Wikipedia:Reference desk/Archives/Science/2008 April 17

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April 17

Suns disappearance and resulting gravitational wave

Hi, Suppose the sun did suddenly disappear. As you know, this is a common thought experiment designed to ask: how long would the earth feel the gravitational tug of the sun after the sun's disappearance?

Let us say it take 8 minutes (or so) for the earth to lose the gravitational tug. What I'm interested in knowing, though, is would we human beings feel the sudden change in acceleration of the earth? That is, since the earth would change from moving in a circular orbit to traveling in a straight line, would that change in direction be felt by us earthlings and a force on our bodies? Or would be feel essentially nothing (except for the fact the light would go out)?

If humans did feel a force, that force would depend on where on the earth you happen to be standing, right? If your on the surface facing the sun, the earth moves out from under you, and you'd seem to feel a lose of attraction towards the earth. If your standing on the surface facing away from the sun, you'd feel a sudden increase in acceleration as the earth moves towards you.

--InverseSubstance (talk) 01:43, 17 April 2008 (UTC)

Numerous rehashes of this can be found by searching the ref desk archives. In short, those on Earth would experience no gravitational difference outside of laboratory instrumentation. Consider that the major source of tides is due to the Moon (the Sun's influence is about 20% of the effect). Do you feel lighter when the moon is overhead? Heavier when it's below the horizon? No. The sun's influence is even less. All this is due to gravitational force diminishing with the square of distance while increasing only linearly with mass. — Lomn

The question isn't directly about gravity, it's about jerk (the derivative of acceleration). Someone would have to work the math to see how much jerk the sudden removal of the Sun's gravity would produce, but humans are very sensitive to jerk (consider elevators) so my gut feel is that we probably could feel it. I also suspect, though, that the sudden onset of darkness would probably cause us to not notice the simultaneous jerk.

Atlant (talk) 13:50, 17 April 2008 (UTC)

An individual human is unlikely to detect the instantaneous difference, because it is so small. However, the instantaneous difference is likely to cause stresses within the earth, and this is likely to cause earthquakes. The difference in stress on the earth as a whole is many orders of magnitude larger than the direct effect on an individual human. -Arch dude (talk) 01:20, 18 April 2008 (UTC)

The jerk would be practically infinite, since the force would disappear instantly, but that still doesn't mean it would be enough to feel. Let's find out. Gravity varies as m/r² where m is the mass of the attracting body and r is the distance to its center. The Sun's center is about 93,000,000/3,960 times as far away as the Earth's center (more or less, depending on the time of year); square that number and you get about 550,000,000. The Sun's mass is a third of a million times the Earth's mass. So the force of gravity on your body due to the Sun is 1/(550/(1/3)) = less than 1/1,500 of your own weight. If you were calmly sitting waiting for it to happen, you might barely feel the effect, but otherwise, not at all. --Anonymous, 18:00 UTC, April 17, 2008.

Approximating the Earth's orbit as circular with a radius of 1 AU and a period of 3.2×10⁷ s (1 year), the centripital acceleration is approximately 5.8×10⁻³ m/s^2. This is approximately $0.0006g$. Somehow I doubt it's going to matter much to the human body how quickly that magnitude of an acceleration is applied. According to this page, an earthquake that is 3.5 on the Richter scale ("detected only by very sensitive people") corresponds to an acceleration of about 1 cm/s^2, or $0.001g$. --Prestidigitator (talk) 18:31, 17 April 2008 (UTC)

Even if the gross acceleration were much larger than that you couldn't feel its disappearance because gravitational acceleration isn't detected by accelerometers (including the accelerometer that is your body). Even in principle you could only detect the disappearance of the tidal force, which is about 10⁻¹⁴ g per meter of height if the sun is directly overhead or directly underfoot, less otherwise. -- BenRG (talk) 20:48, 17 April 2008 (UTC)

Hmm, good point. Because the change of acceleration would apply equally to all parts of your body, there'd be nothing for the sensors in your sensory system to sense. Thanks for the correction. --Anonymous, 18:00 UTC, April 18, 2008.

