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Author Topic:   A science question
Silent H
Member (Idle past 5819 days)
Posts: 7405
From: satellite of love
Joined: 12-11-2002


Message 136 of 148 (191267)
03-13-2005 6:30 AM
Reply to: Message 134 by Percy
03-12-2005 2:14 PM


Re: journey to the center of the earth
No one thinks the atmosphere should be hotter than the core, and I can't think of anything I or Sylas said that would make you think we were arguing for that. If you can find the place where we said something that led to this misimpression then maybe I can clarify.
This is definitely the impression I was getting. You had mentioned it at least twice but we can deal with the last comment you made (which renewed my interest)...
We know this because the temperature gradient within the earth goes from cool on the outside to hot on the inside. If the earth were getting hotter then the reverse would be true, with the outside being hotter than the inside.
The only way this reads to me is that if the earth was getting hotter, the outside must be hotter than the inside.
Heat cannot be trapped. Short of a vacuum insulating layer, a temperature gradient is *always* indicative of conductive heat flow.
Now you're a stickler for language? I have already said I know that the only perfect insulator and so perfect "heat trap" (at least as far as conductivity is concerned) is a vacuum. I even used the styrofoam cup as an example of the kind of "trapping" I am talking about.
While a gradient certainly indicates that heat flow will occur, the amount of heat flow can be minimized (not negated) due to material properties, and movement of materials. This means that due to a slower rate of convection than if it was simply a solid block of the same material, we can have a situation where temperatures are higher in certain regions for long periods of times. I am suggesting this kind of "trapping". It does not have to be perfect or permanent, just a slowing such that a gradient is retained for a long time.
Heat flows along the gradient, but the temperature of the layers doesn't change.
I do not believe we can say that at all. According to the history of the atmosphere at Wikipedia it suggests that layers have changed in temperature and as a result, composition, which further alters temperatures.

holmes
"...what a fool believes he sees, no wise man has the power to reason away.."(D. Bros)
"...don't believe I'm taken in by stories I have heard, I just read the Daily News and swear by every word.."(Steely Dan)

This message is a reply to:
 Message 134 by Percy, posted 03-12-2005 2:14 PM Percy has not replied

Replies to this message:
 Message 137 by Sylas, posted 03-13-2005 8:10 AM Silent H has replied

  
Sylas
Member (Idle past 5260 days)
Posts: 766
From: Newcastle, Australia
Joined: 11-17-2002


Message 137 of 148 (191271)
03-13-2005 8:10 AM
Reply to: Message 136 by Silent H
03-13-2005 6:30 AM


Re: journey to the center of the earth
While a gradient certainly indicates that heat flow will occur, the amount of heat flow can be minimized (not negated) due to material properties, and movement of materials. This means that due to a slower rate of convection than if it was simply a solid block of the same material, we can have a situation where temperatures are higher in certain regions for long periods of times. I am suggesting this kind of "trapping". It does not have to be perfect or permanent, just a slowing such that a gradient is retained for a long time.
I don't see what you are getting at.
If heat is "trapped", then whatever is being heated gets hotter. This increases the gradient, until the flow is balanced again. The temperature gradients are driven to whatever is needed for energy to escape the system at the same rate it is produced. Any excess or underflow will inevitably alter change the temperature gradient; it is a feedback loop.
In the atmosphere, the situation is more complex due to radiation, and also transport of water vapour; but it is still a balance of input and output.
Within the Earth it is simpler. The temperature gradient below the Earth's surface MEANS that heat is flowing out. I'm not sure if you understand that bit yet. Insulation around a source of energy (like the core, with radioactive decay producing energy) does not actually "slow" the release of energy. It just forces a higher gradient to drive the heat flow required to release the energy.
Indeed, the temperature gradient was used by Lord Kelvin in the late nineteenth century to "prove" that the Earth was only twenty to a hunred thousand years old. In fact the Earth is several billion years old, because it is not "cooling". It is releasing heat being generated by decay. We can calculate how mush by knowing the temperature gradient and also the insulating properties of the rock.
Cheers -- Sylas

This message is a reply to:
 Message 136 by Silent H, posted 03-13-2005 6:30 AM Silent H has replied

