Het "kleine wetenschappelijke vragen" topic #4

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Op dinsdag 15 augustus 2006 19:03 schreef Menn0 het volgende:
Wat gebeurt er als je een perfect halfgeleidend spiegelbolletje zou maken waar alleen maar licht in kan komen en er niet meer uit? En je legt deze vol in de zon?
Het gevangen licht kaatst als een gek rond en er komt alsmaar meer en meer bij...
Knalt hij uit elkaar? Smelt hij? Gebeurt er helemaal niks?

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Op dinsdag 15 augustus 2006 19:01 schreef helldeskr het volgende:

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Hmm
Kweenie of ik het daar wel mee eens ben..
De kinderen uit de buurt hier op t pleintje doen t ook..
Maar dan terwijl ze rondjes om t klimrek fietsen of weet ik wat..
Is dat om aan te geven dat ze blij zijn of weet ik wat ?
Het is meestal niet omdat ze iets perse willen want dat is meer schreeuen en dat wordt ook afgestraft... drift buitjes en dergelijke kunnen we natuurlijk niet hebben

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Op dinsdag 15 augustus 2006 19:07 schreef Aitan het volgende:

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Is het mogelijk een voledig spiegelvlak te maken wat geen enkel licht doorlaat aan 1 kant en ander de andere kant wel? Volgensmij zijn de meeste spiegelruiten vanaf de spiegelkan ook lichtdoorlatend

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Op dinsdag 15 augustus 2006 19:04 schreef Aitan het volgende:
Misschien is dat vrij stug maar een schrikreactie is instinctief dus als je snel met je hand naar de baby toebeweegt? (sta niet achter die methode btw)

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Op dinsdag 15 augustus 2006 19:24 schreef Aitan het volgende:
licht is een vorm van energie (zoals alles) dus theoretisch zou er inderdaad een ontlading moeten komen...

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Op woensdag 16 augustus 2006 17:28 schreef Aitan het volgende:
Nee licht heeft geen volume, zodra je een bron van licht wegneemt is het licht ook gelijk weg. Licht is niets meer dan trillingen...

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Op vrijdag 11 augustus 2006 13:38 schreef nein het volgende:

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is dat zo? dus ik kan beter mn ijsklontjesbakje vullen met heet water? wahahaha

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Op donderdag 17 augustus 2006 16:53 schreef Aitan het volgende:
water bestaat uit watermoleculen stroop uit veel meer soorten moleculen met allemaal een andere nulwaarde? denk ik...

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Op donderdag 17 augustus 2006 21:43 schreef Walter81 het volgende:

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Inderdaad. OMdat water een zuivere stof is. Dit heeft een smelt- en kookPUNT, en stroop een smelt- en kookTRAJECT.

Wanneer de temperatuur van het ijs stijgt, en op 0 graden komt, gaat alle energie op in het verwarmen tot water van 0 graden. Dan zal het pas verder opwarmen. En andersom werkt het ook zo dus (bevriezen).

Het schijnt trouwens ook zo te zijn dat wanneer men ijsblokjes maakt van warm water, deze veel meer doorzichtig zijn dan koud water wat bevriest.

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Op zaterdag 19 augustus 2006 12:43 schreef VonHinten het volgende:
Nieuwe vraag

1. Waarom wordt het (soms) wit voor je ogen als je te snel opstaat?
2. En waarom komt dit vaker voor als je een lege maag / honger hebt? Wat is het verband?

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Op donderdag 17 augustus 2006 21:43 schreef Walter81 het volgende:
Het schijnt trouwens ook zo te zijn dat wanneer men ijsblokjes maakt van warm water, deze veel meer doorzichtig zijn dan koud water wat bevriest.

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Op zaterdag 19 augustus 2006 15:46 schreef speknek het volgende:
Ah, dat klinkt logischer. Dat laatste althans, want het is nou ook weer niet zo dat als je ijsblokjesmal uit de diepvries haalt, dat de blokjes half verdampt zijn ofzo.

