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Difference between IR and heat

  1. Oct 8, 2015 #1
    This is about a debate (argument??) I am having on another forum in which people are claiming heat can be transferred from greenhouse gases in a cooler atmosphere to a warmer surface (that allegedly warmed it). The so called back-radiation is presented as a positive feedback which warms the surface, that warmed the GHGs. That is not only a contradiction of the 2nd law it represents perpetual motion.

    Came across a thread on this forum about radiation and the 2nd law of thermodynamics. The link is here:


    There was a response from Andrew Mason, I think it was, which went as follows:

    "An even better example would be a laser. The temperature of the laser device used to cut steel is much less than temperature of the steel that the laser strikes. But this does not violate the second law since the energy flow from the laser to the steel is not a transfer of thermal energy".

    That response is in agreement with my understanding of the general difference between infrared radiation and heat. IR transfers thermal energy from a warmer body to a cooler body but IR is NOT the heat itself. Heat is the kinetic (internal) energy of atoms in the respective bodies and does not flow through the air. IR is EM, and EM has distinctly different properties than heat.

    However, if you read certain textbooks on this subject they describe an infrared interchange between bodies of different temperatures and insist that the IR flow is heat. It can't be heat since IR, which is EM, has no property related to heat. IR is the transporting agent only and heat always remains local to the bodies, increasing or decreasing depending on the situation.

    Heat in atoms of solids like metals is largely related to valence shell electrons and resembles electric current more than it does IR. Heat is transferred in metals via electrons and in insulators via phonons. Either way, heat is an energy flow related to the vibrations of atoms and the waves they propagate. As such, it cannot travel through air or a vacuum as can EM.

    The 2nd law is clear as written by Clausius that heat can only be transferred from a warmer body to a cooler body without compensation. Put another way, by Clausius, heat cannot of itself transfer from a cooler body to a warmer body.

    Writers using pure radiative theory, a la Boltzman, are clearly ignoring the 2nd law. Some of them claim that the 2nd law is satisfied as long as a net energy (IR) flow between the bodies is positive. However, Clausius did not reference IR when he wrote the 2nd law, he referred only to heat transfer, Q, the temperature at which the transfer took place T, and work. He did reference internal energy and work but claimed they were not required to calculate external heat and work.

    In models they present of bodies exchanging heat, they do not state the temperature of the bodies or whether the bodies are independent heat sources. I have no idea what would happen if two stars (modelling blackbodies) with temperatures in the millions of degrees were close to each other but at temperatures of the Earth, where the surface allegedly warms greenhouse gases in the atmosphere, that is a far different story.

    Some people are getting entropy mixed up with the 2nd law and that convolutes the matter. However, Clausius made it clear when he coined the term entropy that it is a summation of infinitesimal quantities of heat dQ, at a temperature T, in a heat process. Whereas entropy has it's uses in chemical reactions it's not helpful when it comes to the 2nd law and radiative heat transfer, which is really about the direction of heat transfer.

    The 2nd law, no matter how it is stated, refers to heat, not EM.
  2. jcsd
  3. Oct 8, 2015 #2

    Andrew Mason

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    Welcome to PF, gordo999!

    The second law means that positive net heat flow, or the net transfer of thermal energy, from cooler to hotter bodies cannot spontaneously occur. If the mechanism for the heat flow is radiation (eg. there is no convection or conduction), the second law still applies. Thermal radiation is radiation from a source by virtue of its temperature. The spectrum emitted follows Planck's law for thermal radiation which produces an energy distribution that parallels a Maxwell-Boltzmann thermal energy distribution for matter. Since the thermal radiation emitted is proportional to ##T^4## (the Stefan-Boltzmann law), the net flow of thermal radiation is always from the hotter to the colder body.

    But a laser is not a thermal source. Neither is a fluorescent bulb. The radiation of a laser is monochromatic: its spectrum does not follow Planck's law. So, essentially, the energy flowing from the laser is not heat flow. Similarly, electricity can spontaneously flow from a cold body to a hot body (a cold battery can cause electricity to flow into a hot coil). This does not violate the 2nd law of thermodynamics because the electrical energy is not "thermal" (i.e. the energy of the moving charges does not follow a Planck or Maxwell-Boltzmann distribution).

