interesting fizyks

[salividor]

couple of questions i came across on my travels:
1) why do molecules of the air not fall to the ground and stay there? (i have a feeling it may be due to convection and pressure but not sure)
2)why does the top of a pond freeze long before the middle and bottom?
3)finally why do stars look red, white or blue but never other colours like green etc.

any ideas would be cool, cheers

[the_marq]

2)why does the top of a pond freeze long before the middle and bottom?

Because that is where the cold air is, touching the top of the water. Heat is drawn out of the top layer of water first, therefore it freezes first. In time that layer of ice may even insulate the lower water from freezing.

3)finally why do stars look red, white or blue but never other colours like green etc.

I would say that they do appear all colours but green may be less common as it is lower on the visible light spectrum than red(roygbiv). The reasons we see coloured stars is because the light coming from stars is filtered via our atmosphere.

Those are my two stabs at your questions, as for the first one.... I'd only have guess as to why that is, bit I am sure some other TFP'er will step up.

[Charlatan]

Originally Posted by the_marq

Because that is where the cold air is, touching the top of the water. Heat is drawn out of the top layer of water first, therefore it freezes first. In time that layer of ice may even insulate the lower water from freezing.

I don't think that is the right answer. It has more to do with the unique property of water. When it freezes it expands whereas most other things when they freeze contract. Ice is less dense than water so Ice can float on the surface...

This is particularyly useful in that if ice sank to the bottom, most of the life there would die... Many people point to this unique property as proof of intelligent design...

[the_marq]

Originally Posted by Charlatan

I don't think that is the right answer. It has more to do with the unique property of water. When it freezes it expands whereas most other things when they freeze contract. Ice is less dense than water so Ice can float on the surface...

This is particularyly useful in that if ice sank to the bottom, most of the life there would die... Many people point to this unique property as proof of intelligent design...

I see your point about ice expanding and such, but the question was simply "Why does the top freeze first?"

The answer, cause it's closest to the cold. Let's not make this more complex than it needs to be.

[kutulu]

Convection has nothing to do with it. Convection is a means of heat transfer. Pressure and temperature dictate the volume of a gas. Gasses disperse in order to equilize pressure and temperature gradients.

The top of the pond freezes because that is the area exposed to the cold temperature. The heat is going from the pond to the air. The ground that surrounds the pond insulates the bottom of the pond.

Don't put ID into this.

[C4 Diesel]

Originally Posted by kutulu

Convection has nothing to do with it. Convection is a means of heat transfer. Pressure and temperature dictate the volume of a gas. Gasses disperse in order to equilize pressure and temperature gradients.

Close, but not quite (or just overly simple)... Yes, pressure and temperature gradients do tend to be equalized in free gases, but the underlying reason is molecular motion. However, due to gravity we have a kind of equilibrium between the molecular motion which disperses the gas (equalizing the pressure) and the force of gravity pulling the molecules down. This is why the lower atmosphere is more dense than the upper atmosphere - to an extent, the air is pulled down and it is staying down. It just happens that the more "down" you go, the more air is there, so the air above it is more likely to bounce back up as there is less room for a particle to fall without hitting another particle.

Oh, and the stars emit different wavelengths of light (thus different colors) based on their material composition and how hot they are. Although part of this is our atmosphere, they still have color outside of our atmosphere. The only colors that I've heard of stars being are white, yellow, blue, red, orange, and brown (rare). You'd think that with the wavelegnth for green being between yellow and blue, which are common colors in stars, that there would be green stars. I'd assume this does not happen because the materials which comprise stars do not have energy levels which, when reducing from one to another, would correspond to an energy release equal to the amount of energy required to produce a photon in the wavelength ranges corresponding to the colors green or purple.

[stingc]

Originally Posted by the_marq

I would say that they do appear all colours but green may be less common as it is lower on the visible light spectrum than red(roygbiv). The reasons we see coloured stars is because the light coming from stars is filtered via our atmosphere.