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Op zaterdag 19 augustus 2006 14:18 schreef Aitan het volgende:

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Is dit verband een feit?
Heeft zowiezo iets met de bloedtoevoer te maken, vergelijk het maar met de pretpark atractie spaceshot ofzo...
Misschien is het ontbreken van voedingsstoffen een verklaring voor het 2e deel...

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Op zaterdag 19 augustus 2006 15:43 schreef Jordy-B het volgende:
Het komt vooral door fouten... twee belangrijke fouten zijn verdamping en het verschil in dichtheid.
Verdamping is een belangrijke, want daardoor hoeft er minder water te bevriezen.
Verschil in dichtheid is een belangrijke, want warm water heeft een lagere dichtheid dan koud water. Als je van beiden een gelijk volume neemt (bij vloeistoffen wel de meest gebruikelijke manier), heb je een lager gewicht aan warm water dan aan koud water... er hoeft dan dus ook minder water te bevriezen.

Convectie is de enige fatsoenlijke reden, imo. Als je een bak warm water in de vriezer zet, koelt vooral het oppervlak snel af. Als het afkoelt, krijgt het een hogere dichtheid dan het warme water en begint te zinken. Als het zinkt, komt er 'nieuw' warm water bij het oppervlak wat vervolgens weer afkoelt en zinkt. Er ontstaat dus beweging in het water en dit noem je convectie. Doordat het verschil in temperatuur tussen de lucht in de vriezer en het water, bij warm water groter is, heb je meer convectie en koelt het makkelijker af.

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Op zaterdag 19 augustus 2006 12:43 schreef VonHinten het volgende:
Nieuwe vraag

1. Waarom wordt het (soms) wit voor je ogen als je te snel opstaat?
2. En waarom komt dit vaker voor als je een lege maag / honger hebt? Wat is het verband?

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Normally, when a person stands up, the blood vessels in the legs constrict in order to support the blood pressure in the standing position.

Orthostatic hypotension – a fairly common condition – occurs in patients whose blood vessels do not constrict normally when they stand, thus allowing the blood to pool in the legs, and the blood pressure to drop precipitously. This condition is most often caused by prescription drugs, but is also seen in diabetes, Parkinson’s disease, with dehydration, and in several other disorders. A modestly alert physician can easily diagnose orthostatic hypotension simply by taking the blood pressure while the patient is lying down, and again while the patient is standing.

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Vasomotor syncope occurs when the blood vessels in the legs dilate, causing a large proportion of a person’s blood volume to pool in the legs. As a result the blood pressure drops, the brain suddenly is not receiving an adequate amount of oxygen, and syncope occurs.

Once the patient falls to the ground, however, gravity no longer keeps the blood pooling in the legs. Blood returns to the heart, blood flow is restored, and the patient regains consciousness within several seconds. Vasomotor syncope is therefore “self correcting” - as long as well-meaning bystanders don't prop the victim up.

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Vasovagal syncope (also known as cardioneurogenic syncope) is the most common cause of syncope, probably accounting for more than 80% of all syncopal episodes. Since vasovagal syncope is simply an exaggeration of a normal neurological reflex, most individuals will experience at least one vasovagal episode in their lifetimes.

The reflex responsible for vasovagal syncope works like this: A person is exposed to some stimulus (such as a needle stick) that initiates the reflex. The “stimulated” nerves (the nerves of the stuck finger, for instance) send an electrical signal to the vasomotor center in the brainstem. (The vasomotor center determines the body’s vascular “tone.”) The vasomotor center, in turn, signals the blood vessels in the legs to dilate, causing the blood to pool in the legs, and producing syncope. This same reflex also causes a drop in the heart rate, but usually it is the pooling of blood in the legs – and not the slow heart rate – that produces loss of consciousness.