Replies to this message:
 Message 139 by Silent H, posted 03-13-2005 6:33 PM Sylas has replied

  
Percy
Member
Posts: 22392
From: New Hampshire
Joined: 12-23-2000
Member Rating: 5.3


Message 138 of 148 (191276)
03-13-2005 9:02 AM
Reply to: Message 135 by Sylas
03-13-2005 2:41 AM


Re: journey to the center of the earth
Sylas writes:
Within the Earth the matter is simpler. There is no radiation...
It occurred to me recently that this probably isn't true. A molecule on the inside of a solid probably emits as much radiation as a molecule on the solid's outer surface. Radiation emitted from molecules within the solid are probably reabsorbed by nearby molecules. Such radiation must also play a role in heat conduction in a solid.
I need a mental image for conveying heat through mechanical conduction in a solid. Does a molecule in a solid that is vibrating with "heat" transmit that vibrational energy to adjacent molecules through its bonds, i.e., through the interaction of its electrons in the outer energy levels? Or is it via some other means?
Now that I think about it, my mental image for radiation being emitted and absorbed as a means of conveying heat isn't that good, either. When a photon strikes a molecule, I imagined it being absorbed by the electrons in the outer levels of the atoms of the molecule. The electrons have the option of jumping to a higher energy level, and they can re-emit a photon by falling back to a lower energy level, but what happens when the molecule absorbs the photon and increases its kinetic energy? How does that happen?
I poked around on the web a bit, and one of the explanations I found said that collisions between atoms can cause an electron to rise to a higher energy level. When it drops back down a photon is emitted. But what is it about the mechanical collision between atoms that causes the electron to rise to a higher energy level. The higher energy level of the electron of one atom has to be matched by a decrease in energy level in some form but in the same amount in the other atom. What is it that happened to that other atom? Did one of its electrons drop to a lower energy level? Is the atom now moving more slowly? If this latter possibility is the case, then how did kinetic energy become transformed into the higher energy level of an electron?
Of course, logic and common sense suggest that atoms are really just miniscule billiard balls and we can skip all this other complicated nonsense!
--Percy

This message is a reply to:
 Message 135 by Sylas, posted 03-13-2005 2:41 AM Sylas has not replied

Replies to this message:
 Message 145 by Brad McFall, posted 03-14-2005 8:58 PM Percy has not replied

  
Silent H
Member (Idle past 5819 days)
Posts: 7405
From: satellite of love
Joined: 12-11-2002


Message 139 of 148 (191330)
03-13-2005 6:33 PM
Reply to: Message 137 by Sylas
03-13-2005 8:10 AM


Re: journey to the center of the earth
If heat is "trapped", then whatever is being heated gets hotter.
Maybe I need to find another word than trapped. Let me try and start clean...
What we are talking about is a temperature gradient moving from the core toward the atmosphere. So yes, it is understandable that the energy which causes the high temps within the core will be moving outward toward the surface and into the atmosphere.
However, the fact that energy is moving toward the surface does not mean that it will always coincide with an equivalent temperature rise as it moves outward.
First, by nature of geometry there is succeedingly greater amounts of material above than below. That means the core is a smaller mass, heating a larger mass.
Second, the material above does not have to simply use the energy it receives to gain in temperature, it does move (usually expanding) and creates currents and upwellings which reduce the amount of energy it has to impart to the next layer.
Third, materials may differ in how much energy they can absorb before ultimately transmitting the energy through to the next layer in the form of raising temperatures. This means that it may take time to get temperatures equalized between layers. As I analogized before, you can put a lighter in the center of the earth and let it run in a closed system, one might not see a rise in any temperature beyond the core even after billions of years.
Fourth, equalization is complicated even more based on the fact that some layers can move and use energy to do something other than increase in temperature.
So when I say temperatures or heat flow may be "trapped", I am suggesting that the flows may be slowed or unevenly distributed based on the physical makeup of the system such that one can see gradients last for long periods of time. It is like the lighter being trapped at the center of the earth... yes heat is always being radiated out, but its progress may be slowed or hindered compared to what we would see in other systems.
I do understand that in any isolated system, eventually temperatures will even out, because eventually energy will even out throughout the system. But the amount of time this will take depends on the system, doesn't it? During the time of "evening out" various pockets of high temps could exist.
I guess what i am saying is that I am not seeing how we have the knowledge that just because there is a gradient we have proved that earth is definitely cooling as an entire system. Do we really know that enough time has passed such that everything should have evened out by now, or that such a large gradient could not exist?
Especially hard for me is to accept any references to the atmosphere in comparison to the core. The atmosphere is free to move and expand and thus "cool" itself by using the energy in ways other than simply staying in place and gaining in temp.
In fact the Earth is several billion years old, because it is not "cooling". It is releasing heat being generated by decay. We can calculate how mush by knowing the temperature gradient and also the insulating properties of the rock.
I can accept this way of handling why we can know if it is cooling or not, though now I am confused as it seems you are saying it is not cooling.

