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October 2

Difference in methanol and ethanol surface tension

Surface-tension_values

How come methanol surface tension is higher than ethanol's? What causes it?Since if we are taking H bond,London dispersion and dipole moment it is all inclined towards ethanol, I mean ethanol's surface tension would be higher. — Preceding unsigned comment added by 115.132.144.146 (talk) 05:39, 2 October 2014 (UTC)[reply]

An important thing to remember here is that these relationships are complex and the heuristics we create to explain them are not always going to neatly match actual experimental results. In the case of methanol and ethanol, two competing factors are at work here. Ethanol, being a larger molecule, has more polarizability, and thus is more susceptible to London dispersion forces. Methanol, since it has a smaller hydrocarbon portion than ethanol, has a more basic oxygen atom, which makes Methanol more susceptible to hydrogen bonding than ethanol; this can be explained by the Inductive effect, such that the extra carbons on ethanol effectively withdraw electron density away from the oxygen atom, making it less basic and thus less strongly affected by hydrogen bonding. This, and several other effects, result in a rather inconsistent relationship between ethanol and methanol with regards to their properties. As you note, methanol has a higher surface tension, which would indicate that the hydrogen bonding has more of an effect here, while methanol has a lower boiling point, which would indicate that the dispersion forces were of more importance to that property. But this is all but speculation; remember most importantly that these descriptions of intermolecular forces and their effects are merely a heuristic tool to explain general trends in molecular behavior. Actual experimental results may vary due to the complex way all of these properties interplay with one another. --Jayron32 06:54, 2 October 2014 (UTC)[reply]

Stellar gravitational collapse

In the last weeks of the gravitational collapse of a star, what style best describes the situation? Do they collapse isotropically, or is it a case where multiple low-pressure zones form, which rapidly grow and join together? Basically, do they shrink like a deflating weather balloon, or do they become dimpled like some bizarre golf ball? Plasmic Physics (talk) 11:45, 2 October 2014 (UTC)[reply]

Depends on type and size. Some dont collapse, some just slowly shrink and become more and more inactive. Others actually dont collaps but inflate and form a nebula. Also i doubt this can be measured in weeks because such changes more often take millions of years. May want to read Stellar evolution. --Kharon (talk) 06:29, 3 October 2014 (UTC)[reply]

Though, I specified a collapsing event, so other types of stellar death is irrelevant. I know that gravitational collapse takes a long time from beginning to end, but I'm specifying the final weeks of such a collapse. Plasmic Physics (talk) 07:04, 3 October 2014 (UTC)[reply]

The gravitational collapse during the end of the life of a massive star is isotropic, except that the final transition to a neutron star or black hole can be so abrupt that the shockwave may propagate asymmetrically. An asymmetric explosion is especially likely if the star is somewhat asymmetric to begin with due to interactions with a close companion. Remember that any pressure differences in a star will balance out at roughly the speed of sound in the material, so only process that happen more rapidly than they can effectively propagate could lead to asymmetries. Dragons flight (talk) 17:35, 3 October 2014 (UTC)[reply]

Perfect answer, thank you. One last question, by how much does a star collapse, starting from the last giant phase, before rebounding? Plasmic Physics (talk) 02:48, 4 October 2014 (UTC)[reply]

~~I'm going to posit a different view:~~ that the collapse giving rise to a supernova can be highly anisotropic. I'm sorry I don't have any links, but I remember reading an article on the subject (it may have been in Scientific American) in which modelling showed that extreme instabilities in the process led to highly anisotropic behaviour, including the remaining neutron star sometimes acquiring a high velocity relative to the original star trajectory. —Quondum 20:49, 4 October 2014 (UTC)[reply]

Like popcorn? Plasmic Physics (talk) 20:53, 4 October 2014 (UTC)[reply]

Yes, nice description. Basically, Dragons flight's description applies. Some papers seem to describe it as "turbulent", and indicate that the resulting neutron star may be accelerated to velocities of up to several hundred km/s in 1 to 2 seconds. See [1] and [2]. This type of anisotropy may be typical, since they speak of "explaining the distribution of pulsar velocities". —Quondum 21:20, 4 October 2014 (UTC)[reply]

Variation and productivity

Whether in work or study, are humans generally more productive when their activities are varied? For example, are they more productive when they switch between activities or work every hour or so than if they do the same activity or work on the same job all day? 194.66.246.20 (talk) 19:46, 2 October 2014 (UTC)[reply]

I think it would completely depend on what people and what activities. The question is too broad. Vespine (talk) 00:20, 3 October 2014 (UTC)[reply]

(OR) I think it's more productive when the activities are varied. Think of a typical school - you don't sit and do English 101 the whole day and then Algebra 101 the next day. 196.214.78.114 (talk) 13:07, 3 October 2014 (UTC)[reply]

Actually there are schools where they study in larger blocks of time, such as all morning in one class and all afternoon in another. StuRat (talk) 16:51, 3 October 2014 (UTC)[reply]

As stated before, the answer will depend on what the "one thing" is. If they can do one thing which is challenging, like say brain surgery, then sticking with that one thing is a good choice, so they increase their expertise in it. But, if the "one thing" is completely unchallenging, like emptying trash barrels, then variation will keep them from getting bored, "tuning out", and making mistakes. There's also repetitive motion injury to worry about. So alternating with washing windows, sweeping, vacuuming, etc., would be a good choice there. StuRat (talk) 16:51, 3 October 2014 (UTC)[reply]