If we assume that somehow the same amount of light and heat reaches the Earth (so the only thing we have to worry about is the loss of gravity), then the first thing that would be particularly noticable would probably be a few weeks later, when the stars don't seem to be following their normal pattern (the astrophysicists would probably notice it much quicker, but after a couple of weeks I think even amateur astronomers would be getting a little suspicious). Then a few months later, when the seasons aren't changing like they should we'd be getting pretty worried, and not long after that we'd find ourselves a whole lot less worried about global warming. Confusing Manifestation(Say hi!) 23:28, 17 April 2008 (UTC)

venturi flowmeters

The venturi flowmeter shown below is used to measure the flow rate of water in a solar collector system. The flowmeter is inserted in a pipe with diameter 1.9 cm; at the venturi of the flowmeter the diameter is reduced to 0.51 cm. The manometer tube contains oil with density 0.88 times that of water. If the difference in oil levels on the two sides of the manometer tube is 1.4 cm, what is the volume flow rate?
________cm³/s

The equations out of my book don't exactly help, given the fact that both of the two equations for initial velocity and ΔP use the other variable, thereby making solving for just one impossible.

What I have so far is this- ΔP=(1/2)*ρ*v_1²*(b²-1)

where b is the ratio of the larger to smaller area

and

v₁=sqrt((2*ΔP)/(ρ*(b²-1)) —Preceding unsigned comment added by 152.7.59.86 (talk) 04:06, 17 April 2008 (UTC)

It's not damaging that the two equations each involve both variables: see simultaneous equations. Unfortunately, your "two" equations are actually the same equation written twice; solving the first for $v_{1}$ yields the second, and solving that for $\Delta P$ yields the first. What you are asked for is a volume flow rate, which has a relationship with velocity that should be obvious. You then need velocity, which means you need $\Delta P$; how can you get that? What does a manometer measure, anyway? --Tardis (talk) 16:28, 17 April 2008 (UTC)

Limits of current-limiting diodes

Current-limiting diodes are two-terminal devices that are internally just FETs with the source connected to the gate. They don't allow any more current through them than IDSS of the FET. It seems from CLD datasheets that they are only a solution for currents below about 15 mA. See [1], for instance.

But if more than one CLD can be placed in parallel on a PCB for higher currents, why can't they manufacture multiple CLDs in parallel on the chip? Wouldn't that be exactly the same as paralleling a bunch of transistors on the same chip to make a power transistor? — Omegatron 18:47, 21 March 2008 (UTC)

I suspect it's a question of demand and power dissipation:

• Demand: There probably aren't enough people who a) want a particular, higher value of CLD and 2) would save enough money (as compared to a discrete-device implementation) to pay for the limited run of "their" value of CLD.

• Power dissipation: Remember that CLDs are linear devices, dissipating VI power. For many higher-current constant current applications, that becomes an unacceptable waste of power. As a result, you see specific LED driver ICs that use the same techniques as switching power supplies to "synthesize" a robustly-stable, very-low-loss constant-current source.

Frankly, I almost never see circuits that use CLDs other than the very rare one you find packaged within an LED. Most constant-current circuits I see are discrete implementations or just the shorted-FET approach. (Constant-current circuits are very common within the integrated-circuit implementations of differential amplifiers, of course.)

Atlant (talk) 13:39, 17 April 2008 (UTC)

The gentleman in your ref [2] Omegatron, has stated the reason thus:

Conventionally, by adding the CLDs in parallel, the limiting current may be doubled, tripled, and multiplied. However, it would be cumbersome and impractical by just adding the CLDs to attain high current levels. For example, it is not uncommon to require a current limit protection at 1 amp or higher at the conventional PC card or system module level. It would not make any practical sense to use 66 CLDs :in parallel to attain 1 amp or 100 CLDs to attain 1.5 Amp. Therefore, the need for a practical solution to :achieve high current limit is evident

I also think its a Q of Power diss. Its easier to use a power transistor off chip than to redisgine the chip to get rid of watts of power.