    Last edited: Oct 8, 2015
  4. Oct 8, 2015 #3
    Andrew...thanks for reply. One point I was trying to make is that infrared energy is not thermal either. The heat it transfers never leaves the body from which it is transferred. The transfer is done through IR being absorbed in the receiving body and raising the kinetic energy of the atoms/molecules locally.

    A question arises, however, regarding the ability of IR from a cooler source to affect the kinetic energy level of atoms at a higher temperature. According to Bohr, energy will only be absorbed in an atom's valence electrons if the IR has the proper resonant frequency and intensity to be absorbed. There is no reason to presume that IR back-radiated from a cooler atmosphere will be absorbed by the surface at all.

    I am open to input here but to me heat is the kinetic or internal energy of moving particles. In solids, that motion is a vibration as atoms vibrate in position. IR, as part of the EM spectrum. is not heat. IR is generated when electrons in an atom change energy levels, or in molecules when atoms in the molecule have electrons change energy levels OR the dipoles in a bond between molecular atoms absorb or emit IR.

    In the case of the anthropogenic warming theory, the most plausible argument is that heat can be transferred from GHGs in a cooler atmosphere to warm the surface BEYOND what it is warmed by solar energy. That represents a positive feedback which disregards losses at the surface when the radiated IR that warms the GHGs leaves the surface. The theory also ignores the scant amount of CO2 in the atmosphere.

    The other argument, that GHGs act as a heat trapping blanket is as physicist/meteorlogist Craig Bohren claims, a metaphor at best, and at worst, plain silly. He agrees that the only plausible explanation for the anthropogenic theory is a model with two bodies radiation against each other.

    Heat cannot be trapped and the large flux of IR leaving the surface has already cooled the surface. There's no way to trap that flux, or slow it appreciably.

    Climate scientist Stephan Rahmstorf has claimed that the former theory is correct, that GHGs back-radiate enough energy to super-warm the surface by adding to incoming solar energy. I think that theory is seriously flawed since it violates the 2nd law. For one, IR from a cooler source is transferring heat to a warmer surface, and secondly, the so-called back-radiated energy has to over-come serious IR flux losses at the surface before it can begin warming it.

    More importantly, you cannot take energy that was produced initially by solar energy, converted to IR in the surface, transmitted to warm GHGs, then recycle that energy by adding it to incoming solar energy so as to super-warm the surface. That's perpetual motion in which losses have been ignored.

    Scientists like Rahmstorf are claiming that the net flow of IR compensates for the contradiction of the 2nd law. In fact, they claim it satisfies the 2nd law. They are obviously confusing IR with heat. The 2nd law is about heat only, not IR. Therefore you cannot sum IR and claim it satisfies the 2nd law.

    This is a conundrum since climate scientists are blatantly applying Boltzmann and Kircheoff while ignoring the 2nd law. I don't know enough about this but I am guessing that Boltzmann and Kircheoff are aimed at very high temperature IDEAL bodies which are independent sources of heat. As such, I don't think they are directly applicable at temperatures found near the Earth's surface where atmospheric GHGs are DEPENDENT on surface radiation. I think the two scenarios are so different that you cannot blindly apply Boltzmann/Kircheoff without considering the 2nd law.

    Overall. I think there is something wrong with the notion that GHGs heated by surface radiation can re-radiate a portion of that energy and have the EM raise the kinetic energy in surface atoms/molecules.
  5. Oct 8, 2015 #4


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    that isn't quite correct ... the emitted solar energy ( well part of it) IS IR radiation. the IR doesn't magically appear when the sun light hits your skin.
    Its the energy in the IR radiation that is being absorbed by you and heating you up

    There's other similar points I could comment on, but that one really stood out

  6. Oct 8, 2015 #5
    There might be some misunderstanding here.

    Bodies do not possess heat.
    Bodies have certain characteristics, temperature, heat capacity, internal energy being just a few.