I'm seeing this as a classic case for school class scheduling. Traditionally, our classrooms followed the eight or seven period class day. However, in the last few decades, we've seen many schools switch to block scheduling which allows students to have more time for focusing on their specific tasks each day. In this case, according to the Center for Public Education, many schools have seen students performance increase. However, their long-term retention of class material suffers, since classes in a block-scheduled school system last for only part of the year, instead of the full length of a school year. In this case, more time on a topic can had positive effects on productivity. This is a hot topic in Psychology and still up in the air. Given that, psychologists are well aware of directed attention fatigue, a term referring to long periods of focus that stress the brain's capacity to work on a single task. In addition, task-switching - or what we call multitasking - is believed to also have a measurable cost on one's executive processing powers and ultimately your productivity, and contributes to directed attention fatigue. According to some studies, the amount of productivity you have when you are varying tasks requires a few factors included, such as, 1) Whether the varying tasks are a routine switch, or each new task is completely random, and 2) How long you have to focus on one task before switching to another. Bgoodwin0922 (talk) 22:01, 3 October 2014 (UTC)[reply]

The efficiency of an assembly line to produce automobiles, for example, as compared to producing craftsmen built autos, would suggest that some humans, at least, are most efficient when they are carrying out simple, repetitive tasks. Richerman (talk) 23:19, 3 October 2014 (UTC)[reply]

mirrored room

If a person is in a room with a lamp and every wall and the ceiling and floor are covered by mirrors, what does he or she see?144.35.254.5 (talk) 23:42, 2 October 2014 (UTC)[reply]

The future! But seriously. You see a recursive set of repeating images in whatever direction you look; the specific set of recursive images you see will depend on the relative positions of the mirrors and yourself. You can see here some examples of the sort of set-up you mention. --Jayron32 23:44, 2 October 2014 (UTC)[reply]

The smaller images will also get increasingly dimmer the "further" away from the source they become, and the light won't build up over time because it will be converted to heat by anything that is not perfectly reflective. μηδείς (talk) 00:29, 3 October 2014 (UTC)[reply]

(edit conflict)If the mirrors were perfectly reflecting, and the same amount of energy leaves the room as what enters it from the lamp, then you should see a Mandelbrot type reflection. However, since real mirrors aren't perfect, the quality of the successive iterations within each reflective surface should gradually deteriorate, as the information loss is compounded. Eventually, the iterations will fade and be indistinguishable from the mirror material itself. For instance, if the mirror material is glass, then the iterations will eventually fade to a blue-green colour.

Interestingly, Einstein says that space-time is curved in such a way that the universe is actually shaped into such a setup. Hypothetically, if you had a pole that was lengthening at the correct rate, faster than light, you could in theory poke yourself in the back. He used a flashlight in his thought experiment, but a pole will do equally well for the sake of argument. Plasmic Physics (talk) 00:43, 3 October 2014 (UTC)[reply]

Some historical cosmological models (including Einstein's static universe) wrapped around like that, but current cosmological models don't. -- BenRG (talk) 20:55, 3 October 2014 (UTC)[reply]

What's a Mandelbrot reflection? —Tamfang (talk) 08:54, 4 October 2014 (UTC)[reply]

I'm referring to the self-similarity property, which Mandelbrot sets are known for. Plasmic Physics (talk) 20:58, 4 October 2014 (UTC)[reply]

One practical issue is that the mirrors are never truly, perfectly parallel - so the line of images of yourself actually goes curves away in one direction or another. So even with really high quality mirrors with high reflectivity, you never get to see more than a few hundred copies of yourself in each direction. SteveBaker (talk) 05:18, 3 October 2014 (UTC)[reply]

When all of the walls (and floor and ceiling) are mirrored, they don't need to be parallel. -- BenRG (talk) 20:55, 3 October 2014 (UTC)[reply]

I have been in such a room, though it would more accurately be described as a pod, as it could only accommodate one person at a time. It was part of an art installation. It was lit with a thin strip-light around the top. Looking up or down was more interesting than straight ahead, as you don't normally see infinite regressions of yourself at those angles.--Shantavira|^{feed me} 08:26, 3 October 2014 (UTC)[reply]

You can experience something like this at some children's science exhibits that I've visited that have a setup with hexagonal 'pod' with six mirrored "walls" suspended a couple of feet off the ground so a kid can get in by crawling underneath. There is no ceiling, so light enters through the top of the exhibit. But lacking floor or ceiling mirrors, certainly reduces the effect. SteveBaker (talk) 13:30, 3 October 2014 (UTC)[reply]

All you need is two mirrors, say a fixed mirror and a hand mirror, to see the effect yourself. It's quite interesting. StuRat (talk) 16:43, 3 October 2014 (UTC)[reply]

Well, the three dimensional case, rather than simple back-and-forth, ought to be much more interesting, since in theory all the walls can reflect from... anywhere. As alluded to in great literature, there must be some strange crystallographic science of how the reflections could possibly be arranged. Maybe someone on the Math desk could explain it to us... provided there's someone around to explain what he said. :)

One thing you can easily deduce is that with 3 mirrors meeting at right angles at a corner, a lightbeam will be reflected in exactly the opposite direction as where it came from. This set up of mirrors has been put on the Moon for the laser ranging experiment. Count Iblis (talk) 21:06, 4 October 2014 (UTC)[reply]