I read that as "it would not be practical to parallel 100 devices on a PCB", and therefore wondered why they couldn't be paralleled in miniature on the chip itself. — Omegatron (talk) 00:09, 18 April 2008 (UTC)

Well certainly it is not practical to parallel 100 devices on a PCB due to the board space it would take. But paralleling a large number of devices on a chip is also not practical because of the difficulty of heat sinking a chip for such large powers. It is (IMO) much better to use the technique described in your ref. —Preceding unsigned comment added by 79.76.181.213 (talk) 01:14, 18 April 2008 (UTC)

histone protein gene

organization and arrengement of histone protein genes?Drsudhirjain (talk) 16:22, 17 April 2008 (UTC)drsudhirjain

Taking a look at histone, you can see that it is a heterooctomer composed of two copies each of H2A, H2B, H3 and H4 for four genes (five if you count the H1 linker histone). The genes are detailed on the pages for those proteins. – ClockworkSoul 02:02, 18 April 2008 (UTC)

A clarification about propanone and iodine - Organic Chemistry

I was recently reading up on the kinetics between propanone and iodine - could anyone further shed light on how the finer details of SN1 manifests itself in this fashion because I got bogged down somewhere in between. Thanks. AlmostCrimes (talk) 16:44, 17 April 2008 (UTC)

What's the overall reaction (starting materials and products)? What are the mechanistic steps involved? Tell us exactly what you got so far...otherwise we're likely to be helping you with a part you already understand or a part that you haven't even gotten up to yet. DMacks (talk) 19:15, 17 April 2008 (UTC)

I'm not sure what reaction you are talking about. But this picture shows a reaction between iodide and an alkane. It is on the SN1 reaction page. Also I don't think that any reaction between propanone (acetone) and iodide would be an addition reaction on the carbonyl carbon, or an SN2 reaction if there is an approriate leaving group attachted to the carbonyl carbon.--Shniken1 (talk) 04:19, 18 April 2008 (UTC)

• 
  SN1 Reaction

A thought occurs, is this question actually about S_N1 use of iodide with acetone as solvent, perhaps as a qualitative-analysis test for tertiary chloride? DMacks (talk) 14:41, 18 April 2008 (UTC)

Electric Potential

Hi. can anyone please tell me the real meaning of potential difference? i mean i know the defination but i want to know what do we mean by saying that "we have applied a potential difference across a rod?" how is this done practically? thanks... —Preceding unsigned comment added by 61.2.19.8 (talk) 19:10, 17 April 2008 (UTC)

Moving from one end of the rod to the other requires going against (or with) an electric field.Em3ryguy (talk) 21:11, 17 April 2008 (UTC)

Practically speaking, you could attach a battery or other voltage source to the ends of the rod. Edison (talk) 23:32, 17 April 2008 (UTC)

When creating a theoretical circuit, an electrical engineer can use a "pure voltage source" or a "pure current source." The theoretical construct thta best matches your question is a "pure voltage source." This would be e.g. a perfect battery that always provides a precise voltage difference between its positive terminal and its negative terminal. In your example: ifhge abstract rod is non-conducting, you can connect the ends ofthe rod to an abstract battery that produces exactly the required voltage difference. In the real world, lets take a piezoelectric rod and a 1.5V AA-cell battery. The rod has a theoretically infinite resistance, but the true resistance is finite (measured im MegOhms). The real battery always generates a 1.5V potential, but the real battery has an internal resistance and a finite energy. the two resistances will eventually drain the battery. -Arch dude (talk) 01:08, 18 April 2008 (UTC)

Huh? Edison (talk) 05:11, 18 April 2008 (UTC)

Exactly! Arch dude, your post is not completely clear in its meaning. Would you care to rephrase? —Preceding unsigned comment added by 79.76.181.213 (talk • contribs)

If the rod is high enough in resistance that it tests with a megohm meter high enough to be called an insulator, then the battery might just sit there until local action causes it to fail from shelf life expiration five years from now, same as if it were in a device which was turned off, with the battery case acting as a just as much an insulator as the hypothetical rod. Edison (talk) 02:33, 19 April 2008 (UTC)

Here's a good site explaining potential difference/voltage [3]

