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  #1  
Old 07-11-2005, 06:26 AM
Charlie Tan Charlie Tan is offline
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Can cold radiate?

My physics classes are at least 25 years back and I slept through them. However, my hazy understanding of thermodynamics say that cold is the absense of heat, i.e. movement of atoms. The less they move, the colder something is.
So, I guess, heat radiates because the atoms that move interact with surrounding atoms. But since cold is things not moving, there can't be interaction. Is this correct?
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  #2  
Old 07-11-2005, 06:39 AM
Crafter_Man Crafter_Man is online now
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Sure, cold things radiate energy, as long as it's above absolute zero.

And energy will be transferred from something cold to something even colder.

Not sure if this answers your question...
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  #3  
Old 07-11-2005, 06:55 AM
CalMeacham CalMeacham is offline
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Having lived in Rochester, N.Y., I can swear that it feels as if cold radiates, but I've always understood this to be heat radiating in the other direction. Sorta like the darkon theory of light.
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  #4  
Old 07-11-2005, 06:56 AM
chrisk chrisk is offline
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Quote:
Originally Posted by The Gaspode
My physics classes are at least 25 years back and I slept through them. However, my hazy understanding of thermodynamics say that cold is the absense of heat, i.e. movement of atoms. The less they move, the colder something is.
So, I guess, heat radiates because the atoms that move interact with surrounding atoms. But since cold is things not moving, there can't be interaction. Is this correct?
Okay, first off, 'atoms that move interact with surrounding atoms' does not really describe the radiation of heat... that is conduction. If you put a pan of cold water on an hot element of an electric stove, for example, heat is efficiently conducted from the element to the pan, from the pan to the water, from the pan to the surrounding air, and so on. Air does conduct heat, but not terribly well.

Cold does not 'conduct' in the same way that heat does, but it can seem to because a cold object serves as such a good heat sink. Heat is conducted from everything nearby into the cold object, making it less cold and everything else cooler.

Radiation works differently, not atoms directly interacting with each other, but atoms sending off tiny little energy particles that make whatever they hit warmer. (And whatever has radiated will be cooler because it has lost that energy.)

Cool does not precisely 'radiate' either, but again there is a psychological perception that it does, because it is not radiating heat as intensely as nearby warmer objects, but is absorbing radiation from them. Everything at room temperature, for instance, is constantly radiating infrared light, bouncing back and forth, and the net effect of this low-level radiation generally cancels out. If you put a big block of cold metal into a room were everything else is normal temperature though, that block will be radiating much less than everything else, and this, (along with the fact that air is conducting heat into it) will make you feel cooler when you stand a few yards away from it.

Hope that this helps.
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  #5  
Old 07-11-2005, 10:14 AM
spingears spingears is offline
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Quote:
Originally Posted by Crafter_Man
Sure, cold things radiate energy, as long as it's above absolute zero.
And the surroundings are at a LOWER temperature.

Quote:
And energy will be transferred from something cold to something even older.
The amount being proportional to the ratio of the 4th power of the absolute temperatures.
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  #6  
Old 07-11-2005, 10:26 AM
chrisk chrisk is offline
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Quote:
Originally Posted by spingears
And the surroundings are at a LOWER temperature.

The amount being proportional to the ratio of the 4th power of the absolute temperatures.
Do you have a cite for that?? I thought that even an object surrounded by higher temperatures would radiate, just that it would, on average, absorb more radiation than it emitted.
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  #7  
Old 07-11-2005, 10:45 AM
scr4 scr4 is offline
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Quote:
Originally Posted by chrisk
I thought that even an object surrounded by higher temperatures would radiate, just that it would, on average, absorb more radiation than it emitted.
That's right. What spingears meant, I believe, is that there's a net loss of heat by radiation if the surrounding objects are colder.
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  #8  
Old 07-11-2005, 11:29 AM
CurtC CurtC is offline
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Isn't it true that you can think of the "cold" radiating, even though it's really just the absence of heat radiating? If you were placed in a cold-walled room (let's say with no air so that you can ignore conduction and convection), it would suck the heat out of you just like if you were in a warm room it would add heat to you. That's because you radiate heat to the walls, and they radiate heat back to you, but since the cold walls radiate less heat, there is a net heat transfer from you to the cold walls, exactly opposite to how warm walls would have a net radiation of heat to you.
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  #9  
Old 07-11-2005, 11:44 AM
Charlie Tan Charlie Tan is offline
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I didn't mean radiation, as in anything having to do with nuclear physics.

