Wikipedia:Reference desk/Archives/Science/2013 August 12

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August 12[edit]

Engine brakes restricted.[edit]

I see such signs on American freeways. Why? My guess is because it is hard on the road? But I want to get a confirmation and perhaps an extended explanation why it is so.

Also why restricted? Why not prohibited? Is restricted the same as prohibited? I don't think so?

Thanks, - Alex174.52.14.15 (talk) 03:09, 12 August 2013 (UTC)

I think it's because of the noise it makes which disturbs the residents. "Restricted" probably means "for God's sake, if you're going to crash otherwise, then go ahead and use them". StuRat (talk) 03:21, 12 August 2013 (UTC)
What are "Engine breaks"?? Or is "Engine braking" what is meant? Rojomoke (talk) 05:39, 12 August 2013 (UTC)
See engine braking for details on how to use the engine to slow down a vehicle. The signs most likely refer to compression release engine brakeing, which is common on heavy vehicles, efficient and very, very noisy... WegianWarrior (talk) 06:19, 12 August 2013 (UTC)
The signs often seen are about brake retarders. And yes, if you see signs, it must be about the noise. For some reason American government has a fixed idea that anything having to do with noise, even on vehicles that travel interstate, has to be handled at the township ordinance level ... but on the other hand, the moment someone makes an electric car that is quiet, the federal government leaps in to regulate them to be louder lest a blind person not hear it. Wnt (talk) 06:42, 12 August 2013 (UTC)
The fed gets into the act because of laws connected with various physical handicaps. Being able to hear well is not considered a handicap. The Congress has the power to regulate interstate commerce, and they might be able to pass such restrictions if there was a lobbying / advocacy group. Which there probably isn't. The hearing-impaired would have lobbyists. The nearing-unimpaired probably don't. ←Baseball Bugs What's up, Doc? carrots→ 13:37, 12 August 2013 (UTC)
I recall there was a lot of concern raised about the noise level of the Concorde, but the article doesn't say what, if anything, was actually done about it, nor whether the federals got involved or not. It reads kind of like they did, but it's not stated explicitly. ←Baseball Bugs What's up, Doc? carrots→ 13:45, 12 August 2013 (UTC)
I believe that the Concorde flight paths into New York were the subject of lengthy negotiations, and it was never allowed to fly across the continent. These negotiations took so long that, although it was clearly designed for the London - NY route, the inaugural flight was to Bahrein. Alansplodge (talk) 15:21, 12 August 2013 (UTC)
"Scheduled flights began on 21 January 1976 on the London–Bahrain and Paris–Rio (via Dakar) routes... When the US ban on JFK Concorde operations was lifted in February 1977, New York banned Concorde locally. The ban came to an end on 17 October 1977 when the Supreme Court of the United States declined to overturn a lower court’s ruling rejecting efforts by the Port Authority and a grass-roots campaign... Scheduled service from Paris and London to New York’s John F. Kennedy Airport began on 22 November 1977." (See Concorde) Alansplodge (talk) 15:26, 12 August 2013 (UTC)

I've been under the impression that the engine braking involves the electrical generator. When the diesel fuel is cut off the truck moves forward under the gained momentum. The rotation of the wheels is eventually transferred to the generator which generates electricity that is stored in the rechargeable battery. The work invested in the process slows the truck down. The article quoted above does not mention this at all. — Preceding unsigned comment added by (talk) 04:50, 13 August 2013 (UTC)

That's regenerative braking, aka KERS. With "normal" cars, it is a parasitic effect that cannot be controlled and is too weak to brake the vehicle significantly. Engine braking makes the engine use kinetic energy to compress air, thus the noise when the air escapes. - ¡Ouch! (hurt me / more pain) 08:18, 13 August 2013 (UTC)
In detail: Engine braking means putting the vehicle into a lower gear and taking your foot off of the gas. When that happens, the cylinders in the engine don't get much fuel - so the down-strokes aren't producing much energy - but the up-strokes still have to compress the fuel-air mixture, and that consumes energy. When the engine is running slowly, the energy from the burning fuel produces more energy than is needed to compress the gasses. But when the engine is forced to run at high RPM (because we're in a very low gear for the speed) and there is too little gasoline present - the energy to compress the gasses by far exceeds the gain from burning that fuel. This energy comes from the kinetic energy of the vehicle - which must therefore slow down. This process is noisy because there is suddenly a hell of a lot more air being pushed out of the exhaust than normal. SteveBaker (talk) 16:31, 13 August 2013 (UTC)

I want to thank everyone. It is very interesting. - Alex174.52.14.15 (talk) 23:38, 14 August 2013 (UTC)

IUPAC additive nomenclature[edit]

How do I choose the central atoms in ambiguous cases?

