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Old 07-31-2009, 05:37 PM
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Science Stupid Science Questions Thread

I had this dumb science question that came up while I was looking up through a skylight.

If the Sun winked out instantaneously and stopped shining light and warmth suddenly, would Earth immediately begin freezing the moment the last rays of light hit Earth? How long would it take for the planet to freeze?

Feel free to ask your stupid science questions here.
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Old 07-31-2009, 06:12 PM
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Originally Posted by MonCapitan2002 View Post
If the Sun winked out instantaneously and stopped shining light and warmth suddenly, would Earth immediately begin freezing the moment the last rays of light hit Earth?
Nope. Half the earth is plunged into darkness at any given time. It goes all night without freezing.

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How long would it take for the planet to freeze?
Dunno. More than 12 hours, though.
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Old 08-01-2009, 09:35 PM
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Default Re: Stupid Science Questions Thread

Definitely more than 12 hours.

There are a few things to consider. First is the fact that most of the planet's surface is covered by water, and water has a tremendous heat capacity. That fact, plus the fact that the atmosphere contains a lot of water vapor that traps heat near the planet's surface, explains why there's so relatively little difference in temperature between day and night.

You can get a hint of that by considering what happens in the interior of large deserts, far from the ocean. It isn't unusual for temperatures to be well over 100 degrees Fahrenheit during the day, only to plunge to around freezing during the night. That's because there are no large bodies of water nearby to release some of their stored heat and so help prevent such drastic temperature fluctuations. Nor is there much water vapor in the air to help trap heat near the surface. (Water vapor is a greenhouse gas.)


Second, and more important on a longer scale, is the fact that a planet is, relatively speaking, enormous. Keep in mind that, if you keep its proportions the same, as something grows larger, its volume (being a cubic function) increases far faster than does its surface area (which is only a square function). You can easily verify this for yourself with some dice.

Consider a single die that's 1 centimeter by 1 centimeter by 1 centimeter in size. The surface area of a rectangle is simply length times width, and the surface of a die is made up of 6 equal-sized squares. So, its total surface area is (1 cm x 1 cm) x 6 = 6 cm2. Its volume is length x width x height -- in this case, 1 cm x 1 cm x 1cm = 1 cm3.

So the die has a surface area/volume ratio of 6:1.

Now double the size of the die, so that each side is 2 cm. The new die has a surface area of (2 cm x 2 cm) x 6 = 24 cm2, and a volume of 2 cm x 2 cm x 2 cm = 8 cm3.

In other words, if you double the die's size, its surface area increases by a square factor and so quadruples (22). But its volume increases by a cubic factor, and so increases by eight times (23).

And so, by doubling the die's size, we've reduced the surface area/volume ratio to 24/8 or 3:1.

If we triple the die's size, its surface area would be (3 cm x 3 cm) x 6 = 54 cm2, and its volume would be 3 cm x 3 cm x 3 cm = 27 cm. So the SA/V ratio is down to 54/27 or 2:1.

If we quadruple the die's size, its SA/V ratio drops to 1.5:1. If we quintuple the die's size, its SA/V ratio is 1.2:1. If we increase the die's size by six times, its SA/V ratio is down to 1:1, and if we increase the die's size by seven times, its SA/V is down to approximately 0.86:1. And so on.



Why is this significant? Because it's the volume of an object that generates (or stores) heat, but it's the surface area that determines how fast that heat can escape. And planet-sized objects have relatively enormous volumes compared to their surface areas. This means it takes a long time for them to cool down. (The planet Jupiter still hasn't completely cooled from the heat of its formation, and its over 4.5 billion years old.)

To further illustrate the point, consider the Sun. Pound for pound, it generates less heat than does a human being. How can that be, given that a human's surface temperature is far less than that of the Sun? Because, even though it generates a relatively small amount of heat (compared to what a human body would generate if it could somehow be enlarged to the same size), the Sun has a relatively tiny surface area, compared to a human being, and so the energy that the Sun releases must be released over a comparatively tiny area. In other words, the energy is, in effect, highly concentrated.



On a related note, nuclear fusion in the Sun's core is the ultimate source of its power. The Sun is so vast (and therefore loses heat so slowly) that if those reactions were to stop tomorrow, it would literally be millions of years before the Sun cooled enough for this to be a serious concern.


***


So, if the Sun were to magically wink out tomorrow (fortunately, it can't), what would happen?