It's just that if I take a popsicle out of the freezer, it's 20C below freezing, and about 40C below room temperature. I have to almost put my hand on it to feel that it's cold. Moving to something that's 40C above room temperature, I'd feel that from further away.

And that room with no air - vacuum is used as insulation in thermos bottles. Why doesn't that function as a heat sink (which vacuum does in space, no?).
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  #10  
Old 07-11-2005, 12:15 PM
scr4 scr4 is offline
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Roughly speaking, there are three ways of heat transfer: conduction, convection and radiation.

Conduction means if two objects are in contact, there's a net flow of heat from the hotter object to the colder one. As a result, the hotter object gets cooled down and the colder object is warmed up. So in a sense, "cold" can be transferred by conduction.

Convection means hot objects transfer heat to the surrounding air, and the air moves around and carries that heat to other objects. Again, this results in a hotter object getting colder, and vice versa.

Radiation is as described above. All objects radiate heat (usually in the form of infrared radiation). If a hot object is surrounded by colder surfaces, the hot object loses more heat by radiation than it gains, so there's a net loss of heat.

Thermos bottles are designed to minimize all three methods of heat transfer. It's a double layer glass bottle, which means the inner bottle is only supported by the mouth of the bottle. This minimizes the heat path for conduction. The space between the two layers is evacuated (made into vacuum), which eliminates convection. And the glass layers are coated with metal, which emits very little infrared radiation and therefore minimizes radiative heating/cooling.

As for why you don't feel "cold" when you put your hand near a cold object, part of the reason is that radiation goes as 4th power of temperature, as already mentioned. The net heat transfer is proportional to the difference between the 4th power of temperature of the objects. So radiative heat transfer between a 340K object and 300K object is 50% stronger than between a 300K object and 260K object
((3404-3004) / (3004-2604) = 1.5).

Also, hot air rises, so if you put your hand above a hot object you get a direct blast of hot air. Cold air sort of dissipates towards the ground. And many "hot" objects we encounter have lots of water (e.g. hot coffee, freshly baked pizza), and steam transfers heat extremely well - it releases heat as it condenses on your hand. And many other "hot" objects around us are significantly hotter than body temperature (toaster oven, frying pan, etc).
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  #11  
Old 07-11-2005, 12:27 PM
chrisk chrisk is offline
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Quote:
Originally Posted by The Gaspode
I didn't mean radiation, as in anything having to do with nuclear physics.
Nothing to do with nuclear physics... but more conventional physics, yes.

Any object will give off 'thermal radiation'... the heat of the object determines how much it radiates and at what part of the spectrum. Think of a piece of metal glowing red-hot or a light bulb filament when the light bulb is turned on. Our furniture (and ourselves) don't give off radiation in the visible part of the spectrum, but they still radiate. (Apparently it has to do with the fact that all matter is made up of charged particles moving towards and away from each other... the motion of electric charge through electrically charged fields stimulates electromagnetic radiation.)