Combining sleeping bags

I have observer that two light sleeping bags (comfort temp. 10 C) are cheaper, more compact and lighter than an extreme sleeping bag (comfort temp. -10 C). Can I just use two of them to get a comfort temp. of -10 C or something? How can I calculate how much protection two sleeping bags combined will offer me? Mr.K. (talk) 20:08, 17 April 2008 (UTC)

Warning: the following is a naive theoretical answer; I have no idea what the answer is in the real world. That said: the rate of heat loss through an insulator is a function of the temperature difference between the sides, so with two identical nested bags the temperature between the bags will be halfway between the inside and outside temperatures. The inside temperature is circa 37 C. For optimum comfort the temperature between the bags should be 10 C, which implies an outside temperature of 10 C − (37 C − 10 C) = −17 C. -- BenRG (talk) 21:05, 17 April 2008 (UTC)

Not too bad. What you are really interested in is the thermal resistance of the insulators. To the inhabitant, the environment on the inside should look the same if the same rate of heat transfer is occuring. Call the comfort rate of heat transfer P, the temperature difference at the heavier sleeping bag's comfort temperature $\Delta T_{1}$, the temperature difference at the lighter sleeping bag's comfort temperature $\Delta T_{2}$, the thermal resistance of the heavier bag $R_{1}$, and the thermal resistance of the lighter bag $R_{2}$. Then we can write the equivalent of an electrical current equation (which is really just the rearrangement of the equations on the thermal conductivity page):

$P={\frac {\Delta T_{1}}{R_{1}}}={\frac {\Delta T_{2}}{R_{2}}}$

And supposing the inside temperature is 37°C as used above:

${\frac {R_{1}}{R_{2}}}={\frac {\Delta T_{1}}{\Delta T_{2}}}={\frac {37^{\circ }C-(-10^{\circ }C)}{37^{\circ }C-10^{\circ }C}}=1.74$

Since resistances in series simply add, you'd need to layer 1.74 of the lighter sleeping bags to get the equivalent of the heavy one. It's going to get hot! :-) --Prestidigitator (talk) 00:07, 18 April 2008 (UTC)

I should add that this assumes the inside of the bag is body temperature (same assumption as the answer above). This may not actually match the definition being used for comfort level, which may be aimed at keeping the inside of the sleeping bag at standard room temperature or something, or might include some kind of thermal resistance value for the human body. It might be interesting to research more thoroughly the meaning of that metric. --Prestidigitator (talk) 00:26, 18 April 2008 (UTC)

The analyses so far have only considered temperature. If I recall correctly, humidity (or at least conveyance of water vapor) is a major consideration with sleeping bags. If this were not the case you could simply wrap yourself in two sheets of aluminumized Mylar. The relative humidity next to the skin is approximately 100%. The most comfortable temperature next to the skin is not 37C, but a somewhat lower temperature (at least in air.) We are not actually interested in temperature. We are interested in the amount of heat that is conducted away from the body. To a first approximation, a quiescent human generates about 125W. Dissipate more than this, and the body feels cold. dissipate more than this, and the body feels warm. -Arch dude (talk) 00:52, 18 April 2008 (UTC)

Nobody has yet mentioned the single most important factor in sleeping bag design, it has to let your farts out. :-) StuRat (talk) 04:03, 18 April 2008 (UTC)

There is also the performance of the bag when it is squashed flat by you lying on it. The fluffy down or fibres with air between are a good insulator, but when you squeeze out air it is not so fantastic. Graeme Bartlett (talk) 11:52, 18 April 2008 (UTC)

Are things 'turned off' really impervious to EMPs?

Hi. Well, that's my question. (...) Okay, I'll elaborate. I'm of course talking about electronics that are 'off' during an Electromagnetic pulse. Electronics that are 'on' during an EMP get fried and whatnot, but it's said that if they were off during the 'attack', nothing would happen to them! Is that really true? Kreachure (talk) 20:49, 17 April 2008 (UTC)

From what I can tell, according to some discussion at Talk:Electromagnetic pulse one of the biggest problems with an EMP is the voltage and current surge which arises and travels through the electric grid. If your device is off at the power switch, especially if it is disconnected from the power socket, then it should be pretty imprevious to surges in the electric grid. In any case, switching a device off by it's own powerswitch will probably often not be enough due to the way the power switch works (for example, if the device is in standby I would expect it still liable to be fried). And whatever the case, the device may still be affected in itself if it's close enough. Nil Einne (talk) 22:00, 17 April 2008 (UTC)

