orbits)? The current orbit of a TNOs does have a specific orbital period after all, a specific aphelion, a specific perihelion, etc. (even if the location of the last aphelion was not the same one as the next one will be). What do you think? The sample data for Sedna (harvested on March 22, 2012), illustrating the problem:


 * This is the English Wikipedia and we have no control over, or even collaboration with, other language projects. We can only answer for what is in our own article.  The five articles you have looked at all have very different quality status.  The en article is FA status (the highest) and will have been thoroughly reviewed and can be expected to be well sourced.  The next highest is the de article with GA status.  The it and fr articles are only B Class and the sp article is unclassified and appears to be entirely unsourced as far as I can tell.


 * The orbital elements in our article are sourced to the JPL Small Body Database Browser which I would have thought is a reliable source. However, looking on that page it seems to have been recently updated (14 March) and now has data that disagrees with all the articles; for instance e=0.8590486217687489.  Spinning  Spark  12:52, 20 March 2012 (UTC)


 * I think what's going on here is that a single source keeps updating the data as it gets more results, and it gets updated in the Wikipedias at random times, either directly or (for smaller Wikipedias) perhaps by copying from this article. The German infobox repeatedly changed, for example, especially around 2006. Wnt (talk) 12:59, 20 March 2012 (UTC)


 * The first report in 2004, based on data from 2001 (precovery) to 2003, gave error margins for their calculations. For example the perihelion was most likely to be within about 7 AU of the 76 AU perihelion estimate. When I see an orbit fit (www.boulder.swri.edu/~buie/kbo/astrom/90377.html) giving 76.13229114 as value (based on 1990 to 2009 data), and all error terms equal to zero, I'm wondering about standards in astronomy...
 * (BTW: Minute of arc gives wrong value for second of arc in lede, third sentence, missing a factor 60) 84.197.178.75 (talk) 14:03, 20 March 2012 (UTC)

Audio oscillators
What is the main cuase of harmonic distortion in audio oscillators? I have a theory that it is the ampliude stabilising system that cuases this and that it is predominantly second harmonic due to the full wave rectifiaction of the the signal in the control circuit. Would i be correct, and can anyone shed any light on design of ultra low distortion oscillators (say -100 dBc)? Thanks --92.25.96.193 (talk) 14:50, 20 March 2012 (UTC)
 * It's a bit useless to discuss the root-cause of distortion in a circuit without specifying the circuit. There are thousands of designs for audio amplifiers, audio-frequency oscillators, low-noise amplifiers, and power amplifiers.  There are thousands of components that you can use to build them.  Each design, and each implementation thereof, will have its own limitations; so if you want to start talking about 100dB of dynamic range, you need to more precisely describe the circuit you are using.  Have you considered starting with a basic text on amplifier design?  Do you need help locating such a text?  Nimur (talk) 18:21, 20 March 2012 (UTC)
 * I think he means sinus wave oscillators. These have to have gain at small signals > 1 but ned to be amplitude limited. When this happens by clipping, there's much distortion. Clever way to solve this was the Wien bridge oscillator that uses an incandescent lamp as PTC thermistor. 84.197.178.75 (talk) 18:50, 20 March 2012 (UTC)
 * This might help: http://www.moorepage.net/RC.html looks at the circuits used in some commercial designs. 84.197.178.75 (talk) 19:02, 20 March 2012 (UTC)