The “stimulus” that triggers a vasovagal episode can be any one of hundreds of things. As already noted, pain is a common cause of fainting. Other common triggering events include the sight of blood, receiving upsetting news, or standing motionless for long periods (such as with soldiers standing at parade rest).

Anyone can have vasovagal syncope given an adequate triggering event, but many people are particularly prone to these episodes, and often with relatively mild triggers. These individuals tend to relate histories of syncope dating back to adolescence, and frequently will describe several different of triggering events. While, as noted, there are scores of possible triggering events for vasovagal syncope, some are quite characteristic and almost always point to vasovagal syncope. Syncope occurring after urinating, defecating, coughing or swallowing, or syncope associated with pain, fright, the sight of blood, or other noxious stimuli, is almost always vasovagal.

In these and other ways, vasovagal syncope tends to be highly situational. It is more likely to occur after a viral illness, after exercise, after a warm shower, or early in the morning – any time that relative dehydration is present, and dilation of the blood vessels in the legs would be more likely to produce a significant drop in blood pressure. Furthermore, vasovagal syncope is often preceded by a few seconds or a few minutes of warning symptoms. Often, these symptoms include lightheadedness, ringing in the ears, visual disturbances, sweating and/or nausea. Because of such “warning symptoms,” people who have had one or two episodes of syncope are frequently able to tell when an event is about to occur. And importantly, if they recognize the warning symptoms, they are able to abort the blackout simply by lying down and elevating the legs. (“Aborting” syncope is not possible with most other forms of syncope.)

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Op woensdag 23 augustus 2006 17:00 schreef Tarrel666 het volgende:
Hoe kan het dat water uit bijv. de zee verdampt naar regenwater zonder dat het 100° C wordt??

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Op maandag 28 augustus 2006 18:15 schreef Aslama het volgende:
Weet iemand hoeveel zwarte gaten er zijn?

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Op maandag 28 augustus 2006 20:14 schreef Fir3fly het volgende:

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Lijkt me moeilijk tellen als je ze niet kunt waarnemen he?

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Op maandag 28 augustus 2006 20:11 schreef ThinkTank het volgende:
-like, *knip* enzo-
_{Beetje rommelig opgeschreven, misschien kan iemand het beter uitleggen...}

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Op maandag 28 augustus 2006 22:11 schreef Jordy-B het volgende:

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'k vond mijn uitleg wel duidelijk, eigenlijk...

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Op maandag 28 augustus 2006 22:01 schreef Aslama het volgende:

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Je kunt ze indirect waarnemen.
Misschien kan ik de vraag anders stellen: hoeveel zwarte gaten zijn er tot nu toe indirect waargenomen of theoritisch bepaald?

Nu een echt antwoord

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The Question
(Submitted November 20, 2001)

How many black holes are known to man that are in the universe? And out of those know black holes what are the names?

The Answer
The answer to your first question depends a lot on how strong an evidence you would want to accept something as a black hole.

Astrophysicists generally agree that when the compact object in an X-ray binary system is shown to be more massive than about 3 times the mass of the Sun, then this compact object is a black hole beyond reasonable doubt. These are called "dynamically confirmed black holes." Dr. Orosz maintains a list of such systems (currently 14) on his home page:
_{zijn er inmiddels 20}
http://mintaka.sdsu.edu/faculty/orosz/web/

If you accept a less strict standard of evidence, then there are many more black holes that (we think) we know of. These include additional X-ray binaries such as Cygnus X-1, the mysterious object at the center of our Galaxy, and the central objects in many (perhaps even most) luminous galaxies.

For example, the Sloan Digital Sky Survey aims to measure the distances to more than a million galaxies and quasars:

http://www.sdss.org/

A large fraction of these galaxies, and all the quasars, are thought to contain a supermassive black hole. Given such a huge number, there is no plan to individually name these black holes; astrophysicists use designations based on their positions on the sky (you see examples on Dr. Orosz's page).