It's still conductive even if it's off, so unless it's shielded, *zap*. Ilikefood (talk) 22:23, 17 April 2008 (UTC)

An AM radio with a built-in ferrite rod antenna or an FM radio with a telescoping rod antenna or the power cord extended as an antenna or a portable FM with the headphones acting as an antenna or a TV connected to rabbit ears, rooftop antenna or cable would receive the same jolt to its first RF stage from an EMP whether it was on or off when the event happened. I would expect that boxed up most electronic devices would be impacted slightly less because of the lack of extended wires to pick up the jolt and because any voltage surges on the power grid would not enter them. A disconnected electronic device stored in a metal cabinet with no external connections should be affected far less than if it were plugged in, whether operating or not. Edison (talk) 23:31, 17 April 2008 (UTC)

The RF stage would see a voltage spike, but wouldn't this voltage have to exceed the breakdown voltage of the device to caue a problem? In contrast, an active semiconductor would amplify the voltage spike and increase the chance of breakdown in the second transistor. Another consideration - if the electronic equipment is not grounded (ie. unplugged), why would it care what voltage it was at? Franamax (talk) 03:25, 18 April 2008 (UTC)

The supply voltage and other factors should limit the actual voltage level of a spike reaching the stage following amplification. "Breakdown voltage" can have more than one meaning. I would expect that the potential difference across semiconductor devices and the currents and voltages seen by through delicate rf transformers would be points of vulnerability. Edison (talk) 23:58, 18 April 2008 (UTC)

Thatcher (Comet)

Hi. When I looked at this comet's orbital diagrams from neo.jpl.nasa.gov, I found something weird. I'd seen this before, but had always wondered why it was like this. On the orbital diagram for this comet, which is the source of the Lyrids meteor shower, (you only need to enter "Thatcher" and the comet's diagram pops up), on December 24, 2017, it shows the comet plunging into the sun, at a distance of 0.0 AU. This only works if you use the intervals by "hours" as well. However, its current orbit does not bring the comet anywhere near the sun. In fact, the orbital diagram suggests it has long left its orbit. However, if you look at the ephemeris, it mentions nothing about the comet speeding near the sun at this time, not predicting any substantial increase in apparent and absolute brightness. What's going on? Is this an anomaly or an error produced by the orbital diagrams, or the nature of the diagrams themselves that produce this error, or is there some possibility that the orbital diagrams are right (and the ephemeris is not)? What could ever send a comet plunging directly into the sun? In fact, I haven't seen the orbital diagrams produce 0.0 AU for either the Earth or the Sun for any other minor object. Can someone explain some possible explainations? Thanks. ~AH1^(TCU) 21:33, 17 April 2008 (UTC)

That's a weird one. The comet is not shown on the orbital track, it seems to be shown on the semi-major axis (which would indeed pass through the sun). I'd be thinking software glitch. In contrast, Halley's comet follows its orbit like a good comet should. As far as how to get a comet to hit the sun, it would need to pass near enough to Jupiter that the planet gravity would perturb the orbit just the wrong way, or some similar set of influences from other sources of gravity than the Sun. Franamax (talk) 01:45, 18 April 2008 (UTC)

Thatcher also passed through the sun in November 1704, apparently without long-term ill effects. In fact if you look at the whole history of its trajectory from the beginning of time (January 1, 1600) to the end (2200), it spends most of it in decidedly non-Keplerian motion, and never even completes a single cycle of its supposed orbit. -- BenRG (talk) 01:57, 18 April 2008 (UTC)

What is this bird?