No. A star's color has to do with its temperature. Any hot object gives off light. This light is spread over a wide range of wavelengths (colors), although it does have a peak. Stars can have a peak intensity in the green part of the spectrum, but the range of wavelengths given off is so wide that our eyes don't interpret that as green. It usually looks more like white or yellow.

A red star is not very hot, so its peak is usually in the (near-) infrared. There is still enough radiation in the red region of the spectrum that we'll see the star as red, but there isn't enough at higher frequencies for our eyes to get "confused." A similar thing happens with blue stars.

As for the pond, it is a combination of what Charlatan and others said. No matter how the water is cooled, the coldest portion will be the least dense, and will therefore rise to the top (this only works near the freezing point - the opposite effect happens in warm weather).

Think about the gas as a collection of very quickly moving molecules bouncing around. They are falling down (and flying up) all the time. Gravity does affect this, which is (one reason) that the air pressure decreases with increasing altitude. The molecules lose speed as they fly upwards and gain speed when falling down. The macroscopic description is rather different, so I won't try to confuse you.

edit: C4 beat me to it Gas molecules don't hit each other very often though.

Also, energy levels have nothing to do with this. Thermal radiation is a separate process. Energy levels only give miniscule corrections to an overall spectrum that is nearly continuous.

[kebo]

just as a side note, a "twinkling" star (you know, the ones that seem to change colors) can sometimes be attributed to the motion of the star causing what is known as the doppler effect. As a star moves towards the earth, the light waves are compressed and the color is shifted toward blue. As it is moving away, the opposite happens and the colors shift toward red. This google has lots of cool info about the doppler effect.
kevin

[C4 Diesel]

Originally Posted by stingc

Also, energy levels have nothing to do with this. Thermal radiation is a separate process. Energy levels only give miniscule corrections to an overall spectrum that is nearly continuous.

Okay, now this confuses me... Well, maybe.

You say the color is a function of how hot the star is, but isn't the light produced still caused by electrons dropping energy states and releasing the energy as photons? I was of the opinion (and I don't know where I got this from) that stars are not comprised of many compounds - eg. the sun is made of primarily helium if I remember correctly. So the colors would be limited to the photons that could possibly be released from drops in the limited number of energy states of the primary element, or it's ionic form which I believe is also commonly the case.

If I have my facts straight, I don't know how energy levels could have nothing to do with a star's color (or only give "miniscule corrections"). I'm fairly certain that the only way to produce a photon is by changing the energy level of a subatomic particle, so the energy levels of the electrons in the atoms (and the frequency at which they are occupied and vacated) would have to directly determine the color.

. . . Someone who knows more about this than me, please set me straight.

[MageB420666]

The more energy being released, the hotter the star is. This helps determine the colour of the star, along with material composition, they aren't just hydrogen and helium you know.

As for the pond thing, the cold water is actually more dense than the warmer water, so it sinks, up until a certain point near the freezing temperature, where the water actually begins to expand. So the colder water at the bottom actually begins to rise to the top at this point where it then freezes, forming a layer of ice, now the ice acts as an insulator for the rest of the body of water, and the thicker the ice, the more it insulates the water underneath, so the colder it gets, the thicker the ice layer gets, and the harder it becomes to freeze the water underneath. And the ground acts as an insulator also, even in some of the more polar regions, the ground is only frozen for a few feet down.

As for the atmosphere, there are many reasons that the air does not just settle. Convection as the air is heated and cooled, molecular motion, and the rotation of the earth are the ones that I can think of right now. Convection causes the gases to heat and sink as they cool and heat, molecular motion causes the molecules to bounce around one another, never becoming still, and the rotation of the earth just mixes up the whole mess, kind of like the blades in a blender.

[Hain]

Well just clarificaion on what everyone else has been saying:

Water at the top of the pond is exposed to the cold air before the bottom is and that wonderful property of ice to float keeps it there.