According to the estimates I've seen, it would take a few million years for the Earth to reach thermal equilibrium with its surroundings, if all solar input ceased. The Earth would stabilize at a surface temperature of about -400 degrees F. (The Earth actually generates heat of its own; it radiates heat because of the decay of radioactive elements in the crust and mantle, so it would remain a few degrees warmer than its surroundings for literally eons.)


After the Sun stopped shining, the average global surface temperature would drop below freezing within a few days. Within a week, the average global surface temperature would drop below 0°F. Within a year, the average global surface temperature would have dropped below -100°F.

By then, of course, the top layers of the oceans would have frozen, but ice is an excellent insulator, so it would greatly slow loss of heat from the deeper layers of the oceans. It would probably take well over 100,000 years for the oceans to completely freeze.

Actually, there would be a few places where the oceans wouldn't freeze at all. There are places at the bottom of the ocean where "cracks" in the Earth's crust allow ocean water to circulate into the crust and to come into contact with heated rock from deep in the crust. (These are known as "hydrothermal vents," and some of the organisms living around these vents can generate food independent of sunlight. In other words, there are a few communities on the planet that might persist for millions of years independent of the Sun, kept going by the heat being released from the Earth's crust.)



So, if the Sun stopped shining tomorrow, virtually all life on land would be dead within a year or so -- the exceptions being some bacteria and archaens that might survive in geothermal hot springs like those in Yellowstone National Park. Life could persist in the deepest parts of the oceans for perhaps a few hundred thousand years, maybe a million years or so.


I suppose that, if we had time to prepare, some humans might be able to survive indefinitely even without the Sun, by going underground and using nuclear power and/or geothermal power to provide for their energy needs. (They'd have to take plants with them, of course, for food; and so they'd have to generate artificial light for purposes of photosynthesis.)

The atmosphere itself would freeze once the planet's surface temperature dropped below about -300 degrees F (this would probably take thousands of years), so I'm not sure that living on the surface would be practical in the long run, unless it was in pressurized and heated domes. It'd almost-certainly be both easier and safer to live underground.


Cheers,

Michael
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Old 08-01-2009, 10:09 PM
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Old 08-01-2009, 10:17 PM
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Default Re: Stupid Science Questions Thread

In a related question, what is the speed of gravity? As in, if the sun winked out of existence altogether (alien abduction or whatever), how long would Earth and the other planets stay in orbit, before the lack of the sun's pull affected them. Is it immediate? The speed of light (8 minutes to Earth, more/less for the others)? Slower?
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Old 08-01-2009, 10:20 PM
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Is that even knowable given todays understanding of gravity? I would think the effect would be pretty much immediate, Earth would leave it's orbit on pretty much a straight line without the gravitational pull of the Sun, tugging it's little moon along for the ride.
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Old 08-01-2009, 10:22 PM
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So the effect, the information, would move faster than light?

I also think it might not be known.
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Old 08-01-2009, 10:24 PM
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Based on relativity, it would take about 8 minutes before the gravity of the sun vanished, since gravitational wave propagate at about the speed of light.
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Old 08-01-2009, 10:25 PM
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Gravity is waves? I did not know that.
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Old 08-01-2009, 10:32 PM
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Gravity is, um I'm not sure, ask the lone ranger :D

In relativity the 'ultimate speed limit' is the speed of light in a vacuum, thus even the disturbance of spacetime caused by the sun would take the speed of light to affect planets.

I think this is the reason spacetime drags behind an object, causing a rotating object to create a spiral drag effect on spacetime (this drag has been measured recently).
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Old 08-01-2009, 11:16 PM
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Based on relativity, it would take about 8 minutes before the gravity of the sun vanished, since gravitational wave propagate at about the speed of light.

Yep.
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Old 08-02-2009, 02:43 AM
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Default Re: Stupid Science Questions Thread

Yes, gravity waves travel at the speed of light. So if the Sun were to disappear right now, it would be about 8 minutes before we could possibly know about it, because no information can be propagated faster than the speed of light -- it would take about 8.5 minutes for the light leaving the Sun right now to reach us, and for the gravity waves as well.


***


As an aside, a heated object normally cools in an exponential manner. That is, it initially cools quickly, but as its temperature falls, the rate at which it continues to lose heat decreases.