Best wishes,

Koji & Georgia
for "Ask an Astrophysicist"

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Op maandag 28 augustus 2006 22:35 schreef ThinkTank het volgende:

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Over deze vraag moet ik gelukkig niet te veel nadenken

Antwoord is al enkele jaren oud, dat wel:
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Op maandag 28 augustus 2006 22:48 schreef Aslama het volgende:
Leuke info, thanks

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Op maandag 4 september 2006 21:04 schreef Fir3fly het volgende:
Ja, de directe afstammelingen van de dinosaurussen...

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Op maandag 4 september 2006 21:03 schreef speknek het volgende:

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Een hypothese stelt zelfs dat er bijkans ontelbaar veel zwarte gaten zijn. Ze weten namelijk nog niet wat ongeveer 80% van de materie in het heelal is. Ze weten wel dat het heelal een bepaalde massa heeft, want het effect daarvan kunnen ze meten met de relativiteitstheorie, maar niet wat die materie inhoudt, omdat het donker is, ze kunnen het dus niet zien.
Een van de hypotheses is nu dat de reden dat ze het niet kunnen zien is, dat het allemaal hele kleine zwarte gaten zijn -denk aan de grootte van micrometers tot enkele centimeters-, maar het effect daarvan is bekend..

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Op donderdag 7 september 2006 20:00 schreef Secretus het volgende:
Kan je eigenlijk, zonder dat je een oogafwijking hebt, het ene oog onafhankelijk van het andere bewegen ?

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Op donderdag 28 september 2006 20:19 schreef ThinkTank het volgende:
Hoe lang zou een mens het luchtledige kunnen overleven? In enkele SF verhalen (oa van Arthur C. Clarke dacht ik) worden reddingsacties beschreven waarin men zonder ruimtepak (of luchtsluis) van het ene ruimteschip naar het andere overstapt. In hoeverre is dit realistisch?

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Op donderdag 28 september 2006 20:19 schreef ThinkTank het volgende:
Hoe lang zou een mens het luchtledige kunnen overleven? In enkele SF verhalen (oa van Arthur C. Clarke dacht ik) worden reddingsacties beschreven waarin men zonder ruimtepak (of luchtsluis) van het ene ruimteschip naar het andere overstapt. In hoeverre is dit realistisch?

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The best data I have comes from the chapter on the effects of Barometric pressure in Bioastronautics Data Book, Second edition, NASA SP-3006. This chapter discusses animal studies of decompression to vacuum. It does not mention any human studies.

page 5, (following a general discussion of low pressures and ebullism), the author gives an account of what is to be the expected result of vacuum exposure:
"Some degree of consciousness will probably be retained for 9 to 11 seconds (see chapter 2 under Hypoxia). In rapid sequence thereafter, paralysis will be followed by generalized convulsions and paralysis once again. During this time, water vapor will form rapidly in the soft tissues and somewhat less rapidly in the venous blood. This evolution of water vapor will cause marked swelling of the body to perhaps twice its normal volume unless it is restrained by a pressure suit. (It has been demonstrated that a properly fitted elastic garment can entirely prevent ebullism at pressures as low as 15 mm Hg absolute [Webb, 1969, 1970].) Heart rate may rise initially, but will fall rapidly thereafter. Arterial blood pressure will also fall over a period of 30 to 60 seconds, while venous pressure rises due to distention of the venous system by gas and vapor. Venous pressure will meet or exceed arterial pressure within one minute. There will be virtually no effective circulation of blood. After an initial rush of gas from the lungs during decompression, gas and water vapor will continue to flow outward through the airways. This continual evaporation of water will cool the mouth and nose to near-freezing temperatures; the remainder of the body will also become cooled, but more slowly.

"Cook and Bancroft (1966) reported occasional deaths of animals due to fibrillation of the heart during the first minute of exposure to near vacuum conditions. Ordinarily, however, survival was the rule if recompression occurred within about 90 seconds. ... Once heart action ceased, death was inevitable, despite attempts at resuscitation....