A star' color is based on its temperature and temperature will create a variety of wavelengths, some are just over powered by others to create different ones. C4, the reason thermal radiation is a different process because that is when you describe electrons emmitting light. The sun produces light from the intense radiation created from the nuclear fires in it's depths. Plasma emmits light because it is stripped of it's electrons. Are you thinking of that?

As to why gas doesn't settle is because it is not dense enough to. There is enough space in between gas molecules to allow it to ZING around. It's density as mentioned is affected by it's relative height above the earth's surface. Just like water in an ocean: the deeper you go the more pressure there is at that depth because it has more water above it. The same applies to gas, its denser here with more pressure because there is more gas above it.

Make sense? Easy as differential equations!

[munchen]

With regard to the stars, stars fundtion like black bodies where the temperature determines the peak wavelenght of light emitted. Someones else will have to explain about black bodies, i'm no physics talkin' guy.

The only colors that I've heard of stars being are white, yellow, blue, red, orange, and brown (rare).

Actually the so called "Brown dwarf stars" are not really stars. They are just massive planets that fall just short of the mass neccessary to start nuclear reactions.

[stingc]

Originally Posted by kebo

just as a side note, a "twinkling" star (you know, the ones that seem to change colors) can sometimes be attributed to the motion of the star causing what is known as the doppler effect. As a star moves towards the earth, the light waves are compressed and the color is shifted toward blue. As it is moving away, the opposite happens and the colors shift toward red. This google has lots of cool info about the doppler effect.
kevin

Twinkling is caused by our atmosphere. The doppler effect would not be noticed because it is not (usually) changing very quickly. It is also extremely small for most stars visible to the naked eye, and requires special equipment to measure. It is recognized by looking at the shift in spectral lines.

C4, the simple picture of electrons jumping energy levels is really only applicable to diffuse gases. Things get much more complicated once the densities get high enough that the different atoms start interacting with each other. The system then acts more like a bunch of charged particles attached to each other with springs (very roughly of course). Quantizing spring-mass systems gives just the right energy levels to get a continuum of radiation in the limit that the number of particles is very large (as it is in any macroscopic object). It may sound counterintuitive that you can get a continuum of radiation with quantized energy levels, yet it turns out that if you try to use the classical theory, you get an infinite amount of radiation! This was one of the things that originally motivated the development of quantum mechanics.

A star is actually a fairly complicated system, and has both continuum and discrete features in its spectrum. This is made even more difficult by the fact that a lot of light escapes from regions with very different temperatures. Anyway, the continuous portion of the spectrum is essentially what determines color for our eyes, and it is mostly independent of the star's composition.

[C4 Diesel]

Okay... I kind of understand. Thanks, stingc.

...Crazy advanced physics is one of those things that I would like to learn, but have no time to at this stage of the game. I've always said I'd get a degree in physics for fun when I retire.

[Paradise Lost]

Originally Posted by MageB420666

The more energy being released, the hotter the star is. This helps determine the colour of the star, along with material composition, they aren't just hydrogen and helium you know.

Umm, I don't think this could be right. A Red Supergiant star is releasing
ALOT of energy, but for the most part, it's no where near the temperature of
lets say, a Blue Giant (the hottest type of star I know of.)

Also, the only things producing reactions in the star, as far as I know, are
deep in the core, and consist of Hydrogen fusing together to make Helium,
sometimes with something left over.

[Master_Shake]

Water at the top of the pond is exposed to the cold air before the bottom is and that wonderful property of ice to float keeps it there.

I don't think that's entirely correct. I think MageB420666 was on the right track and that it has to do with pressure. The water below the surface is under more pressure. Pressure reduces water's freezing point, thus the water at the bottom may actually be colder than the water at the top but not freeze due to pressure.

http://www.lsbu.ac.uk/water/explan2.html#Pvisc

Interestingly enough, the minimum temperature that liquid water can exist without ever freezing is -21.985°C at 209.9 MPa.