Hope this helps clarify what I meant by radiation.
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  #12  
Old 07-11-2005, 12:58 PM
David Simmons David Simmons is offline
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Quote:
Originally Posted by CurtC
Isn't it true that you can think of the "cold" radiating, even though it's really just the absence of heat radiating? If you were placed in a cold-walled room (let's say with no air so that you can ignore conduction and convection), it would suck the heat out of you just like if you were in a warm room it would add heat to you. That's because you radiate heat to the walls, and they radiate heat back to you, but since the cold walls radiate less heat, there is a net heat transfer from you to the cold walls, exactly opposite to how warm walls would have a net radiation of heat to you.
Not really. There is no "absense of heat radiating" at a temperature above the minimum of 0 K. Everything above that temperature radiates to some extent. If the thing is a perfect radiator/absorber (black body) it radiates according to the Stefan-Boltzman equation, I (radiation intensity) equals a constant multiplied by T4 (temperature). The value of the constant depends upon the units used for I and T.
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  #13  
Old 07-11-2005, 02:46 PM
chrisk chrisk is offline
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Quote:
Originally Posted by David Simmons
Not really. There is no "absense of heat radiating" at a temperature above the minimum of 0 K. Everything above that temperature radiates to some extent. If the thing is a perfect radiator/absorber (black body) it radiates according to the Stefan-Boltzman equation, I (radiation intensity) equals a constant multiplied by T4 (temperature). The value of the constant depends upon the units used for I and T.
for "absence of heat radiating", try substituting "relative scarcity of radiated heat"?
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  #14  
Old 07-11-2005, 04:03 PM
Napier Napier is offline
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>Isn't it true that you can think of the "cold" radiating, even though it's really just the absence of heat radiating?

Yes, this is true. I have worked this from differerent starting points and am sure of the conclusion. In fact it is fun to try to think of an experiment that would disprove the hypothesis that cold radiates.

There are limits as to how hot and how cold things can get, but our practical experiences are much closer to the cold limit. This is the closest thing I can find to a difference in the hot/cold symmetry. You can also do quantum experiments with thermal radiation photons, and the corresponding 'cold photon' things don't work. So for instance putting a 'cold radiator' near a metal target wouldn't create a photoelectric effect on it. I think these are pretty obscure details that might not be obvious to someone developing a reasonable theoretical system of radiant cold.

Light is a good metaphor for this (as well it should be, since the two are EMR with wavelengths about an order of magnitude different). I have a thermographic camera that sees thermal radiation, and have played with it in a variety of settings, and have the same difficulty you have with light proving that darkness doesn't radiate.

Electrons and the plus and minus of electricity are another good metaphor.

Can anybody here describe an experiment that would make you say cold doesn't radiate? Theoretical explanations that start with the assumption that the thing that radiates is heat aren't really fair, are they?
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  #15  
Old 07-11-2005, 04:24 PM
scr4 scr4 is offline
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Quote:
Originally Posted by Napier
Can anybody here describe an experiment that would make you say cold doesn't radiate?
Point a thermographic camera at a cold object, and then start accelerating towards the object. If the object were radiating "cold," you should get more "cold" flux as you move faster towards the object, and therefore the temperature reading should drop. In reality the temperature reading would increase, because the flux of "heat" from the object will increase (i.e. blueshift of infrared light).
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  #16  
Old 07-12-2005, 09:15 AM
spingears spingears is offline
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Quote:
Originally Posted by chrisk
Do you have a cite for that?? I thought that even an object surrounded by higher temperatures would radiate, just that it would, on average, absorb more radiation than it emitted.
What do you mean by cite?
It's fundamental physics/engineering.
Heat radiates.
There is no such thing as "COLD." i.e. an object may be colder that another, it is a comparative term.
You and the OP need to review the fundamentals.
Heat always flows from one region to one of LOWER temperature absent a "heat pump" type device.

If you were in an Ice Cave you might be misled to believe the cold feeling was being radiated by the surroundings. In reality you feel colder becase you are radiating heat to the surrounding and sense the drop in body temperature.
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  #17  
Old 07-12-2005, 09:35 AM
chrisk chrisk is offline
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Quote:
Originally Posted by spingears
What do you mean by cite?
It's fundamental physics/engineering.
Heat radiates.
There is no such thing as "COLD." i.e. an object may be colder that another, it is a comparative term.
You and the OP need to review the fundamentals.
Heat always flows from one region to one of LOWER temperature absent a "heat pump" type device.

If you were in an Ice Cave you might be misled to believe the cold feeling was being radiated by the surroundings. In reality you feel colder becase you are radiating heat to the surrounding and sense the drop in body temperature.
Okay... wtf?? Where in my request for a cite, or anywhere else, did you see me arguing for a literal 'radiation of coldness'?? That wasn't even something you were denying directly before. The only line there that even comes close to my point is the line about heat always flowing to a region of lower temperature.