Hi. Now before someone screams at another one of my bird questions, this isn't based on song. I took three pictures, and uploaded them because I wasn't sure which one was the best. It's a red bird, and it somewhat resembles a chicken, and sings the song sort of like the one that I described as "Whree? Whree? woo-woo-woo-woo-woo-woo". However, the "woo" sounds more like a short "dřoū", as I am not good at describing birdcall and do not know how these accents work. Anyway, the images are below. Could it be a Northern Cardinal, which I considered as one of the possibilities that the song may be from? It seems to also have a hint of black or at least dark red or rufous, but I'm not sure. Thanks. ~AH1^(TCU) 22:05, 17 April 2008 (UTC)

Hi. WHERE IN THE WORLD WERE YOU WHEN YOU TOOK THESE PICTURES? (Get it? Screaming for a different reason.) General region of the planet is about the most important bit of data after general description for the indentification of any living thing, and most non-living things. Are you the one in southern Ontario? Anyway, we're in luck, because nothing else looks like the Northern Cardinal. --Milkbreath (talk) 22:30, 17 April 2008 (UTC)

I have a backyard full of Northern Cardinals. They sound like car alarms. They call: "who-it chee who-it chee who-it chee woop woop woop woop wee wee wee wee boo-dee boo-dee boo-dee" - repeat ad naseum. To make it worse, I have mocking birds in the front yard who repeat it back to them. -- kainaw™ 00:26, 18 April 2008 (UTC)

Dear AH1: since you seem to be interested in bird identification by sound, and since you seem to be digitally competent at least to the extent of using a digital camera, have you considered using a digital audio recorder of some sort? they are cheaper than digital cameras, and a sound file is a lot smaller than a picture. -Arch dude (talk) 00:34, 18 April 2008 (UTC)

Guide to Animal Sounds on the Net-http://members.tripod.com/Thryomanes/AnimalSounds.html Em3ryguy (talk) 07:36, 19 April 2008 (UTC)

Strange cloud

fuzzy, box-shaped cloud, surrounded by normal puffy clouds

Can anyone explain this cloud? Possibly classify it? It was more interesting looking before I got the photo (it had dissipated slightly by then), but it's still odd looking. I know it's not the best photo either; I got a few others, but they all kind of suck. -- Consumed Crustacean (talk) 23:49, 17 April 2008 (UTC)

Could the box shape be caused by rainfall? --Elliskev 00:42, 18 April 2008 (UTC)

Argh, yeah, looks that way, may just be momentary dumbness affecting me. The only thing is that I can't see an originating cloud, and this block of rain stayed up there for a decent amount of time. I don't know if the cloud is just obscured, or if it needed a cloud to appear like that, or if the cloud just fell in its entirety. -- Consumed Crustacean (talk) 00:53, 18 April 2008 (UTC)

What was the temperature at the time? It looks to me maybe like fog rising from a swamp or cooling pond. Franamax (talk) 01:27, 18 April 2008 (UTC)

It was warmish, I think around 12C? I don't think it was rising fog; it was big, and I don't know of any ponds in that direction. It hasn't rained in several days, so everything is decently dry. -- Consumed Crustacean (talk) 01:32, 18 April 2008 (UTC)

Could it be dust?--Shniken1 (talk) 04:07, 18 April 2008 (UTC)

Combine dry conditions and Shniken's suggestion, havat Dust storm though only the biggest examples appear, there are links. Julia Rossi (talk) 07:56, 18 April 2008 (UTC)

Looks like it could be smoke from some sort of field fire, it seems to be trailing to the left as it gets to a certain altitude. Richard Avery (talk) 14:11, 18 April 2008 (UTC)

It does look like smoke from a field burn, the boxiness is due to a stable atmosphere, a relatively warm layer of air above the fire, similar to an atmospheric inversion. As the heated gasses near the fire rise, they will expand and cool (see lapse rate). In a stable atmosphere, the rising smoke will eventually hit a layer of relatively warm air, and the column will flatten out as shown in the picture. You can see a small bulge over the hottest part of the fire, where the smoke is densest. Here the air was heated enough to "punch through" the layer.—eric 14:46, 18 April 2008 (UTC)

I don't know. to me it looks like a rain cloud. There seem to be definite lines that follow what I'd expect to see with sun-rays. The cloud is a little low on the horizon. The haze could be refraction. I wonder if it was a still, high-pressure kind of day. --Elliskev 23:11, 18 April 2008 (UTC)