The shit they didn't teach me in high school astounds me.

[C4 Diesel]

No, shake... While the thought of the water below the surface being under high pressure is a noble thought, that's not why it doesn't freeze first. The reason the top freezes first IS because it gets cold first. The cold comes from the air, which is above the water, and the resulting temperature profile would indicate that the water on the surface is the coldest, and would also therefore freeze first. The water on the top insulate the water on the bottom and it does not cool evenly.

Not to mention that 210 MPa is about 2,000 atm. Not even deepest parts of the oceans comes under that much pressure from water "weight". If I remember correctly, it takes 33ft of water to increase the pressure by one atmosphere, and one (or a few) atmosphere(s) will not significantly lower the freezing point of water.

[Hain]

Originally Posted by C4 Diesel

No, shake... While the thought of the water below the surface being under high pressure is a noble thought, that's not why it doesn't freeze first. The reason the top freezes first IS because it gets cold first. The cold comes from the air, which is above the water, and the resulting temperature profile would indicate that the water on the surface is the coldest, and would also therefore freeze first. The water on the top insulate the water on the bottom and it does not cool evenly.

Not to mention that 210 MPa is about 2,000 atm. Not even deepest parts of the oceans comes under that much pressure from water "weight". If I remember correctly, it takes 33ft of water to increase the pressure by one atmosphere, and one (or a few) atmosphere(s) will not significantly lower the freezing point of water.

Actually the increase of pressure would allow for ice to freeze quicker. But that is a patsy answer since to increase pressure would lower the temperature on a closed environment. And you can treat the oceans for instance as a closed environment because of the depths involved. But again, the TOP FREEZES FIRST BECAUSE IT IS EXPOSED TO COLD AIR FIRST. Like any good chemistry student you should know that to increase the surface area where a reaction is taking place is to speed it up (like stirring!). Since the top of the pond is not stirring it freezes and then the ice as it gets colder forces the water beneath it to freeze in layers.

[C4 Diesel]

Originally Posted by Augi

Actually the increase of pressure would allow for ice to freeze quicker. But that is a patsy answer since to increase pressure would lower the temperature on a closed environment. And you can treat the oceans for instance as a closed environment because of the depths involved.

A little off, Augi... First off, an increase in pressure would, if anything, INCREASE the temperature, however the relation between T and P is generally only significant for gases, not liquids. Secondly, the increased pressure does allow for freezing point depression in water since ice, unlike most solids, is LESS dense than its liquid counterpart (increased P favors the more dense state).

[Jrock]

Originally Posted by Paradise Lost

Umm, I don't think this could be right. A Red Supergiant star is releasing
ALOT of energy, but for the most part, it's no where near the temperature of
lets say, a Blue Giant (the hottest type of star I know of.)

Also, the only things producing reactions in the star, as far as I know, are
deep in the core, and consist of Hydrogen fusing together to make Helium,
sometimes with something left over.

You are partly correct. A red giant produces more energy than a white dwarf. But a red giant is huge, we're talking as big as the orbit of mars, so the energy is more spread out, so the temperature at it's surface is lower than that of a white dwarf. The dwarf produces less total energy, but it's only the size of a city, so the temperature at it's surface is very high.

Yes, the only thing producing energy is nuclear fusion in the core, but the rate at which it fuses hydrogen is a function of a stars mass. A more massive star has more gravity, which pulls the molecules in harder. More pressure means a faster rate of fusion, which means more energy. More massive stars burn hotter, and are usually blue in color. A mid sized star like our sun isn't quite as hot and is yellow. A low mass star will be red dwarf.

Red giants are a different thing, though. When a sunlike star fuses enough hydrogen into helium, it will start fusing the helium. Helium fusion produces a lot more energy. The star's gravity can't overcome this energy, so the star expands. It is producing a lot more energy, but at it's surface, it's spread out over a larger area so it's not as hot at the surface, and thus it's red color. When the red giant is done fusing helium, it's done. No longer producing any energy, it colapses into a white dwarf. Not puting out nearly as much energy, but much lower surface area, and thus hotter at the surface.