I was asking for a cite on the implied statement that 'a [cold] object will not radiate heat energy unless it is surrounded by colder things.' I was taking that literally, and it seemed inconsistent with what I knew of thermal radiation. scr4 suggested that you were speaking generically, saying that there's no net loss of energy due to radiation if the surrounding objects are hotter.


Would you care to clarify your point without building straw men that have nothing to do with the statements I have made in this discussion??

My position is that literally anything above absolute zero will radiate thermal energy. Just about anything made of matter will also absorb radiant thermal energy, as long as there is something else in its universe. Whether there is a net gain or loss of energy through these two effects does not change the truth of the statements.
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  #18  
Old 07-12-2005, 09:48 AM
Charlie Tan Charlie Tan is offline
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Quote:
Originally Posted by spingears
You and the OP need to review the fundamentals.
If I knew the answer, I wouldn't be asking the question.

chrisk - don't bother with spingears' sense of superiority. It often shows up in his posts in GQ, and IIRC has resulted in at least one pitting. It's just not worth the trouble though
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  #19  
Old 07-12-2005, 12:19 PM
Kimstu Kimstu is offline
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Cecil's classic column on heat transfer.
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  #20  
Old 07-12-2005, 01:00 PM
chrisk chrisk is offline
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Quote:
Originally Posted by The Gaspode
If I knew the answer, I wouldn't be asking the question.

chrisk - don't bother with spingears' sense of superiority. It often shows up in his posts in GQ, and IIRC has resulted in at least one pitting. It's just not worth the trouble though
Ahh, okay. I see you have personal experience with one of those pittings.

As far as being worth it... well, I think it was worth it to defend my own position, especially since I didn't realize he was considered to be somethink of a ____. (fill in appropriate term.)

and now...

Quote:
Originally Posted by scr4
Point a thermographic camera at a cold object, and then start accelerating towards the object. If the object were radiating "cold," you should get more "cold" flux as you move faster towards the object, and therefore the temperature reading should drop. In reality the temperature reading would increase, because the flux of "heat" from the object will increase (i.e. blueshift of infrared light).
(deliberately puts on pseudoscientist hat)

See, what you don't realize is, cold already radiates so fast that you don't get more of it hitting you as you travel towards the cold thing... in fact, then it's coming at you so fast that most of it goes right through your camera... and it knocks some of the cold that was already there away as well. Yeah, that's the ticket.

(/pseudoscientist)

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  #21  
Old 07-12-2005, 02:55 PM
Chronos Chronos is online now
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I was talking over the design of such an experiment at lunch with a few fellow physicists, and we came up with a few more:

1: Put an object out in deep space, and let it radiate. If it radiates heat, it'll get colder, but if it radiates cold, it'll get hotter. The problem with this is that truly empty space is experimentally unachievable: If nothing else, you've always got the cosmic microwave background radiation, and one could argue that the CMB is adding cold to our "isolated" object.

2: As the temperature of an object increases, it's presumably emitting less and less coldons. Eventually, you would reach some finite maximum temperature, where an object would emit zero coldons. Problem with this is that there's no a priori way to know what this maximum temperature would be, and it might well be far outside the range of practical experimentation.

3: The one we finally settled on. If you put a test object in radiative contact with a themal resevoir, the temperature of the test object will change in discreet steps, corresponding to the absorbtion and emission of individual photons (or coldons). Suppose, for instance, that you have a cold object in a hot room. Occasionally, the cold object will make a small step downwards as it emits a photon (or absorbs a coldon), and more often, it'll make a big step upwards as it absorbs a photon (or emits a coldon). By experiment, the size of those upward steps depends on the temperature of the walls of the room, but not on the temperature of the test object. This is consistent with the photon explanation (the hotter the walls are, the "bigger" the photons they prooduce), but not with the coldon explanation, since it would require the object radiating coldons to know its surroundings, to know how "big" the coldons it radiates should be.
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  #22  
Old 07-12-2005, 08:07 PM
spingears spingears is offline
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Quote:
Originally Posted by chrisk
My position is that literally anything above absolute zero will radiate thermal energy. Just about anything made of matter will also absorb radiant thermal energy, as long as there is something else in its universe. Whether there is a net gain or loss of energy through these two effects does not change the truth of the statements.
It appears we are having problems with semantics.
Cold is a measurable propety called temperature.
Heat is also a measurable propety called temperature.
Heat energy is an entity which is measured in BTU's or Calories or ?
Heat energy or thermal energy flows from a high temperature domain to a lower temperature domain.
Heat can not flow (radiate) from a low temperature domain to a higher temperature domain. Such would be a violation of the second law of thermodynamics and you could achieve perpetual motion.

