# Difference between IR and heat

• gordo999
In summary, the conversation is about a debate on a forum regarding the transfer of heat by radiation and the 2nd law of thermodynamics. Some people are claiming that heat can be transferred from greenhouse gases in a cooler atmosphere to a warmer surface, which goes against the 2nd law and represents perpetual motion. However, this is based on a misunderstanding of the difference between infrared radiation and heat. While IR can transport thermal energy, it is not heat itself, which is the kinetic energy of atoms. The 2nd law states that heat can only be transferred from a warmer body to a cooler body without compensation, and writers using pure radiative theory are ignoring this law. Additionally, the use of entropy in this context is not helpful as
Andrew Mason said:
The OP seems to believe that the proponents of this theory think that the magnitude of the negative Q exceeds that of the positive Q. That seems to be the basis for his criticism of the theory, which of course is nonsense.

Hi Andrew:

Thanks for your post.

I do not know what the OP thinks, but my thought is that the mechanism you describe has nothing to do with the greenhouse effect, which involves CO2 and other greenhouse gases. The conduction -> convection path of heat, is followed by thermal radiation from the atmosphere of which about half goes up and escapes from the earth, and half goes down back to earth. This mechanism involves the entire atmosphere, which is about: 78% nitrogen, 21% oxygen and 1% everything else. CO2 is part of this 1%, about 0.036% of the atmosphere. It is entirely a thermal process, unlike the greenhouse mechanism, which is not thermal at all.

Regards,
Buzz

willem2 said:
My point here, is that if the upper atmosphere radiates away black body radiation, it must also absorb it. See the wikipedia on kirchhofs law

Hi willem:

Thanks for your post.

The quote above is entirely correct about the upper atmosphere needing to absorb the black body energy it radiates. You are mistaken that the form of this energy is absorbed radiation. The absorption starts with conduction into the lowest atmosphere, and then by convection to the upper atmosphere.

You also said:
The page about Einstein Coefficients gives no quantitative information about the likelyhood of re-emission at the same frequency.​
The following are relevant quotes from the paper.
A photon with an energy equal to the difference E2 - E1 between the energy levels is released or absorbed in the process. The frequency ν at which the spectral line occurs is related to the photon energy by Bohr's frequency condition E2 - E1 = hν where h denotes Planck's constant.​

Spontaneous emission is the process by which an electron "spontaneously" (i.e. without any outside influence) decays from a higher energy level to a lower one. The process is described by the Einstein coefficient A21 (s−1) which gives the probability per unit time that an electron in state 2 with energy E2 will decay spontaneously to state 1 with energy E1, emitting a photon with an energy E2 − E1 = hν.

Due to the energy-time uncertainty principle, the transition actually produces photons within a narrow range of frequencies called the spectral linewidth.​
I suppose one might interpret this language as a bit careless insofar as it does not explicitly say that the E1 and E2 in these two quotes are the same energy levels of an atom or molecule. The "likelihood of re-emission at the same frequency" (or possibly two frequencies both within the same spectral linewidth) is 100%.

Regards,
Buzz

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Buzz Bloom said:
The quote above is entirely correct about the upper atmosphere needing to absorb the black body energy it radiates. You are mistaken that the form of this energy is absorbed radiation. The absorption starts with conduction into the lowest atmosphere, and then by convection to the upper atmosphere.

Kirchhoffs's law for radiation, states that any material that radiates at a certain wavelength must also absorb at that wavelength.

The Idea here is that there's a net input of energy from non-radiation sources, there must be net radiation radiating this energy away. But this implies that radiation must also be absorbed at the same wavelengths where it is radiated. There will of course be less radiation absorbed than radiated away, because one side (space) is very cold and there's no incoming IR from that side.

willem2 said:
Kirchhoffs's law for radiation, states that any material that radiates at a certain wavelength must also absorb at that wavelength.
Hi Willem:

Thanks for your post:

I would much appreciate it if you could cite the source with the text that your interpret with the quote above, and also include a quote of the context. With my admittedly inadequate research skills, I could not find by searching the internet anything that approximates this language.

My guess is that you misinterpreted something you read about Kirchhoff's law for radiation. I believe the law means that:
any material that radiates at a certain wavelength must also be able to absorb at that wavelength.​

Regards,
Buzz

Buzz Bloom said:
My guess is that you misinterpreted something you read about Kirchhoff's law for radiation. I believe the law means that:
any material that radiates at a certain wavelength must also be able to absorb at that wavelength.​
That's certainly what I meant also. I got that quote from the wikipedia page about Kirchhoff's law of radiation.
Since the atmosphere does emit IR radiation, it must also be able to absorb it. There's plenty of radiation about. Why doesn't this happen? You still have given no reason why it doesn't happen.

willem2 said:
That's certainly what I meant also. I got that quote from the wikipedia page about Kirchhoff's law of radiation.
Since the atmosphere does emit IR radiation, it must also be able to absorb it. There's plenty of radiation about. Why doesn't this happen? You still have given no reason why it doesn't happen.
Hi willem:

Thanks for your post. Sorry if I misunderstood your previous question.