holmes
"...what a fool believes he sees, no wise man has the power to reason away.."(D. Bros)
"...don't believe I'm taken in by stories I have heard, I just read the Daily News and swear by every word.."(Steely Dan)

This message is a reply to:
 Message 137 by Sylas, posted 03-13-2005 8:10 AM Sylas has replied

Replies to this message:
 Message 140 by Sylas, posted 03-13-2005 7:08 PM Silent H has replied

  
Sylas
Member (Idle past 5260 days)
Posts: 766
From: Newcastle, Australia
Joined: 11-17-2002


Message 140 of 148 (191337)
03-13-2005 7:08 PM
Reply to: Message 139 by Silent H
03-13-2005 6:33 PM


Re: journey to the center of the earth
I guess what i am saying is that I am not seeing how we have the knowledge that just because there is a gradient we have proved that earth is definitely cooling as an entire system. Do we really know that enough time has passed such that everything should have evened out by now, or that such a large gradient could not exist?
While acknowleding the various complexities you mention about how heat is transported; we do know from the large temperature gradient that Earth is radiating more energy than it receives. There is a flow of energy from the core to the surface, as shown by the gradient, and energy is not accumulating at the surface, as shown by the more or less fixed temperature over a scale of years.
I can accept this way of handling why we can know if it is cooling or not, though now I am confused as it seems you are saying it is not cooling.
There could be two possible sources of the heat flowing up to the surface from the core. One possibility is that the Earth is cooling from an initially molten state. Another is that there is some continuous source of energy within the Earth which is maintaining its high temperature.
If there was no source of energy, the Earth would have long since cooled to solid cold rock. It would take hundreds of thousands of years; but the Earth is over 4 billion years old. This was a problem for geology in the nineteenth century; geology made the Earth seem old, physics and thermodynamics suggested it was much younger.
But there is a source of energy. The heat of the core is maintained by radioactive decay, and this energy transports to the surface as heat flow; by both conduction and convection. The temperature gradient is a consequence of this steady flow of heat, and the gradient is an infallible indicator of a flow of heat energy from the core to the surface.
Cheers -- Sylas

This message is a reply to:
 Message 139 by Silent H, posted 03-13-2005 6:33 PM Silent H has replied

Replies to this message:
 Message 141 by Silent H, posted 03-14-2005 7:49 AM Sylas has not replied
 Message 142 by sfs, posted 03-14-2005 10:28 AM Sylas has replied

  
Silent H
Member (Idle past 5819 days)
Posts: 7405
From: satellite of love
Joined: 12-11-2002


Message 141 of 148 (191404)
03-14-2005 7:49 AM
Reply to: Message 140 by Sylas
03-13-2005 7:08 PM