The Second Law

How would you measure the radiation (energy flow) from the colder domain to the hotter one? Or are you making the assertion that it just does and the net effect is what matters in the long run?

The Second Law

It's a one way street!
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  #23  
Old 07-12-2005, 08:59 PM
bizzwire bizzwire is online now
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To answer the OP:

No.
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  #24  
Old 07-13-2005, 03:27 AM
matt matt is offline
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Quote:
Originally Posted by spingears
It appears we are having problems with semantics.
Yes.

Quote:
Originally Posted by spingears
Heat can not flow (radiate) from a low temperature domain to a higher temperature domain. Such would be a violation of the second law of thermodynamics and you could achieve perpetual motion.
And here is the problem - flow and radiate are not the same thing. All objects above absolute zero radiate heat. If you have a "cold domain" and a "hot domain" they both radiate heat, but the hot domain radiates more heat onto the cold domain than vice versa, so there is a net flow of heat from hot to cold. The Second Law is not broken.

Quote:
Originally Posted by spingears
How would you measure the radiation (energy flow) from the colder domain to the hotter one?
Take a black 50 gallon drum of hot water and plot its cooling curve. Then do it again with a second 50 gallon drum of warm water beside it but not touching, and show that it takes longer for the hot drum to cool down. This is because the cooler object (the drum of warm water) was nevertheless radiating heat onto the warmer object (the drum of hot water). The difference in the cooling curves gives a measure of the intercepted energy radiated by the cooler object.

Quote:
Originally Posted by spingears
Or are you making the assertion that it just does and the net effect is what matters in the long run?
How does a rock at 100 deg. C "know" to radiate its heat in room at 20 deg C, but stops radiating it if you put it in a furnace pre-heated to 500 deg. C? To make this a decent thought experiment, we'll say there's no atmosphere in both cases and have the rock levitating without touching any walls, to eliminate conduction.
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  #25  
Old 07-13-2005, 05:10 AM
chrisk chrisk is offline
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Quote:
Originally Posted by spingears
It appears we are having problems with semantics.
Cold is a measurable propety called temperature.
Heat is also a measurable propety called temperature.
I think a lot of people here are using cold or heat in their common senses, rather than their scientific senses... heat being the tendency of something to have a relatively high temperature, cold being the tendency to have a relatively low temperature. They aren't measurable as such, when used in the common sense, though I'll agree that they are both underpinned by the measurable property of temperature.

Quote:
Heat energy or thermal energy flows from a high temperature domain to a lower temperature domain.
Heat can not flow (radiate) from a low temperature domain to a higher temperature domain. Such would be a violation of the second law of thermodynamics and you could achieve perpetual motion.

The Second Law

How would you measure the radiation (energy flow) from the colder domain to the hotter one? Or are you making the assertion that it just does and the net effect is what matters in the long run?

The Second Law

It's a one way street!
matt has brought up a few good points here. I'll say that I'm not entirely sure how to reconcile thermal radiation versus second law, except to guess that the second law is more general, as opposed to the specificity of thermal radiation, and that the second law generally seems of the type that "the net effect is what matters in the long run."

I admit I haven't measured the temperature increase in the sun, say, from the light of a cold emission nebula shining onto it. I'm working based on theoretical abstracts here, coupled with the certainty, as matt said, that the cold emission nebula has no way of knowing whether the sun is hotter than it is. It just radiates in all directions.
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  #26  
Old 07-13-2005, 07:25 AM
matt matt is offline
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Quote:
Originally Posted by chrisk
I'll say that I'm not entirely sure how to reconcile thermal radiation versus second law, except to guess that the second law is more general, as opposed to the specificity of thermal radiation, and that the second law generally seems of the type that "the net effect is what matters in the long run."