This is because energy often behaves as if it is flowing. The greater the temperature difference between two objects, the faster that heat flows from the warmer object to the cooler one. By the same token, as they approach the same temperature, the rate of heat exchange between the objects decreases dramatically.

That's why it would take only a few days for the Earth's surface to cool so much that life on land wouldn't be possible, but it would take literally millions of years to reach thermal equilibrium with its surroundings.

Here's what a typical cooling curve looks like:


Cheers,

Michael
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Old 08-02-2009, 11:34 AM
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Default Re: Stupid Science Questions Thread

But, but... that's just bizarre! The planets veering off orbit gradually, one after another?! I want to see instant pool table chaos!
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Old 08-02-2009, 11:46 AM
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With a ratio of ten women chosen for their sexual desirability to every men, we could reach the gross national product in about thirty years. . . .

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Old 08-02-2009, 03:21 PM
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Interesting article on gravity.
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Old 08-03-2009, 01:49 AM
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Default Re: Stupid Science Questions Thread

Here's one. Actually, here's two.

I was pondering this the other night. What keeps planets and stars spinning? Is it the inertia from the creation of them or is it the electromagnetic field.

Also, what is fire? Is it a particle, a wave or something else?
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Old 08-03-2009, 01:55 AM
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When I went to see Monsters vs Aliens in 3D, and I couldn't make out the 3D effects almost at all. When I went to see HP6 in 3D, the girl told me that these glasses had a different polarization. I could see these effects pretty well (but not perfect).

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Old 08-03-2009, 01:57 AM
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Originally Posted by Deadlokd View Post
I was pondering this the other night. What keeps planets and stars spinning? Is it the inertia from the creation of them or is it the electromagnetic field.
The conservation of angular momentum. Which is closer to your first guess. I would try to explain it in a fantastic description like TLR, but I suck at that. So perhaps he will provide a more elegant answer.

Quote:
Also, what is fire? Is it a particle, a wave or something else?
Fire is energy released from a chemical reaction, I believe. Though it does emit light energy waves, which is what we see; as well as heat energy waves, which is what we feel. Both of those are in the EM spectrum.
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Old 08-03-2009, 03:32 AM
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I was pondering this the other night. What keeps planets and stars spinning? Is it the inertia from the creation of them or is it the electromagnetic field.
In effect, it is inertia.

According to the standard model of stellar/planetary formation, solar systems form from nebulae, gigantic clouds of gas and dust in space. (In astronomy, hydrogen and helium are referred to as "gases"; all other elements and compounds are referred to as "dusts.")

A key concept to keep in mind, as Demimonde indicated, is the angular momentum of this cloud. Angular momentum is conserved, and it's all but impossible that this cloud will happen to have zero angular momentum. This means that, considered as a whole, it will be spinning, however slowly.


Perhaps triggered by the shock wave from a relatively nearby supernova, a nebula begins to collapse under the combined gravitational pull of its component particles. This nebula might be several light years across; gravity is by far the weakest force in nature; the atoms and molecules in a typical nebula are much further apart, on average, than are the atoms and molecules that make up the Earth's atmosphere -- so in other words, this process of gravitational collapse is very slow, and will take hundreds of thousands of years, at least.

As the nebula undergoes gravitational collapse and so grows smaller, its rate of spin will begin to increase, since angular momentum is conserved. [This is the same reason that an ice-skater spins faster as she pulls her arms inward.]


Most of the mass of the nebula will condense into the center. As more and more matter condenses into that central mass, it becomes denser and hotter. Since the nebula is, inevitably, made mostly of hydrogen and helium, when the central mass becomes sufficiently hot and sufficiently dense, nuclear fusion will begin in its center. Hydrogen nuclei will fuse to form helium nuclei. At this point, the central mass becomes a star, radiating away as light and heat the electromagnetic energy produced as a result of the nuclear fusion occurring in its core.

More than 98% of the mass of our solar system is contained in the Sun.

Actually, in most solar systems, not one star forms, but two or more. Our solar system is unusual in that it contains only one star.



As the nebula condenses and most of its mass concentrates into a central mass that will ultimately form one or more stars, the "centrifugal force" generated by the spinning mass causes the matter that is not in the center to flatten out into a disk orbiting the central protostar(s).

This disk is known as an accretion disk. The relatively cool matter in the disk will condense into dust grains. These grains will occasionally collide and stick together. Eventually, by chance, some will form large-enough structures (called planetesimals) that have enough gravity to attract each other. These planetesimals will begin to "sweep" up local matter, including other planetesimals. (When an object grows by sweeping up smaller particles and incorporating them into itself, the process is known as accretion.)