[on recompression] "Breathing usually began spontaneously... Neurological problems, including blindness and other defects in vision, were common after exposures (see problems due to evolved gas), but usually disappeared fairly rapidly.

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The Bioastronautics Data Book answers this question: "Some degree of consciousness will probably be retained for 9 to 11 seconds.... It is very unlikely that a human suddenly exposed to a vacuum would have more than 5 to 10 seconds to help himself."

A larger body of information about how long you would remain conscious comes from aviation medicine. Aviation medicine defines the "time of useful consciousness", that is, how long after a decompression incident pilots will be awake and be sufficiently aware to take active measures to save their lives. Above 40,000 feet (12 km), the time of useful consciousness is 12 to 25 seconds. (The shorter figure is for a person actively moving; the longer figure is for a person sitting quietly.) For complete vacuum, this will be slightly less; 9 to 11 seconds quoted by the Bioastronautics data book, 9 to 12 seconds quoted by Fischer.

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Your blood is at a higher pressure than the outside environment. A typical blood pressure might be 75/120. The "75" part of this means that between heartbeats, the blood is at a pressure of 75 Torr (equal to about 100 mbar) above the external pressure. If the external pressure drops to zero, at a blood pressure of 75 Torr the boiling point of water is 46 degrees Celsius (115 F). This is well above body temperature of 37 C (98.6 F). Blood won't boil, because the elastic pressure of the blood vessels keeps it it a pressure high enough that the body temperature is below the boiling point-- at least, until the heart stops beating (at which point you have other things to worry about!). (To be more pedantic, blood pressure varies depending on where in the body it is measured, so the above statement should be understood as a generalization. However, the effect of small pockets of localized vapor is to increase the pressure. In places where the blood pressure is lowest, the vapor pressure will rise until equilibrium is reached. The net result is the same.)

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On a page on human survival in vacuum formerly posted to the /medlib server at NASA JSC, further details are given about the vacuum-exposure accident mentioned by Roth in NASA CR-1223:

"At NASA's Manned Spacecraft Center (now renamed Johnson Space Center) we had a test subject accidentally exposed to a near vacuum (less than 1 psi) in an incident involving a leaking space suit in a vacuum chamber back in '65. He remained concious for about 14 seconds, which is about the time it takes for O2 deprived blood to go from the lungs to the brain. The suit probably did not reach a hard vacuum, and we began repressurizing the chamber within 15 seconds. The subject regained conciousness at around 15,000 feet equivalent altitude. The subject later reported that he could feel and hear the air leaking out, and his last conscious memory was of the water on his tongue beginning to boil." .

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"Immediately following rapid decompression, it was noted that he began to cough moderately. Very shortly after this he was seen to lose consciousness, and the picture described by the physicians on duty was that the patient remained deeply cyanotic, totally unresponsive and flaccid during the 2-3 minutes [to repressurise the altitude chamber] down to ground level.
... "Manual artificial respiration was begun immediately... The patient at no time breathed spontaneously; however, at the moment ground level was reached he was seen to give a few gasps. These were very irregular and only two or three in number.
...
"The conclusion of the [autopsy] report was as follows: "The major pathologic changes as outlined above are consistent with asphyxia. It is felt that the underlying cause of death in this case may be attributed to acute cardio-respiratory failure, secondary to bilateral pneumothorax..." "

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Many other cases of death following decompression are noted in the aviation literature, including one spaceflight incident, the Soyuz-11 decompression accident, in 1971. A recent analysis of this accident can be found in D. J. Shayler, Disasters and Accidents in Manned Spaceflight.