It does happen. I confess that the concepts in my elaboration below are guesses, since I haven't studied anything in detail about this particular physical phenomenon.

When an atmospheric molecule interacts with another atmospheric molecule, a thermal photon will/may be emitted, and possibly two photons, one from each molecule. This would be characterized as stimulated emission. Such photons are subsequently likely to be absorbed by a similar molecule, and then soon after, spontaneously re-emitted. These re-emitted photons and then also likely to be absorbed by a similar molecule.

This emission -> absorption -> re-emission -> absorption ->... sequence will continue perhaps many times until instead of a photon:
(1) being absorbed, it will finally (a) hit the Earth, or (b) escape into space; or
(2) being re-emitted, the excited molecule will hit another molecule.​
The number of photons that hit the Earth in this scenario is about the same as the number which escape into space. This is because such a re-emitted photon is equally likely to be headed downward as upward.

The next scenario is the normal (non-greenhouse) effect which increases the Earth's temperature from what it would be with no atmosphere. Following the heating of the atmosphere by conduction-convection, all of these emissions/re-emissions and absorption of the thermal black-body photons are non-thermal, in that a molecule's being excited from its absorption of a photon does not make the gas warmer. However, when two molecules collide, and no photon is emitted, the energy of the excited state will add to the kinetic energy of one or both of the colliding molecules, and this will make the atmosphere warmer. So, both the thermal and non-thermal re-emitted photons that hit the Earth return some of the energy from the Earth back to the Earth. This is NOT a violation the thermodynamics law about a cold body not being able to heat a warmer body. The warmer Earth is in thermal equilibrium with the cooler atmosphere, and there are net exchanges of heat from the Earth warming the atmosphere, with some of that heat being returned to the Earth, reducing what would otherwise be a warmer atmosphere.

When a greenhouse molecule absorbs a photon, it also is involved in a similar scenario of repeated re-missions and absorptions. Although it is rare for an excited molecule to collide before it re-emits a photon, it can happen. When it does, the small number of cases when it does results in some slight warming of the atmosphere.

So far I have ignored clouds. When a photon from Earth, or re-emitted from a water vapor molecule, hits a water droplet in a cloud, and is absorbed by a water molecule, the excited molecule is much more likely to collide before it re-emits a photon. This is because the molecules in a liquid drop are much closer together than they are in a gas. Therefore, these photons heat the water droplets, which in turn by conduction thermally warm the atmosphere gasses.

Regards,
Buzz

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gordo999 said:
The notion that heat is only energy transfer is is an incorrect interpretation that has become en vogue recently.

So, now it's an argument over the meaning of a word? Fine. Physicists use the word heat to mean a transfer of energy between objects because of a difference in temperature between those objects. You won't be able to convince us that it's become en vogue recently when it's been that way for a very long time with no serious discussion of any reason to change it.

Consult the introductory college-level physics textbooks. Look at the new ones. Look at the ones that were in use 60 years ago. They will support what we're telling you.

Review the physics education literature. They will reveal that this definition is alive and well and is indeed the one in use, and has been for at least the last 60 years.

Internal energy is what you appear to be confusing with heat. Understandable in view of the way the terms are tossed around in less formal circumstances.

davenn
Buzz Bloom said:
Hi Andrew:
I do not know what the OP thinks, but my thought is that the mechanism you describe has nothing to do with the greenhouse effect, which involves CO2 and other greenhouse gases. The conduction -> convection path of heat, is followed by thermal radiation from the atmosphere of which about half goes up and escapes from the earth, and half goes down back to earth. This mechanism involves the entire atmosphere, which is about: 78% nitrogen, 21% oxygen and 1% everything else. CO2 is part of this 1%, about 0.036% of the atmosphere. It is entirely a thermal process, unlike the greenhouse mechanism, which is not thermal at all.
I am not talking about convection or conduction.

We are concerned with the mechanism by which CO2 absorbs IR in the frequency range close to the peak of the IR spectrum emitted by the Earth and then emits IR radiation in the same frequency range isotropically, about half of which is directed back toward the Earth surface. CO2 does this because of its molecular structure. N2 and O2 are transparent to this IR radiation. That must mean that O2 and N2 do not absorb or emit IR radiation in this range.

But this does not mean that the radiation emitted back to the Earth by the CO2 does not result in heat flow into the earth. It does. The thermal effect of this radiation on the Earth must obey thermodynamic laws. That is all I am saying. The OP stated that the greenhouse mechanism can't be correct because it would violate thermodynamic laws. No one is saying that the greenhouse mechanism involves heat flow into or out of the atmospheric CO2. It doesn't. But it does involve heat flow out of and back into the earth.

AM

Andrew Mason said:
No one is saying that the greenhouse mechanism involves heat flow into or out of the atmospheric CO2.
Hi Andrew:

I apologize for misunderstanding your post #36. I agree with everything in your last post, #43.

Regards,
Buzz

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