Re: journey to the center of the earth
There is a flow of energy from the core to the surface, as shown by the gradient, and energy is not accumulating at the surface, as shown by the more or less fixed temperature over a scale of years.
Okay this makes more sense. Like I said I was having problems that the gradient itself proves that the earth system is cooling.
If what is being said is that the gradient shows that heat/energy is moving from the core (a continual source of energy) to the surface and we know that the energy has not been accumulating (fixed temperature at surface being different than just saying there is a temperature difference between core and surface) then clearly it must be cooling.
Here are my theoretical issues with saying we do know that the atmosphere is not accumulating energy:
1) As was stated in the wikipedia history of our atmosphere, this is essentially our third atmosphere. The first one was theorized to have been blown off in part because of the energy being pumped into it. The second and third show that our atmosphere has been continually changing and perhaps it is due to its "handling" of energy being pumped into it. Stable temps may not necessarily indicate that energy is not being accepted by a system.
2) What do we really know about temperatures within our atmosphere? While we may have good data regarding surface temperatures for some time (not sure if it stretches to a billion years though) we cannot say what the total atmospheric temperature was like. It could be that surface temps stay relatively stable as any increases in temp cause a movement upward to higher layers. Other atmospheric layers vary in temperature and I believe the last one gets very hot. The thickness and temperature of that or other layers may have been changing over time.
In any case, I at least see where the argument is coming from if we are talking about stability of temps, rather than just that there is still a gradient.

holmes
"...what a fool believes he sees, no wise man has the power to reason away.."(D. Bros)
"...don't believe I'm taken in by stories I have heard, I just read the Daily News and swear by every word.."(Steely Dan)

This message is a reply to:
 Message 140 by Sylas, posted 03-13-2005 7:08 PM Sylas has not replied

  
sfs
Member (Idle past 2533 days)
Posts: 464
From: Cambridge, MA USA
Joined: 08-27-2003


Message 142 of 148 (191423)
03-14-2005 10:28 AM
Reply to: Message 140 by Sylas
03-13-2005 7:08 PM


Re: journey to the center of the earth
quote:
While acknowleding the various complexities you mention about how heat is transported; we do know from the large temperature gradient that Earth is radiating more energy than it receives.
While this is a reasonable conclusion, do note that it does not actually follow rigorously. The sun is also radiating more energy than it receives, but it has the reverse temperature gradient, at least in the chromosphere and corona. That is, the temperature rises with increasing distance from the center of the sun.

This message is a reply to:
 Message 140 by Sylas, posted 03-13-2005 7:08 PM Sylas has replied

Replies to this message:
 Message 143 by Percy, posted 03-14-2005 12:06 PM sfs has not replied
 Message 144 by Sylas, posted 03-14-2005 3:26 PM sfs has not replied

  
Percy
Member
Posts: 22392
From: New Hampshire
Joined: 12-23-2000
Member Rating: 5.3


Message 143 of 148 (191446)
03-14-2005 12:06 PM
Reply to: Message 142 by sfs
03-14-2005 10:28 AM


Re: journey to the center of the earth
The chromosphere and corona are outer solar layers above the photosphere (the sun's surface). The temperature of the sun's core is around 13 million oK, while the temperature of the sun's surface is around 6000oK, so that's a large and significant temperature gradient. The reasons behind the high temperature of the corona, around 5 million oK, are not yet fully understood, but it is far less dense than the sun, it produces far less radiation, and it's fairly transparent to much of the energy leaving the sun. It's a blip - or maybe I can use my new favorite term, vanishingly dinky - contribution to the thermodynamic profile of the sun. Most of the energy from the sun reaches the surface by convection and conduction - once the energy reaches the surface and can radiate away as EMR the largely transparent chromosphere and corona have little impact.
--Percy

This message is a reply to:
 Message 142 by sfs, posted 03-14-2005 10:28 AM sfs has not replied

  
Sylas
Member (Idle past 5260 days)
Posts: 766
From: Newcastle, Australia
Joined: 11-17-2002


Message 144 of 148 (191489)
03-14-2005 3:26 PM
Reply to: Message 142 by sfs
03-14-2005 10:28 AM