    Heat, designated Q, is the flow of energy from one body to another due to a "difference in temperature".

    The flow of energy can be by contact ( conduction ), by means of a medium ( convection ), or remote (radiation ).

    With conduction and convection, the treatment is that the hot body has a 'one-directional' heat flow to the colder body.
    With radiation, and not just necessarily black bodies, both bodies exchange heat, and the net heat flow can be analyzed and determined.

    Other energy exchanges not due to the difference in temperature of the bodies can result in temperature rise or fall in either, as explained previous post.

    I have no comment on the GHG treatment of heat flow and surface warming.
  7. Oct 9, 2015 #6
    I agree with you there. In fact, 52% of solar energy reaching the Earth is IR. I am talking about what happens after the short wave and long wave EM warms the surface.

    The surface becomes an IR radiator due to the equilibrium temperature it reached due to solar heating minus the heat lost due to IR radiation. There is likely a factor in the equilibrium due to internal heat generated by the Earth's core, although some claim it's not that significant.

    Once the Earth radiates its own IR due to it's ambient temperature in a locale, that's where one of the AGW theories comes in. It is claimed in the Kiehl-Trenberth heat budget of the Earth that some 300+ watts are radiated back to the surface as heat. I think that's pretty ludicrous considering the scant amount of CO2 in the atmosphere and the total GHG content which is around 1% of atmospheric gases.

    As an example, consider a greenhouse with 100 panes of glass. To get the equivalent Earth greenhouse effect with 1% of atmospheric gases, one would have to remove 99 panes of glass from the 100 pane greenhouse.
  8. Oct 9, 2015 #7
    Internal energy of atoms is heat.

    Agreed, but let's be more specific. According to Clausius, Q is the heat transferred into or out of a body. So the energy to which you refer is thermal energy, which Clausius called heat.

    Some people today claim that energy, in radiation theory, is IR. It is not.

    This is where I am having a problem. When Clausius described radiative and conductive heat transfer he did not distinguish between them wrt to heat transfer. He made it clear that radiative heat transfer had to obey the 2nd law, which forbids heat transfer from a cooler to a warmer body unless there is compensation to replace the heat lost by the cooler body. Under normal heat transfer that compensation is not available, it has to be supplied externally.

    When bodies radiate IR, they are not aiming the IR at another body, they are radiating isotropically to space. If another body intercepts a small angle of that radiation, it can either absorb it or not absorb it.

    In your response, you talk about heat flow as if it's IR flow. Heat does not flow through space radiatively, it requires matter. I suppose one might argue that there is matter in the atmosphere, which is 99% nitrogen and oxygen molecules, but that molecule to molecule flow is not how heat moves through a gas. It depends on the transfer of kinetic energy due to molecular collisions.

    IR does not operate like that.

    Heat has very different properties than IR, which is EM. EM is defined by it's frequency and intensity and there is no reference to heat in the definition of EM other than a colour temperature which relates a frequency range of EM to the colour of super-heated steel.

    EM contains no colour, not even light contains colour. Colour is added by the human eye when EM in light frequencies stimulate the retina in the human eye. EM is nothing but an electric field with a magnetic field perpendicular to it which vibrates over a certain frequency range. IR appears as red because its frequency range corresponds to the colour of steel heated a cherry red.

    Heat is something else altogether. Heat is the kinetic (internal) energy of atoms and is controlled by valence electrons in conductors and phonons in insulators. Heat can never leave the matter upon which it is defined. Therefore, when IR is radiated into space, the heat remains in the atoms that radiated the IR, albeit at a lower level based on the amount of IR radiated.

    Based on that, I think it's wrong to suggest that Boltzmann and Kircheoff equations cover all conditions where heat is transferred. There has to be something else going on.

    Perhaps at very high temperatures in bodies which are independent sources of heat, the IR radiated to space can be intercepted by each body and affect the heat in them. However, at temperatures in the range of the Earth's surface, I don't think that holds, especially when one body (the surface) is radiating and warming the other body (atmospheric GHGs), which has no source of heat of its own.