For the sake of argument, an example: H
- how would I decide between μ-fluorido-dihydrogen(1+), dihydridofluorine(1+), or dihydrogenfluorine(2 HF)(1+)? Plasmic Physics (talk) 03:29, 12 August 2013 (UTC)

Is this a technically correct name for [Be]=B[Be]B1[Be]B([Be]B=[Be])[Be]1: di-μ-beryllido-1κB-bis(μ-beryllido-beryllido-2κB-diboron)? Plasmic Physics (talk) 12:55, 12 August 2013 (UTC)

Or is it di-μ-beryllido-1:1ʹκ4Be-bis(μ-beryllido-beryllido-2κBe-diboron)? Plasmic Physics (talk) 03:06, 16 August 2013 (UTC)

Creep Rate Tensor[edit]

I am modeling a pressurized cylinder at high temperature, for which i have a creep rate equation (which involves hoop stress) to calculate hoop strain and it states that stresses in other directions are not important. Now i have to prepare a creep rate tensor (3x3 matrix). Can somebody tell me which components will be entered in the matrix and which will be zero?Brahmarishiraj (talk) 04:23, 12 August 2013 (UTC)

Found this by Googling. Mentions creep rate equations and tensors. You may understand. -- (talk) 16:02, 14 August 2013 (UTC)

>>Thank you for the effort, but i had that paper, could not get much from that.Brahmarishiraj (talk) 07:09, 15 August 2013 (UTC)

Lymph node removal and blood pressure measurement[edit]

In cases of breast cancer, depending on the surgeon's judgement, some or all of the armpit lymph nodes will be removed. This will reduce the effectiveness of fluid drainage of the arm on the affected side. Women are advised to not allow blood pressure to be checked on the affected arm. But if the woman some time later develops breast cancer in the other breast, she may lose the lymph nodes for the other arm as well. How does this affect the measurement of blood pressure, given that it is done by gradually inflating a cuff on an arm while listening for the aparent start and stop of the pulse? Is the high reading increased due to back pressure or something? Or do you get a false low reading from cutting off the perceived pulse early? (talk) 05:45, 12 August 2013 (UTC)

How do I compute the charging time for Li-ion battery?[edit]

I read that the simple formula is Battery_capacity x Battery_type_coefficient / Charger_current_supply. But I couldn't find what coefficient I should use for Li-ion. Zarnivop (talk) 09:16, 12 August 2013 (UTC)

For safe charging that preserves battery life, a Lithium-ion battery requires a non-linear charging cycle. Many have some control circuitry built into the battery, but you need to study charging graphs before attempting to charge this type of battery on a home-made charger. Dbfirs 11:03, 12 August 2013 (UTC)
But the charger is not home made... Zarnivop (talk) 14:38, 12 August 2013 (UTC)
If it's designed for that particular battery, then it will stop charging (and most likely indicate this) when it has finished the charging cycle. The time will vary depending on how deeply discharged the battery was, and what charging rate it deems appropriate. The charging sequence usually measures the rising voltage of the battery to determine when to stop charging. Dbfirs 20:38, 12 August 2013 (UTC)
It does. I wanted to know beforehand, and deduced erroneously that the formula for Ni-Cd batteries will work for Li-ion. The fact that the charger states "Output: 4.2V, 350mA±50mA" supported my error. My bad. Zarnivop (talk) 11:04, 13 August 2013 (UTC)

plasma of blood and plasma of television[edit]

what is the connection between the plasma of blood to plasma of television? (talk) 18:43, 12 August 2013 (UTC)