 * Oscillators such as Wein bridge that utilise a temperature sensitive resistance (thermistor or tungsten filament) to vary feedback and control amplitude are indeed subject to 2nd harmonic distortion. The temperature sensitive resistance tends to be at a slightly different temperature during the + half cycle compared to the - half, leading to 2nd harmonic distortion, worsening at low frequencies.
 * Oscillators using a semiconductor variable resistance (eg FET) to control amplitude also have predominately 2nd harmonic distortion as the control FET or transistor also inherently has a resistance proportional to the oscillation voltage across it. In theory this can be cancelled out by injecting a fraction of the oscillation voltage onto the gate in addition to the feedback control voltage.  However it is virtually impossible to get the cancellation exactly right.
 * The amplifier used to make the oscillator distorts of course like any amplifier. As an amplifier in an oscillator will normally be class A, the distortion will again be mainly 2nd harmonic.
 * Oscillators based on the tranditional Wien configuration have an additional distortion mode. Both the + and - inputs to the amplifier together swing at signal frequency a significant voltage above and below earth.  So amplifier common mode distortion adds to the problem.  The differential input stage of such amplifiers adds odd-order harmonic distortion.
 * Oscillators based on state-variable circuits use only inverting input amplifier stages, thus eliminating common mode distortion, and eliminating the impossible-to-cancel odd-order distortion of differential inputs. It amuses me to see just about every text on audio oscillators devote much sapce to wein bridge oscillators, which should have been consigned to the rubish bin decades ago, for this reason.
 * If a fixed frequency not too low is required, an LC tuned circuit approach can be used. Since the output can be taken frm the LC tank, and not from the amplifer output, any amplifier distortion (which should already be at least 40 dB down, is attentuated by the LC tank, and such attenuation exceeding 60 dB or more is quite easy.  Things like LCR bridges were often built with LC oscillators, as a very simple one-transistor circuit built cheap as chips could reliably give lower distortion than carefully built wein oscillators requiring 4 or more transistors.
 * With state variable oscillators, outputs are available in-phase and out-of-phase. These can be summed with an opamp.  The fundamental will cancel, leaving the even order distortion.  This can be injected back in to cancel the even order distortion at the user output.  Some extremely low distortion oscillators have been made with this technique.
 * By using an integrator circuit, an output 90 degrees out of phase is available. The zero crossings of this can be used to synchnonously gate the peaks of the main output into a sampel & hold circuit, giving a DC voltage directly proportional to output. If the frequency determining circuit has a high Q, it can be driven by a shaped square wave, derived from the DC voltage from the sample and hold. In this way the 2nd harmonic ditortion from temperature dependedent amplitude control is completely eliminated.  However the action of the sample and hold means that energy at sub-harmonics may be present.  Never the less, circuits of oscillators with distortion 120 dB down have been built with this technique.  The magazine Wireless World / Electronics World has covered all these techniques over recent years.
 * Keit124.182.163.120 (talk) 03:23, 21 March 2012 (UTC)

Why does biodiesel have less energy density than regular diesel?
So according to this, http://en.wikipedia.org/wiki/Energy_content_of_biofuel

Biodiesel has really good energy density, but it's still less than regular diesel. Why is this? Isn't petroleum derived from ancient biomass like algae that has been compressed over millions of years? Is the compression a factor? If we compress biodiesel or the oil it's derived from, can we increase the energy density of biodiesel and other derived biofuels? ScienceApe (talk) 16:23, 20 March 2012 (UTC)
 * Crude-oil components "come from" ancient biomass, but it's a lot different than just various chemicals leached out of the dead stuff...there is a whole slew of chemistry that occurs to transform it into crude oil (see Petroleum). Diesel fuel notes that the chemicals in the petroleum-based diesel fuel are simple alkanes and related hydrocarbons, whereas biodiesel is esters (contains oxygen) and often has alkenes in addition to alkanes on the chains. DMacks (talk) 16:43, 20 March 2012 (UTC)
 * In order to have high energy density, what kind of molecular structures should it be most abundant in? ScienceApe (talk) 18:05, 20 March 2012 (UTC)
 * There are several ways to measureenergy density, either per unit of mass, or per unit of volume (see tables); if you want it more specific, see heat of combustion, the values in MJ/L. You see for example that cyclopentane has a considerably higher value than pentane, which is still higher than isopentane. But looking at MJ/kg, cyclopentane has the lowest energy density. All these figures don't automatically give you the best fuel since efficiency of a motor depends upon the temperature, and some fuels burn hotter than others; the reaction products are CO2 and H20 mostly, with water vapor containing considerable amounts of energy that you don't use. 84.197.178.75 (talk) 19:41, 20 March 2012 (UTC)
 * That's not really what I'm asking, I'm asking what chemical structures make a substance have higher energy density (energy per unit of volume). DMacks said that petroleum diesel fuel has alkanes while biodiesel has esters and alkenes. If a substance is abundant in alkanes, does its energy density increase? ScienceApe (talk) 00:21, 21 March 2012 (UTC)
 * From the different heating values I'd say that for a given number of carbon atoms, energy density drops with the number of hydrogens. Aromatics seem to have higher energy density and alkynes > alkenes > alkanes. Oxygen containing molecules (esters, alcohols) will have lower energy. highly strained molules will have higher energy, but does that also influence the density of the liquid? I don't think there are simple rules to calculate energy density per volume of molecules. 84.197.178.75 (talk) 13:35, 21 March 2012 (UTC)