    There is absolutely no proof that such a process takes place, especially in the direction from atmosphere to surface. It's all theory, some of which has been programmed into climate models as fact. It has been measured in labs using much higher concentrations of CO2, but not in the atmosphere.

    Of course, GHGs in a cooler atmosphere will be radiating anyway. All matter radiates right down to the vicinity of 0 Kelvin. The question is, does that IR have sufficient intensity and frequency to stimulate the electrons in surface atoms at a higher temperature? Based on the 2nd law, I somehow think they don't.

    Bohr has stated that electrons in atomic shells will only absorb radiation that has the proper frequency and intensity to stimulate the electrons. It is absorption of EM by electrons that raise them to a higher level of kinetic energy, hence heat. I am theorizing that IR radiation from a cooler source lacks that frequency and radiation, and I think that's why the 2nd law is explicit about heat being transferred from only a warmer body to a cooler body, even through radiation.

    You know, a lot of this stuff we read in climate science about the greenhouse effect being determined by the laws of Boltzmann and Kircheoff is purely theoretical. No one has ever measured it. There has to be exceptions to Boltzmann and Kircheoff whose equations are written for high temperature ideal blackbodies.

    The IPCC has admitted recently that no global warming trend has been detected since 1998 despite an alleged increase in atmospheric CO2. It would appear that emissions from GHGs due to anthropogenic CO2 are having no effect on surface temperatures the past 18 years.
  9. Oct 9, 2015 #8


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    That is not correct. Heat is a process of energy transfer. Perhaps you are thinking of thermal energy?

    That is also incorrect.

    IR doesn't appear red. It doesn't appear as any color because it cannot be seen. Cherry red steel appears a deep red because part of the spectrum of EM radiation it emits happens to fall in the reddish part of the visible range.

    All of this is wrong. As has been stated already, heat is a type of energy transfer. This is basic stuff that can be found in an introductory physics textbook.

    Given that you don't appear to have the slightest clue what heat is, I find the rest of your post to be... lacking.
  10. Oct 9, 2015 #9
    High temperature is anything above 0K.

    Black bodies are ideal, and none does exist.
    The closest would be the night sky.

    Even so, the physics for opaque, totally reflective, totally transparent, semi reflective, etc is a well established science.
  11. Oct 9, 2015 #10

    Andrew Mason

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    The issue is not whether the 2nd law of thermodynamics is being applied. It is really about black-body radiation. The Stefan-Boltzmann law (which is a statistical law) obeys the 2nd law.

    To maintain a roughly constant temperature over time, the earth has to reflect or re-radiate all of the energy that is incident upon it. There is no other mechanism for getting rid of that energy.

    Earth reflects about 30%. So it has to radiate about 70% of the sun's energy that is incident upon the earth. The earth does not radiate EM in the visible spectrum (all light coming from the earth is reflected, not radiated). It radiates in the infra-red spectrum.

    If we were to assume that the earth's surface did all the radiating (ie. no radiation came from the atmosphere) we could determine the earth's surface temperature by simply applying the Stefan-Boltzmann law . If you do the calculation the earth's surface temperature would be about 279 K. We know that the earth's average surface temperature is about 288K. So we can immediately say that the earth's atmosphere must play a factor in warming the earth surface.

    The molecules in the atmosphere do not absorb EM radiation in the visible spectrum. But some of the molecules do absorb EM radiation in the infra-red spectrum (water molecules, CO2, CH3). This results in some of the IR radiation energy from the surface being absorbed and then being radiated by them in all directions - including back to the earth surface. This returning IR radiation effectively reduces the rate of radiant heat loss from the surface. This has the effect of allowing the surface to maintain a higher temperature than it would otherwise have.

    So the net result is that the atmosphere lowers the net IR radiation flow from the earth surface, not that there is net heat transfer from the atmosphere to the surface.

    If you wish to continue this conversation, perhaps you should post on the Earth board where this subject has had a lot of discussion.

    Last edited: Oct 24, 2015
  12. Oct 9, 2015 #11
    Thermal energy and heat are the same energy. The notion that heat is only energy transfer is is an incorrect interpretation that has become en vogue recently.