Through accretion, planets will form in the accretion disk. Like the central star(s), it's all but impossible that a planet will happen to have zero angular momentum, so it will be spinning.

Eventually, all the planetesimals will be swept up by the star(s), planets and moons of the young solar system. (You can see the evidence of this in all the impact craters on the planet Mercury and on our Moon; almost all of these craters were formed by impacts early in the solar system's history.)


Once the star ignites, the radiation it emits will clear out most of the remaining dust and gas in the interior of the solar system, leaving just relatively-large objects such as planets, moons and asteroids in orbit.



In any event, once they have formed, the stars and planets will continue to spin, because angular momentum is conserved. The only way to slow their spins, much less halt them, would be to transfer their angular momentum to something else. Incidentally, some of the Earth's angular momentum is gradually being transferred to the Moon, because of gravitational interaction between the two bodies. As a result, the Earth's rotation is gradually slowing.


Quote:
Also, what is fire? Is it a particle, a wave or something else?
What we see as a "fire" is the energy being released by an exothermic chemical reaction. Chemical bonds contain energy, and when those bonds are broken, the energy stored in them is released.

Chemical reactions involve the forming, breaking, and re-forming of chemical bonds between atoms and molecules. The chemicals that go into a reaction are known as the reactants, and the chemicals that result from the reaction are known as the products. If there is more energy in the chemical bonds of the reactants than in the chemical bonds of the products, the "extra" energy is released as the reaction proceeds. Such a reaction is known as an exothermic reaction, because it releases energy (usually in the form of heat) as it progresses.


Most fires occur as a result of oxidation -- when oxygen in the atmosphere combines with carbon, hydrogen, and other elements in wood, paper, or other substances -- forming products such as carbon dioxide (CO2) and water (H2O).

A certain amount of energy must be supplied to get the reaction started (this is known as the activation energy), but once the reaction is started, the heat it gives off is sufficient to keep the reaction going, so long as fuel is available. As the wood (for example) is heated, gases and small particles are given off, which react with oxygen in the air to form CO2, H2O, etc. As the reaction occurs, energy is released in the form of heat and visible light.

A typical flame from a wood or paper fire releases relatively little energy, and so glows orange/yellow. A hotter flame (such as from burning magnesium) will glow white. A really hot flame will glow blue. A SR-71 spyplane burns its fuel at such high temperatures that it can actually produce purple flames.


I should clarify that a lot (probably most) of what you're seeing when you see a flame isn't directly due to energy released by the oxidation reactions (most of that is invisible infrared radiation). Instead, what you're seeing is light given off by the unburned substances. As the wood (for example) burns, some of it is vaporized, and the heated particles escape without being burned. These particles can absorb so much of the energy released by the oxidation reactions that they, themselves, begin to glow in the visible part of the spectrum. As these unburned substances cool, they stop glowing, and so are referred to as smoke. [You can see this happen; as a "tongue" of flame leaps up from a fire -- that is, as some superheated, glowing gas is released -- it glows yellow-white at first, then yellow as it loses energy, then orange as it continues to lose energy; then red-orange as it continues to lose energy; and finally dull red before it stops glowing. It stops glowing when it has cooled to the point that the energy it's releasing is no longer in the visible portion of the spectrum.]

That is mostly what you're seeing when you see a fire -- superheated gases and particles that are glowing as a result of all the heat energy they have absorbed from the oxidation reactions going on around them.


As an aside, pure hydrogen burns so cleanly (that is, with little or no smoke), that its flames are usually invisible. That's precisely what makes it so deadly. Firefighters could, conceivably, walk right into a hydrogen fire without realizing where it was until it was too late. (If you've ever seen a hydrogen-burning apparatus, there is a region just above the burner where it looks like there's no flame. That isn't the case; it's just that the flame is invisible. The visible flame above it is due to the hydrogen fire heating atmospheric gases to incandescence.)



I've sometimes heard people claim that fire is a plasma -- an ionized gas. That isn't true. A normal fire does not burn hot-enough to ionize the gases involved.