Has the Sun met blinding stars in its travels?
It is amazing to read in Sirius that in 60,000 years it will brighten noticeably, and in a mere 250,000 years, it will no longer be the brightest star in the sky. These are numbers smaller than the lifespan even of ours, one of the most recent of species. This makes me wonder: has a star such as Rigel, not 25 but rather 96,000 times brighter than the Sun, ever passed by our star during its rounds of the Milky Way? Bear in mind that Sirius has magnitude -1.46 at 8.6 ly, the full Moon -12.74; five orders of magnitude = a 100-fold difference. So Rigel, 3840 times brighter than Sirius, would appear as bright as the full Moon at a distance of 3.28 light-years - but all concentrated into a single point of light. That is (I think) quite unlikely, but it illustrates that life on this planet might have had to deal with unusual conditions - conditions where for whole seasons of the year night would not have been absolute.

How much is now known of the Sun's travels, of its recent neighbors among the stars? How far back can the interactions of local stars be modeled with any confidence?

Oh, and Solar neighborhood is a redlink. Anyone want to get it started? Wnt (talk) 18:19, 20 March 2012 (UTC)


 * Stellar neighborhood redirects to List of nearest stars. Would you be happy with just another redirect? Jason Quinn (talk) 18:51, 20 March 2012 (UTC)
 * Hmmm, that list article looks like it is overflowing its bounds - notably List of nearest stars is just the sort of thing I was curious about (though I wish there were a video...). It is incomplete, though, as evidenced that checking its entry for Gliese 710 I arrived at Algol, not on that list, which according to its article was a -2.5 magnitude star 7 million years ago.  (I doubt it was an eclipsing binary back then, but if it were it might have really mindfreaked our ancestors. ;) )
 * Judging by the point that Gliese and Algol are now over 50 light-years away, I can see that predicting the close calls past 10 million years may be quite difficult. Still, it might be possible to do on a purely statistical basis...
 * To give an example of what I mean by "statistical basis", the list gives 5 stars expected to pass within 4 light-years in the next 50,000 years. According to stellar classification, 0.13% of all main sequence stars are class B - 25-30,000 times brighter than the Sun.  Taking 50,000/5*0.0013 gives one class B star passing within 4 light years every 7 million years ... well, shoot, that's just balderdash!  Some nitpicky little thing about independent observations must apply.  I think it's time to let the king's men have another crack at this egg... Wnt (talk) 19:21, 20 March 2012 (UTC)
 * I think it's probably within an order of magnitude. What makes you think it's balderdash? 70.59.28.93 (talk) 20:03, 21 March 2012 (UTC)
 * While it doesn't directly address the question, the Local Bubble is of interest, as is Geminga, which may or may not be responsible for the bubble: both articles give some insight into relatively recent history in this part of the galaxy.  Acroterion   (talk)   03:13, 21 March 2012 (UTC)
 * Well, technically, all about half of the metals heavier than iron here on Earth came from one or more supernova(e) close enough to deposit nearly Earth-sized chunks of metal in the solar system. But that was of course before the formation of Earth, so it's probably not what you mean. [Edit: the other trans-ferrous metals came from red giants or planetary nebula.] 70.59.28.93 (talk) 00:50, 22 March 2012 (UTC)