    Clausius talked about 'heat transfer', not as heat as an energy transfer mechanism. He used Q to represent the amount of heat transferred either into or out of a body. He defined entropy as the integral of dQ/T over a heat process, and in words, defined it as a summation of the infinitesimal transfer of heat, dQ, at a specific temperature, T, over a process. If the process is reversible, entropy = 0. If the process is irreversible, entropy is < 0.

    It is clear that Clausius spoke of heat as a quantity that could be transferred between bodies and an 'energy' that existed in atoms. He referred to the vibrations of atoms internally as work, as the atoms moved back and forth in their bond paths. He made it clear that work is equivalent to heat.

    What else could heat be and why would you differentiate it from thermal energy? Heat is the energy associated with the motion of atoms. As atoms in a gas move faster, they have a higher kinetic energy. However, KE is a generic energy of energy in motion and is relative to the type of energy in question. When applied to the motion of atoms in a substance the KE in question is the energy we know as heat.

    I mistated what I had intended. I said elsewhere in the post that EM has no colour. I did not mean to imply IR was red in colour, I meant it is represented by red in spectral drawings of EM.

    Then you are claiming Clausius did not have a clue what heat is. Either that or you don't. I'll go with Clausius, Planck, and all other scientists who recognize heat as energy.
  13. Oct 9, 2015 #12
    Thanks Andrew, I'll look into that.
  14. Oct 9, 2015 #13


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    this is also incorrect, as Drakkith said, you don't seem to quite have a handle on what heat or thermal energy is

    thermal energy ( commonly known as heat) is transferred in 3 ways, either singularly or as a mixture of all 3

    by conduction, convection or radiatively
    conduction and convection need a medium ... eg. solid, liquid, gas
    EM radiation doesn't

    As 256bit said ... Any object above 0 K produces/emits thermal energy ( and going on from that) that will be radiated by EM radiation and if the object is in contact with another medium, then there will be conduction and possibly convection occurring as well

  15. Oct 9, 2015 #14

    Andrew Mason

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    Drakkith is correct. In thermodynamics "heat" has a particular meaning. It refers to "heat flow", which is the transfer of energy from one body to another by virtue of temperature difference. A body cannot contain "heat" any more than it can contain "work". Both terms refer to transfers of energy.

    The term "heat" may have a variety of meanings in every-day use. We may say "turn up the heat" to mean increase the temperature. We say this because it is more concise and familiar than saying "increase the internal kinetic energy of the air molecules". But in thermodynamics, heat has a very precise meaning: the transfer of energy due to temperature difference.

    Before matter was understood to consist of atoms, physicists thought of "heat" as a substance that a body contained to give it its temperature and that flowed from one body to another due to temperature differences.

    Then Kinetic Theory was developed and it was realized that differences in temperature corresponded to differences in the internal kinetic energy of a body's molecules. However, the term "heat" was kept. But instead of heat being a substance that a body "contains" to give it its temperature and that flowed due to temperature differences, it refers to the flow of energy due to temperature differences. This energy flow may be from a change in internal energy and/or mechanical work being done.

  16. Oct 10, 2015 #15
    I said:
    The only thing wrong here is a typo. I was doing it late at night. I said, "[Some people claim]...energy, in radiation theory, is IR. It is not".

    That should have read, "[Some people claim]...energy, in radiation theory, is heat. It is not.

    The energy in radiation theory is electromagnetic energy and there is no thermal energy associated with EM. EM can cause heat in atoms, but EM by itself has no thermal energy. The heat is brought about when EM affects electrons in atomic orbitals. In a solid conductor, it is valence electrons that transfer heat through the solid. Electrons cannot travel through space under normal circumstances if they are part of a solid, but they can transmit EM, which can transfer heat by affecting the electrons in another body.

    Therefore it is incorrect to imply that a mutual flow of IR is heat. Heat must obey the 2nd law, which states clearly that heat cannot of itself be transferred from a colder body to a warmer body. Radiative flow of IR cannot meet the requirements of the 2nd law.