[Charlatan]

Originally Posted by kutulu

The top of the pond freezes because that is the area exposed to the cold temperature. The heat is going from the pond to the air. The ground that surrounds the pond insulates the bottom of the pond.

Don't put ID into this.

God! I would never bring ID into this...

Honestly I was just poking fun.

I agree that the surface freezes first because it is exposed to air, BUT the reason it stays at the surface is because ice floats (which is unique)

[fckm]

Originally Posted by C4 Diesel

Okay, now this confuses me... Well, maybe.

You say the color is a function of how hot the star is, but isn't the light produced still caused by electrons dropping energy states and releasing the energy as photons? I was of the opinion (and I don't know where I got this from) that stars are not comprised of many compounds - eg. the sun is made of primarily helium if I remember correctly. So the colors would be limited to the photons that could possibly be released from drops in the limited number of energy states of the primary element, or it's ionic form which I believe is also commonly the case.

If I have my facts straight, I don't know how energy levels could have nothing to do with a star's color (or only give "miniscule corrections"). I'm fairly certain that the only way to produce a photon is by changing the energy level of a subatomic particle, so the energy levels of the electrons in the atoms (and the frequency at which they are occupied and vacated) would have to directly determine the color.

. . . Someone who knows more about this than me, please set me straight.

You are correct for systems with few electrons, but stars have way too many electrons to model it that way. Instead, stars are blackbodies and radiate accordingly.

[C4 Diesel]

<-- never learned anything about blackbodies and/or blackbody radiation.

[fckm]

http://en.wikipedia.org/wiki/Black_body

Eventhough it's called a black-body, most macroscopic objects can be considered blackbodies and high temperature objects even more so. The idea is that light radiated from an object (as opposed to reflecting off an object) is related to it's thermal energy. An object is 'black' only at zero degress. At any finite temperature, it emits a spectrum of light that is dependant on the temperature.

[fckm]

A better way of thinking about it is thus. Helium has a certain spectrum. That is, it only radiates light at certain wavelengths. However, the emitted spectrum covers a pretty hefty span. At lower temperatures, the higher energy transitions don't occur as often, so the color emited by helium would correspond to larger wavelength colors. At higher and higher temperatures, larger energy transitions become more likely, so sorter wavelength light is emitted in addition to the longer wavelengths. This changes the apparent color of helium.
Also, remember that in a star, the energy is large enough to ionize atoms. Ionized atoms/free electrons no longer have quantized energy, so they can radiate at whatever wavelength they want.

[Stompy]

As for the color of stars.. doesn't it have to do with how fast they're moving in relation to us?

I was reading something that said.. if you could go the speed of light, things in front of you would be blue while things behind you would be red. It later went to give examples of red/blue stars.

[Jrock]

Originally Posted by Stompy

As for the color of stars.. doesn't it have to do with how fast they're moving in relation to us?

I was reading something that said.. if you could go the speed of light, things in front of you would be blue while things behind you would be red. It later went to give examples of red/blue stars.

How an object is moving relative to us changes it's color, but isn't what causes it's color. Stars can be almost any color based on temerature and composition. An object moving toword us will be bluer than normal, and an object moving away will be redder.

[MageB420666]

Originally Posted by Stompy

As for the color of stars.. doesn't it have to do with how fast they're moving in relation to us?

I was reading something that said.. if you could go the speed of light, things in front of you would be blue while things behind you would be red. It later went to give examples of red/blue stars.

That is the doppler affect and it just causes a shift towards that color, not for it to be that color. To sum it up as simply as I can:

A star emits light waves, if the star is moving towards the observer, the waves get compressed together, increasing their frequency, if the star is moving away from the obsever, the waves are stretched, decreasing their frequency. And since light color is based upon it's wavelength, this shifts the color of the light either towards blue or red.