None whatsoever, except that the two words are based on the same Greek word. See [[1]]. Dominus Vobisdu (talk) 18:55, 12 August 2013 (UTC)
The root is also found in words such as protoplasm and ectoplasm. AndrewWTaylor (talk) 18:58, 12 August 2013 (UTC)
Not so fast. In some people's worlds, televisions drip blood. -- Jack of Oz [pleasantries] 20:53, 12 August 2013 (UTC)
I don't know much about blood plasma, but the other plasma is any matter where the bulk phase has a certain set of properties. The underlying property, which gives rise to others, is the existence of charge separation or presence of a net charge. The most basic type of plasma would be a substance where some of the electrons are not bound to atoms, but move freely within the bulk phase. To become a plasma, a substance needs to undergo ionisation. Plasma is often called a state of matter, but unlike melting, and boiling, ionization does not happen at a precise temperature, it is a gradual process. As temperature increases, a larger fraction of the bulk ionised. This process is also reverable, though a process called recombination, where electrons are literally recombined with ionized atoms. Usually, ionisation and recombination occurs in an equilibrium. There exists a large variety of plasmas: solid plasmas, liquid plasmas, gaseous plasmas and more; of which gaseous plasmas feature the most often. Metal is considered an obscure example of plasma, since its electrons are free to move throughout the bulk, although the atoms are more or less bound within the crystal lattice, but don't be fooled, the atoms also slowly diffuse throughout the lattice. Plasmic Physics (talk) 01:33, 13 August 2013 (UTC)

A weird 100 F happen around Cape Horn[edit]

How can this every happen. I thought 100 F is a b-s joke at the tip of South America. But when I look at 42 F is the dew point, 14% humidity, a high temperature is possible. Is this possible a sudden spike jump in temperature in just one hour. The weather report said 7:00 PM is 53 F but 100 F is 8:00 PM but 9:00 PM is 51. Do 30-40 degree jump in temperature ever happens? Is this possible to start as 75 F at the midnight. At Southern hemisphere January is their summertime, at the high latitude can peak temperature happen at midnight hours, could it be due to constant daylight, and only 4 or 5 hours to release heat make peak temperatures occur at night hours. At low latitudes we never have peak temperature at night or midnight or sudden 20-30 F jump in just one hour.-- (talk) 20:27, 12 August 2013 (UTC)

While a temperature like that could in principle happen, a cursory reading of the data strongly suggests that the report of 100F is in error. First, as you've noted, the temperature on either side of that one reading on the day in question is far lower. Second, the all-time high temperature for any of the days nearby is around 62F. Between those two pieces of data, it's reasonable to assume that the 100F reading is not reliable. — Lomn 20:44, 12 August 2013 (UTC)
Could the inaccurate bizarre reading be due to the thermometer is malfunctioning? Maybe the instrument may not be working properly at that time. Is this possible sometimes there is something technological wrong with thermometer somebody just caught it and fix it.-- (talk) 03:33, 13 August 2013 (UTC)
One of the downsides to a "Web 2.0" data aggregator like Weather Underground Weather Underground is that the data is rarely checked by human; and its source is totally opaque. A comment on the website implies that the website is aggregating METAR data. Now, I only trust one source for my METARs: the National Weather Service (or any of their other authorized reporting services) - and since station data is already available for free online, and the website will even translate it into plain English for you... I can't possibly understand why anyone should ever choose to visit dubious second-hand sources like and Weather Underground - websites chocked full of pretty (useless) graphics and animations and loads of extra cruft and advertisement. - provided by the United States National Weather Service - has excellent worldwide data - even for Southern Argentina. (Where do you think gets their free data!?) NOAA also archives weather and climate data on their website.
As a last note: a METAR ending in $ indicates malfunctioning equipment. If you don't see a $ then there was no diagnosed equipment failure. This is documented in the METAR format specification in Advisory Circular AC-0045G Aviation Weather services, available for free online at NWS, NOAA, and FAA's websites, none of which work well with a hot-link to the PDF. Personally, I keep a paper copy of Aviation Weather Services in the small bookshelf next to my bed at all times, for quick reference. Nimur (talk) 05:05, 13 August 2013 (UTC)