    I have a very sound understanding of what heat is from reading Clausius and Planck, and from having studied it as part of an engineering program. My specialty is electronics. I have found several parallels between the flow of electric current and heat transfer. Good electrical conductors are also good conductors of heat and that's not a coincidence. From having studied electronics for several decades I have gained an intuitive understanding of different energies and some atomic processes. Kinetic energy and potential energy are fundamental to electronics and heat transfer is an important part of electronics.

    Clausius wrote at a time when scientists tended to explain themselves subjectively as well as mathematically. Today, physics tends to be presented mathematically, with subjective explanations ignored. I fear that many people today have been mislead with regard to subjects like thermodynamics and entropy. Quantum theory has lead scientists to abandon reality while developing abstract theories about reality that are based on probability. There comes a time when you have to face reality and cut the abstractions. To claim heat is 'something' that transfers an undefined energy is not only wrong, it is an abstraction.

    Feynman once stated that he would not give a lecture if he could not explain subjectively what he was teaching based on math. David Bohm, a brilliant physicist in the 20th century, made a statement that equations without a reality that can be explained are garbage. Based on that, tell me what I don't understand about heat given the correction I offered and the quote you made.

    Heat is energy but it is not IR, which is electromagnetic energy. You admit yourself that heat is thermal energy. I clearly understand how it is transferred. In radiation, it is transferred through IR as a medium, but that's akin to transmitting audio via a high frequency RF carrier. As the signal goes through the air, the audio component of the signal cannot be heard since it is in the form of an amplitude-varying RF signal, in amplitude modulated transmission. The audio is recovered at the receiver by removing the RF carrier and recovering the audio (demodulation).

    It's not quite the same with heat since heat does not leave the transmitting body. Heat is the internal energy of atoms in the transmitter and it is reduced slightly as the IR is transmitted. On the receiving end, provided the energy and frequency aspects are right, the IR can be absorbed into the electrons in atoms, causing the electrons to move to a higher energy level. That increase in energy is kinetic energy, which is heat.

    If you think that's wrong, then tell me what else heat could be. There's no point calling it thermal energy unless it has definable properties that differentiate it from other forms of energy. Referring to heat as the transfer of energy is completely wrong since heat 'IS' the energy transferred. Describing heat as energy in motion infers that heat is not energy but some kind of abstraction.

    Besides, we already have a name for energy in motion...kinetic energy. The kinetic energy of atoms in motion is heat. Since KE has a velocity component, when you increase the speed of atoms, the KE rises and it manifests as heat.

    That's what heat is.
  17. Oct 10, 2015 #16

    Andrew Mason

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    If the radiation is black-body radiation, such as EM radiation from the sun, then its characteristics are inextricably tied to the thermal energy of the matter in the sun that emits it.

    There is very little similarity. An RF carrier consists of a single frequency. It is not blackbody radiation. IR emitted from a body by virtue of its temperature has a spectrum that follows a well understood distribution given by Planck's law.

    That may be how you would like to define heat, and that is fine. But that is not how the term is used in thermodynamics. What you are calling heat is generally referred to as "thermal energy" which is the internal energy that a body has by virtue of its temperature. See: https://en.wikipedia.org/wiki/Heat or https://en.wikipedia.org/wiki/Thermal_energy

    Last edited: Oct 10, 2015
  18. Oct 10, 2015 #17


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    that's a better clarification, thanks Andrew
  19. Oct 10, 2015 #18


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    The 2nd law is perfectly compatible with thermal radiation (let's not use IR here, as IR is a specific band within the EM spectrum, while thermal radiation encompasses a wide range of frequencies, not just IR). As an example, just look at the Sun and the Earth. Heat flow happens from the Sun to the Earth, not in reverse, because the energy emitted in the form of radiation is MUCH greater for the Sun than for the Earth. In other words, the Earth is received much more energy from the Sun than the Sun receives from the Earth, so there is a net flow of energy away from the Sun to the Earth (and to other planets and objects in space).