Re: journey to the center of the earth
While this is a reasonable conclusion, do note that it does not actually follow rigorously. The sun is also radiating more energy than it receives, but it has the reverse temperature gradient, at least in the chromosphere and corona. That is, the temperature rises with increasing distance from the center of the sun.
To the best of my limited knowledge, the same principles as apply for the Earth also apply for the Sun. There is a large temperature gradient through the portions of the Sun that are opaque to radiation. This means that the Sun's surface is receiving energy from the inner levels, and that from there it radiates out.
The temperature profiles for the atmosphere are different, in both the Sun and the Earth, because they are in a region where there is considerable radiative transfer. We know that the radiation transfer is outwards given the gradients within Sun and Earth.
Cheers -- Sylas
PS. Added in edit. This post should not be taken as disagreement... sfs is right that temperature profiles in the atmospheres of Sun and Earth sometimes go in reverse. For example, the very outmost parts of Earth's atmosphere have temperature rising with altitude; and the same effect is seen in the stratosphere. The causes of this are beyond my scope to explain; but the fact that there is a of energy flowing through the amosphere without stopping (radiation) is significant. There are also other effects; such as the latent heat in water vapour being carried into the atmosphere. It is also significant that the actual internal energies involved are very low, since the atmosphere has a very limited capacity to absorb heat. The heat flow by conduction is negligible by comparison with the radiation passing through.
Most of the atmosphere gets heated from the surface of the Earth, but the outer layers have a larger input direct from solar radiation. This (I think) basically accounts for the gradients.
For the Sun, the major cause of heating for the chronosphere seems to be magnetic fields. The high temperatures of the corona have long been a serious puzzle for physicists. They knew from thermodynamics that something was heating the corona; but they could not identify what it was. Thermal transfer is ruled out because of the gradient. The puzzle recently took a major step towards solution. See Solar Mystery Nears Solution With Data From SOHO Spacecraft.
This message has been edited by Sylas, 03-14-2005 04:37 PM

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Brad McFall
Member (Idle past 5033 days)
Posts: 3428
From: Ithaca,NY, USA
Joined: 12-20-2001


Message 145 of 148 (191564)
03-14-2005 8:58 PM
Reply to: Message 138 by Percy
03-13-2005 9:02 AM


Re: journey to the center of the earth
JC Maxwell attributed heat to a footnote of Clausius where James C was seemingly in a process of seperating "molecular potential" from the "vis viva" he quoted from Clausius. He wrote that in an essay on TAiTES Thermodyanmics textbook. Is it correct that(then?) we have the notion of heat in the kinetic theory of gas DIFFERENT from Clausius' prior VISVIVA? I found this in A SOURCE BOOK IN PHYSICS by Magie of Princeton published by Harvard in 65 under "The Second Law of Thermodynamics"
quote:
To this(the careful investigations of Joule) must be added that other facts have lately become known which support the view, that heat is not a substance, but consists in a motion of the least parts of bodies. If this view is correct, it is admissible to apply to heat the general mechanical principle that a motion may be transformed into work, and in such a manner that the loss of vis viva is proportional to the work accomplished."
I can think about this LOSS in terms of melanin and behavior in organisms but other (in)solids without life (earth, sun, etc) it seems that the quantum replacement notion(from kinetic theory of heat etc) for the clearer meaning of "least parts" of Clausius might actually be an error which physics CAN NEVER OVERCOME (as long as it is inadequately integrated with(in) biology? It seems the popular tendency has been to equate the loss of this isveevaaa with disorderand entropy increase???
where the inside of Maxwell's quote might be
This message has been edited by Brad McFall, 03-14-2005 09:13 PM

This message is a reply to:
 Message 138 by Percy, posted 03-13-2005 9:02 AM Percy has not replied

  
DS650Heavy
Inactive Member


Message 146 of 148 (348304)
09-12-2006 4:09 AM
Reply to: Message 39 by Percy
03-06-2005 9:30 AM


Re: IR and EM
"Naturally a vacuum presents no obstacle to EMR. Heat (actually, energy which only becomes heat when absorbed at the other end) can easily be transmitted across a vacuum using EMR."
I see a couple of inconsistencies here. Granted I am not an expert, but my understanding is that an object cannot possess heat, therefore EMR, specifically Infrared is heat because it IS in transit. When it reaches the other process it again becomes Temperature or internal energy. The last of your statement that "heat can easily be transmitted across a vacuum" via EMR contradicts your previous premise that
Since a vacuum is the absence of matter, heat conduction cannot take place through a vacuum, and therefore a vacuum is a perfect insulator". A perfect insulator resists the transfer of any form of energy. And even if we were to limit the statement to "Since a vacuum is the absence of matter, heat conduction cannot take place through a vacuum, and therefore a vacuum is a perfect thermal insulator" infrared radiation is a transfer of energy and, therefore, heat.
This has been an interesting read so far. I know I will have to do some brushup on my physics, but I'll be back
Thanks