I admit I haven't measured the temperature increase in the sun, say, from the light of a cold emission nebula shining onto it. I'm working based on theoretical abstracts here, coupled with the certainty, as matt said, that the cold emission nebula has no way of knowing whether the sun is hotter than it is. It just radiates in all directions.
The second law states that the entropy of a closed system can't decrease. The rule about heat not flowing from a cold to a hot object follows from that.

In fact, heat can be transferred from a cold object to a hot object via a heat pump. That's how heat can be moved from the inside of a refrigerator to the warmer room outside. It's just that the entropy decrease from this heat transfer is balanced by an entropy increase from the heat pumping operation - the energy degraded by your refrigerator motor-compressor.

If the Sun is able to intercept the heat radiated from a cold nebula, then the cold nebula is also able to intercept the heat radiated from the Sun - it's a two-way path. The net energy flow is from the Sun to the cold nebula, the total entropy increases, and there is no 2nd law violation.
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  #27  
Old 07-13-2005, 08:24 AM
bizzwire bizzwire is online now
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Quote:
Originally Posted by spingears
Cold is a measurable propety called temperature.
Heat is also a measurable propety called temperature.
slight nitpick.

Temperature !=heat.

Consider a block of ice at 0 degrees Celsius in which has been embedded a thermometer. Put said block of ice in a copper saucepan, and place it on your stove over a high flame.

You are now dumping much heat into the ice, but until it has melted, you will notice that the temperature does not change.

Similarly, take a quantity of water at 100 degrees celsius over a hot flame. Even though you are adding much heat to the boiling water, its temperature does not increase.
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Old 07-13-2005, 08:57 AM
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Quote:
Originally Posted by spingears
How would you measure the radiation (energy flow) from the colder domain to the hotter one? Or are you making the assertion that it just does and the net effect is what matters in the long run?
This will be clear if you derive the equation for radiative heat transfer from first priciples. If you look at the final equation typically used, the temperatures of two bodies appear like (T1)4-(T2)4 but this comes after combining two independent terms. If you want to know the radiative heat transfer to a colder body, say the one at T2, you have two terms each representing the radiation from one of the bodies. One of these terms becomes negative if you're looking at the heat transfer in a particular direction and you get the combined (T1)4-(T2)4. But it's clear that this is a net effect; the colder body is radiating energy but it's absorbing more than it emits.

You could look at the net radiation to the hot body. This simply reverses the negative signs in the equation and, since the hot body's temperature is higher, you get a negative result. Negative heat transfer to the hot body means a net loss from the hot body to the cold body.

This is a typical case where using only the derived equations may hamper real understanding. It's always good to go back and look at the derivations of those equations so you understand what the terms represent and what assumptions are implicit in the result.
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Old 07-13-2005, 09:38 AM
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Quote:
Originally Posted by Chronos
3: The one we finally settled on. If you put a test object in radiative contact with a themal resevoir, the temperature of the test object will change in discreet steps, corresponding to the absorbtion and emission of individual photons (or coldons). Suppose, for instance, that you have a cold object in a hot room. Occasionally, the cold object will make a small step downwards as it emits a photon (or absorbs a coldon), and more often, it'll make a big step upwards as it absorbs a photon (or emits a coldon). By experiment, the size of those upward steps depends on the temperature of the walls of the room, but not on the temperature of the test object...
Good one. Out of curiosity, can this be done with today's technology? I don't know what the state of the art is for infrared bolometers.

Another scheme that comes to mind: Put two bolometers in far ends of a very large cryogenic vacuum chamber with low-emissivity high-reflectivity interior walls. Occasionally one of the bolometers will emit a photon or coldon, which travels to the other bolometer and gets absorbed. So temperature of one jumps down, and the other jumps up by the same amount. In theory, it should be possible to tell which occurs first. If one is emitting a coldon which is absorbed by the other, the temperature rise of one bolometer should happen before the temperature drop of the other. If it's hotons, temperature drop would happen before the temperature rise.
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Old 07-13-2005, 05:38 PM
Napier Napier is offline
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scr4 said: Point a thermographic camera at a cold object, and then start accelerating towards the object.