    Yes, well, it turns out that the best experiments ever devised support the ideas of Quantum Theory. The evidence is so overwhelming that I can't understand why you'd have a problem with it. The very device you're using to visit and post on PF is designed and built using the laws of quantum physics. If quantum theory is wrong, it's the most correct wrong theory every devised.

    Heat is not 'something' that does the transferring, it is literally the transfer of energy in a particular way. Going by the explanation below, heat is literally energy in the process of being transferred from one system to another, or between objects within a system, without performing useful work. And the transferred energy isn't undefined at all. It's typically either radiation or thermal energy.

    Per wiki:
    In physics, heat is energy in a process of transfer between a system and its surroundings, other than as work or with the transfer of matter. When there is a suitable physical pathway, heat flows from a hotter body to a colder one.[1][2][3][4][5][6] The pathway can be direct, as in conduction and radiation, or indirect, as in convective circulation.[7][8][9]

    Because it refers to a process of transfer between two systems, the system of interest, and its surroundings considered as a system, heat is not a state or property of a single system. If heat transfer is slow and continuous, so that the temperature of the system of interest remains well defined, it can sometimes be described by a process function.

    It does. From wiki again:

    In thermodynamics, thermal energy refers to the internal energy present in a system by virtue of its temperature.[1] The average translational kinetic energy possessed by free particles in a system of free particles in thermodynamic equilibrium (as measured in the frame of reference of the center of mass of that system) may also be referred to as the thermal energy per particle.[2]

    Microscopically, the thermal energy may include both the kinetic energy and potential energy of a system's constituent particles, which may be atoms, molecules, electrons, or particles. It originates from the individually random, or disordered, motion of particles in a large ensemble. In ideal monatomic gases, thermal energy is entirely kinetic energy. In other substances, in cases where some of thermal energy is stored in atomic vibration or by increased separation of particles having mutual forces of attraction, the thermal energy is equally partitioned between potential energy and kinetic energy. Thermal energy is thus equally partitioned between all available degrees of freedom of the particles. As noted, these degrees of freedom may include pure translational motion in gases, rotational motion, vibrational motion and associated potential energies. In general, due to quantum mechanical reasons, the availability of any such degrees of freedom is a function of the energy in the system, and therefore depends on the temperature (see heat capacity for discussion of this phenomenon).

    A device consisting of several rotating flywheels has internal energy made up of both the rotational energy of the flywheels and the translational, rotational, vibrational, and possibly the potential energy of the particles that compose the device. The latter four make up the thermal energy. If I exposed the device to a near-zero kelvin environment, the thermal energy would be transferred from the device to the environment until both are in thermodynamic equilibrium. But the flywheels would just keep on spinning until something else slowed them down.

    I believe this is only true when considering large numbers of particles moving about randomly where the distribution of their kinetic energy follows the equipartition theorem. The KE of electrons emitted from an electron gun in a CRT would not be considered heat since they don't meet this criteria. Besides, heat includes more than just KE, as I explained above.
  20. Oct 11, 2015 #19
    Andrew...you need to think this through with respect to 'flow' and transfer'. Flow suggests a vector field and transfer suggests moving something from one location to another. Heat does not flow through space but it can be transferred via EM. The heat transfer process is not in any way the same as the flow of EM.

    When you talk about transferring energy between bodies via radiation, what energy is being transferred? You are not 'transferring' electromagnetic energy because EM does not exist as an energy entity in a body. EM is generated by electrons in atoms, it does not exist till an electron drops to a lower energy state and EM is transmitted. Therefore you are not transferring EM from one body to another, you are transferring a different kind of energy, thermal energy.

    If anything flows between bodies, it is EM, as IR. Heat does not 'flow' between bodies radiatively. Heat can be transferred as kinetic energy but the change in heat takes place locally. Kinetic energy does not flow through space, it changes based on the absorption of electromagnetic energy. I don't pretend to understand the process by which the EM is absorbed, causing the electron to move a to higher state of kinetic energy, hence thermal energy. Probably no one understands it subjectively. However failing to distinguish the energy flowing between bodies in a field as EM, from the kinetic energy already present in a receiving body, strikes me as being wrong.