This message is a reply to:
 Message 39 by Percy, posted 03-06-2005 9:30 AM Percy has not replied

Replies to this message:
 Message 147 by MangyTiger, posted 09-12-2006 4:18 PM DS650Heavy has not replied

  
MangyTiger
Member (Idle past 6353 days)
Posts: 989
From: Leicester, UK
Joined: 07-30-2004


Message 147 of 148 (348470)
09-12-2006 4:18 PM
Reply to: Message 146 by DS650Heavy
09-12-2006 4:09 AM


Re: IR and EM
Welcome to the forum DS650Heavy - I'm sure a friendly admin will be along presently to point you at helpful links showing how to use quote boxes and other nifty features to make posts easier to follow.
Since a vacuum is the absence of matter, heat conduction cannot take place through a vacuum, and therefore a vacuum is a perfect insulator.
(Bolding mine)
This statement by Percy seems right - heat conduction is by definition the transmission of heat across matter. Since a vacuum contains no matter there can be zero conduction across it and so it is perfect insulator with regard to heat conduction.
On the other hand a vacuum is clearly not a perfect insulator with regard to electromagnetic radiation (the clue is that big yellow thing you see in the sky when you look up on a sunny day ).
A perfect insulator resists the transfer of any form of energy. And even if we were to limit the statement to "Since a vacuum is the absence of matter, heat conduction cannot take place through a vacuum, and therefore a vacuum is a perfect thermal insulator" infrared radiation is a transfer of energy and, therefore, heat.
As I said above, you need to change this to "a vacuum is a perfect thermal insulator with regard to conduction".
Disclaimer - I haven't done any physics for 26 years so I may well be wrong !

Oops! Wrong Planet

This message is a reply to:
 Message 146 by DS650Heavy, posted 09-12-2006 4:09 AM DS650Heavy has not replied

  
fallacycop
Member (Idle past 5520 days)
Posts: 692
From: Fortaleza-CE Brazil
Joined: 02-18-2006


Message 148 of 148 (348509)
09-12-2006 7:42 PM
Reply to: Message 99 by Percy
03-07-2005 9:21 PM


Re: Have I Got It?
Heat is very definitely the kinetic energy of molecules. No matter how complex and detailed the definition, no matter what particular terminology you use, there is no other way to look at it. (Somebody is bound to think of a context to illustrate that I've overstated the case, but ignore them and just accept that for any situation you'll ever encounter in your life on this earth, heat is just the kinetic energy of molecules.)
I'm not sure about the definition of internal energy. It probably has a very definite context-dependent definition that I'm not going to bother looking up, but I don't think it is useful in a simple context. Even if we got the definition right today, it is certainly at a minimum vulnerable to being misremembered at a later date. For instance, say internal energy *is* just kinetic energy. Next month am I going to remember that internal energy doesn't include the energy of the chemical bonds of the molecule? Will I recall that it doesn't include the atomic energy? I don't trust my memory that much, so I prefer to stay with the familiar everyday concept of kinetic energy. If it were wrong to say heat is the kinetic energy of molecules then I'd make an effort to get it right, but heat *is* the kinetic energy of molecules.
Percy, this is not how heat is defined in physics. While you can have your own definition of heat if you want (it's a free world), it's usually considered a good idea to stick to old definitions to avoid confusion. What you are describing as heat is actually closer to the definition of internal energy which includes not only the kinetic energy of molecules but also any potential energy due to the interaction of these molecules (not too important for gases, but essential for understanding liquids and solids)
Sylas had it right. Heat is defined as energy in transit in either of three possible ways.
1) radiation (that is by definition EM energy and yes, that may include energy in the visible spectrum)
2) conduction (that is energy being transfered directly by colisions between molecules when two bodies come in contact)
3) convection (that is energy being caried by the flow of a fluid - no convection happens in a solid)
cheers

This message is a reply to:
 Message 99 by Percy, posted 03-07-2005 9:21 PM Percy has not replied

  
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