Cheers,

Michael
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Old 08-03-2009, 10:11 AM
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(In astronomy, hydrogen and helium are referred to as "gases"; all other elements and compounds are referred to as "dusts.")
Just a minor quibble, but this isn't quite right. It's often joked that astronomers know only three chemical elements: hydrogen, helium and metals. Which makes a lot of sense for them, since the heavier elements make up only a tiny fraction (~0.1% by mass) of the matter in the Milky Way, and their only role in stellar spectra seems to be "create a lot of absorption lines". Of course, the people studying interstellar chemistry and molecular line emission tend to have a more nuanced view.

Dust in an astronomical context means mostly macromolecules (like polycyclic hydrocarbons) and solid state particles composed mostly of graphite and silicate. The sizes of these dust grains range from several nanometers to micrometers in interstellar conditions; in planet forming regions, the densities are high enough that they can efficiently coagulate to form larger particles, at least to a couple of millimeters.

Presumably dust grains grow beyond millimeter sizes, but at that point we can't see them anymore; as an analogy, it's hard to see a brick from a couple of miles away, but if you grind it to dust and scatter it in the air it's much easier to spot. From models, we can be pretty sure that the coagulation process continues at least to the meter scale. What happens after that is still a bit of a mystery, since meter sized dust grains aren't good at sticking to each other (again, try it with bricks), and they're not heavy enough that gravity can help them out. It's still an outstanding question how particles grow from meter sized objects to kilometer sized planetesimals.

Once planetesimals form the accretion process is very efficient in forming planets. At this point there are still two processes that stand in the way of a planetary system like the one we're familiar with. First, there is the gaseous disk that still surrounds the parent star. The friction caused by this disk, and the tidal effects it induces may cause a newly formed planet to spiral in towards the central star, or eject them from the system. Second, a solar system with more than one planet is notoriously chaotic, and we have no way of knowing whether it will be stable in the long run. Interaction among the planets may again eject some of them from the system or send them into the star.

---

Apart from that, this was another excellent refresher course on planet formation, TLR. I'd just like to add this cartoon I made a couple of years ago, outlining the whole process from collapsing molecular cloud core via accretion disk with an acive outflow and passive disk to planetary system.
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Old 08-03-2009, 06:25 PM
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There are tidal forces tending to slow any rotation of the planets, and so lengthen their 'days'.

The tidal forces have slowed the rotation of our moon to the point where it rotates once on its axis for each orbit it makes of the earth, and so always shows the same face to earth.

The tides raised on the earth by the moon tend to slow the rotation of the earth, and through the conservation of angular momentum this pushes the moon further away from the earth.

This process has already run to completion in the case of Pluto and its moon, Charon - both now always present the same face towards each other.

Other planets that don't have moons, such as Venus, still experience some tides due mainly to the sun.

At one time, there was a theory that a planet's atmosphere, heated by the sun on the 'day' side, and therefore expanding, could act as a kind of engine and cancel out the slowing due to tidal forces. This theory is now discredited.

It was also believed at one time that Venus might be in a tidal lock with earth! Although the tides on Venus due to the mass of the earth are tiny, it was known that whenever Venus made her closest approach to earth, she would present the same face towards us! It was thought that Venus had exactly five solar days between each closest approach, but this figure has now been found, by more accurate measurement, to be 5.001444 Venusian days, so it's probably just a coincidence.
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Old 08-03-2009, 07:41 PM
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It was also believed at one time that Venus might be in a tidal lock with earth! Although the tides on Venus due to the mass of the earth are tiny, it was known that whenever Venus made her closest approach to earth, she would present the same face towards us! It was thought that Venus had exactly five solar days between each closest approach, but this figure has now been found, by more accurate measurement, to be 5.001444 Venusian days, so it's probably just a coincidence.
Still doesn't rule it out. Things don't just spring into tidal lock, they have to get there. Now we just have to measure the rate of change of Venus' spin... ;)
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Old 08-03-2009, 08:58 PM
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As an aside, pure hydrogen burns so cleanly (that is, with little or no smoke), that its flames are usually invisible.
I was under the impression its color was ultraviolet.
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I've sometimes heard people claim that fire is a plasma -- an ionized gas. That isn't true. A normal fire does not burn hot-enough to ionize the gases involved.
good to know. Explains why I couldn't measure the resistance of a candle flame.
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Old 08-03-2009, 09:15 PM
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Default Re: Stupid Science Questions Thread

Why are there no natural blondes with brown eyes?
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Old 08-03-2009, 10:27 PM
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Why are there no natural blondes with brown eyes?
None at all?

None?
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