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In 1960, during a high-altitude balloon parachute-jump, a partial-body vacuum exposure incident occurred when Joe Kittinger, Jr. lost pressurization in his right glove during an ascent to 103,000 ft (19.5 miles) in an unpressurized balloon gondola, Despite the depressurization, he continued the mission, and although the hand became painful and useless, after he returned to the ground, his hand returned to normal. Kittinger wrote in National Geographic (November 1960):
"At 43,000 feet I find out [what can go wrong]. My right hand does not feel normal. I examine the pressure glove; its air bladder is not inflating. The prospect of exposing the hand to the near-vacuum of peak altitude causes me some concern. From my previous experiences, I know that the hand will swell, lose most of its circulation, and cause extreme pain.... I decide to continue the ascent, without notifying ground control of my difficulty."
at 103,000 feet, he writes:
"Circulation has almost stopped in my unpressurized right hand, which feels stiff and painful."
But at the landing:
"Dick looks at the swollen hand with concern. Three hours later the swelling will have disappeared with no ill effect."

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One of the potential dangers during a rapid decompression is the expansion of gases within body cavities. The abdominal distress during rapid decompression is usually no more severe than that which might occur during slower decompression. Nevertheless, abdominal distention, when it does occur, may have several important effects. The diaphragm is displaced upward by the expansion of trapped gas in the stomach, which can retard respiratory movements. Distention of these abdominal organs may also stimulate the abdominal branches of the vagus nerve, resulting in cardiovascular depression, and if severe enough, cause a reduction in blood pressure, unconsciousness, and shock.

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Usually, abdominal distress can be relieved after a rapid decompression by the passage of excess gas.

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Whenever a rapid decompression is faster than the inherent capability of the lungs to decompress (vent), a transient positive pressure will temporarily build up in the lungs. If the escape of air from the lungs is blocked or seriously impeded during a sudden drop in the cabin pressure, it is possible for a dangerously high pressure to build up and to overdistend the lungs and thorax. No serious injuries have resulted from rapid decompressions with open airways, even while wearing an oxygen mask, but disastrous, or fatal, consequences can result if the pulmonary passages are blocked, such as forceful breath-holding with the lungs full of air. Under this condition, when none of the air in the lungs can escape during a decompression, the lungs and thorax becomes over-expanded by the excessively high intrapulmonic pressure, causing actual tearing and rupture of the lung tissues and capillaries. The trapped air is forced through the lungs into the thoracic cage, and air can be injected directly into the general circulation by way of the ruptured blood vessels, with massive air bubbles moving throughout the body and lodging in vital organs such as the heart and brain.

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Op vrijdag 27 oktober 2006 19:50 schreef densmee het volgende:
ik denk minder dan een seconde luchtledig dus een vacuum
je zult zeer snel exploderen

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Op dinsdag 31 oktober 2006 14:25 schreef Catbert het volgende:

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Euh, nee, dat is dus complete onzin. Je explodeert absoluut niet. Je kunt het best even uithouden. Je gaat eerder dood aan verstikking dan aan iets anders. Op den duur zal je lijk gevriesdroogt worden, maar je bloed gaat niet koken ofzo, dat zijn hollywood fabeltjes.

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Would Your Blood Boil?
No.

Your blood is at a higher pressure than the outside environment. A typical blood pressure might be 75/120. The "75" part of this means that between heartbeats, the blood is at a pressure of 75 Torr (equal to about 100 mbar) above the external pressure. If the external pressure drops to zero, at a blood pressure of 75 Torr the boiling point of water is 46 degrees Celsius (115 F). This is well above body temperature of 37 C (98.6 F). Blood won't boil, because the elastic pressure of the blood vessels keeps it it a pressure high enough that the body temperature is below the boiling point-- at least, until the heart stops beating (at which point you have other things to worry about!). (To be more pedantic, blood pressure varies depending on where in the body it is measured, so the above statement should be understood as a generalization. However, the effect of small pockets of localized vapor is to increase the pressure. In places where the blood pressure is lowest, the vapor pressure will rise until equilibrium is reached. The net result is the same.)