Wow, this is pretty good. I can't see any reason your approach wouldn't work and let you figure out that the positive sense of the thing radiated correlates with the positive sense of hotness and the negative sense of coldness. I also think that if you figured out it was all electromagnetic radiation, you could do the same experiment with light, which might be easier (in fact astronomers do this with the earth's orbit redshifting and blueshifting the same source at times a half year apart).


Chronos said: If you put a test object in radiative contact with a themal resevoir, the temperature of the test object will change in discreet steps, corresponding to the absorbtion and emission of individual photons (or coldons).

This should also work in principle, so hats off to your band of physicists. But not very many hats. Coldons would be awfully damn small - that is, the temperature jump associated with individual coldons would be impossibly small to measure. scr4's approach certainly works with visible light, and to the extent that we can test relatively hotter and colder things that still radiate in the visible, we could say it's practically useable.


spingears said:It's fundamental physics/engineering.
Heat radiates.
There is no such thing as "COLD." i.e. an object may be colder that another, it is a comparative term.
You and the OP need to review the fundamentals.

Wow, spingears, what made you so irritating? Look, I am a physicist and my focus right now is in heat transfer and thermal physics. Next to the bed I have two books on thermal physics, one on statistical mechanics, 3 on heat transfer, and on and on. Last night I was writing out the uncertainties in the Stefan-Boltzmann and Planck and Wien laws for a 4 hour course I am giving tomorrow morning at 8:00. And I have an international reputation for being strong on the fundamentals. I gotta say, I have almost no idea what you are getting at with the above quote. And I also think the OP was a fascinating and open-ended question and many of the replies have been ingenious. Most of the relevant things I can think of to say to you aren't of the General Questions nature - but the most involved posters here don't need remedial studies; rather, they could better use exactly what they are getting from each other. What an excellent discussion!
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  #31  
Old 07-13-2005, 06:56 PM
Scupper Scupper is offline
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I believe that "cold" can radiate, in the sense of one's perception of it.

To set up our experimental area, we need a large, round room with rings of couches all facing the center. Label the rings of couches A, B, C, etc., starting from the innermost.

1) Take an extremely cold object (which to make this observable, you'd likely have to continue to refrigerate over time and pump the heat to some other discussion) and place it in the center of the room.
2) The couches in ring "A" would cool rapidly, due to the large differential in temperature between themselves and the Cool-O-Matic
3) Those couches, having been cooled, would create a greater differential in temperature between themselves and Ring B's couches, which would begin to cool more rapidly than they did when A was still nice and toasty.
4) The cooling of Ring B would create a temperature differential with Ring C, causing it to begin cooling at an accellerated rate.

The radius of the zone of coolness has increased. The cold has "radiated" out from the center of the chamber and will soon pose a danger to the city, and only a crack team of Navy Seals can stop it!
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  #32  
Old 07-15-2005, 04:38 PM
Napier Napier is offline
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Location: Mid Atlantic, USA
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You know, if you want to feel the cold radiating off of something, you'd have to be pretty sensitive.

The total radiation of heat varies as the fourth power of absolute temperature, if it's a blackbody or greybody (that is, if its spectral emissivity were constant over wavelength).

You could radiate into a cold object from your body temperature which is about 37 degrees C or 310 K. How much energy would this be? The same as you would receive from a somewhat warmer object, one that radiated twice the energy you do. So it'd have to have a temperature of 310 K times the fourth root of two which is 1.189, or 369 K, or 96 degrees C. That is, if there were a blackbody at absolute zero on your left, and another blackbody at 96 degrees C on your right, you'd feel as many watts per square meter leaving you on the left as you felt entering you on the right.

Bear in mind that this would be the maximum cold radiation you could ever feel, given that absolute zero and your body temperature and an emissivity of 1 are all fixed. It isn't like radiant heat where the source could be arbitrarily hot.

This isn't very much, eh? I mean, I don't sense a lot of radiant heat thrown off by things at that temperature.
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