    You cannot refer to the energy 'flowing' between bodies radiatively as heat.

    Below you refer to the Kinetic Theory where a difference in temperature corresponds to a difference in internal energy. However, temperature is not a natural phenomenon, it is a scale of relative temperatures developed by humans based on the freezing point and boiling point of water. Temperature measures the relative degree of thermal energy, which is the relative internal energy, or kinetic energy.

    What you guys are missing is that thermal energy is heat. You will see heat defined on many university sites as the internal energy of atoms, or the kinetic energy of atoms.

    In The 1879 version of the Mechanical Theory of Heat, at the beginning of chapter 1, Clausius refers to heat as follows: "Heat is in reality a mode of motion. According to this view, the heat found in bodies and determining their temperature is treated as being a motion of their ponderable atoms, in which motion the ether existing within the bodies may also participate; and radiant heat is looked upon as an undulatory motion propagated in that ether".

    We can excuse Clausius the use of the word ether but modern thermodynamics has identified a similar action, like a wave of energy that flows through atoms as heat. That has a counterpart in electronics where a wave of charge flows through a conductor independent of electron flow.

    Essentially, Clausius was correct, that heat is a mode of motion related to atoms. The temperature of a body is directly related to the kinetic energy of its atoms and KE has a velocity component in it. Therefore heat is related to the motion of atoms and the degree of heat is related to the velocity of the atoms, either as particles in a gas or as atomic vibration in a solid.

    With regard to definitions in physics, many people make presumptions that are not warranted. In his 1913 book on The Theory of Heat Radiation, Planck clarifies some of these issues (page 206):

    "Thus the units of length and time were derived from the present dimensions and motion of our planet, and the units of mass and temperature from the density and the most important temperature points of water, as being the liquid which plays the most important part on the surface of the earth, under a pressure which corresponds to the mean properties of the atmosphere surrounding us".

    It is clear that time has no existence other than as a definition created by humans based on the the rotational period of the planet. Length in metres is defined as a fraction of the distance from the Equator to the North Pole. Temperature and density are defined based based on the freezing point of water.

    Heat as such is not based on a human definition, it exists as a phenomenon, as energy. We have no idea what energy is but we have defined different types of energy, each having unique properties. Maybe there's a fundamental connection between the different types but for now we know heat as energy, although some call it thermal energy.

    In the following quote from experts in thermodynamics, Gerlich and Tscheuschner, in their paper on the greenhouse effect (page 16), they state:

    "Heat is the kinetic energy of molecules and atoms and will be transferred by contact or radiation. Microscopically both interactions are mediated by photons. In the former case, which is governed by the Coulomb respective van derWaals interaction these are the virtual or o-shell photons, in the latter case these are the real or on-shell photons. The interaction between photons and electrons (and other particles that are electrically charged or have a nonvanishing magnetic momentum) is microscopically described by the laws of quantum theory. Hence, in principle, thermal conductivity and radiative transfer may be described in a unified framework. However, the non-equilibrium many body problem is a highly non-trivial one and subject to the discipline of physical kinetics unifying quantum theory and non-equilibrium statistical mechanics".

    You will see that definition of heat on many university sites on the Net.

    An example from an MIT article:


    "To understand how heat spreads through a material, consider that heat — as well as sound — is actually the motion or vibration of atoms and molecules: Low-frequency vibrations correspond to sound, while higher frequencies correspond to heat. At each frequency, quantum mechanics principles dictate that the vibrational energy must be a multiple of a basic amount of energy, called a quantum, that is proportional to the frequency. Physicists call these basic levels of energy phonons.

    In a sense, then, “phonon” is just a fancy word for a particle of heat".
  21. Oct 11, 2015 #20

    Andrew Mason

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    As I said, there are many uses of the term "heat". But in thermodynamics, e.g. the second law of thermodynamics, heat (Q, heat flow), has a particular meaning that is distinct from the internal energy that a body has by virtue of its temperature. So if you want to apply the second law, you have to use heat in that sense.

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