A science question

You are mistaken, in fact. A body gives off radiation depending on its temperature. The hotter the object, the more its emitted radiation shifts from infrared into higher frequencies. A sufficiently hot object will radiate primarily in the ultraviolet.Electromagnetic radiation includes radiowaves, visible light, hard gamma radiation, and everything in between and also beyond. "Infrared" radiation is just light at a longer wavelength than what we can see directly.We tend to associate infrared radiation with heat because warm objects in our normal experience are radiating at that wavelength.I can't makes sense of "all heat is light". No one wavelength of light stands out as have any special association with heat in general. Light is just a form of energy.Cheers -- Sylas

Your link is a rather simplified non-technical description, but it is quite correct. It says that

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Far infrared waves are thermal. In other words, we experience this type of infrared radiation every day in the form of heat. The heat that we feel from sunlight, a fire, a radiator or a warm sidewalk is infrared.

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In other words, in our experience, the things we consider to be "warm" are emitting in the far infrared; and more particularly our bodies are adapted to detect heat at these kinds of temperatures.The peak wavelength in centimeters of heat radiation times the temperature in degrees Kelvin of the emitting blackbody is a constant 0.2898. This is Wien's Law. A warm kitten is around 310 Kelvin. A fire is around 600 Kelvin. The peak wavelengths for these warm things is thus 10^-3 to 5*10^-4 centimeters, which is infrared.As something gets hotter, it starts to glow. This just means that the emitted heat is starting to include visible light. But long before this point we are reaching temperatures which are too hot for us to have any useful sensory detection. Things that hot burn instantly, and no special heat sense is required. Our finer senses are tuned to work well with cooler temperatures.Visible light is around 5*10^-5 centimeters. A blackbody at 6000 degrees will peak at around this temperature; and that is indeed the temperature of the Sun. So the Sun's heat radiation is actually visible light. We see this with our eyes, but it does not appear to us as heat, even though it is. The heat we feel on a warm day is (I think?) mostly the indirect infrared radiation from our environment which has been heated up by the Sun.Added in edit: Put another way; our vision system has adapted to work well with solar radiation; and our heat sense has adapted to be most sensitive to the range of temperatures in our local environment on Earth.If you get something even hotter, like around 60,000 degrees, it will radiate heat in the ultraviolet.Cheers -- SylasThis message has been edited by Sylas, 03-07-2005 01:59 AM

Your link correct, but it is not in conflict with the information people have been trying to explain to you. You need to understand that a light source has a spectrum, with a range of frequencies, and this spectrum depends on (amongst other things) temperature. There is no one frequency which means "heat". This is the first and most important think people are explaining to you.I have said that the Sun (being much hotter than most things in our normal experience) radiates at shorter wavelengths, with a peak at the visible wavelengths. Your link says that most of the power output of the Sun is infrared. How can both these things be true?The solar spectrum is quite close to a blackbody spectrum, with a temperature of about 5800 degrees Kelvin and a peak in the power spectrum at a wavelength of around 500 nm. Blackbody is a very important kind of spectrum; you can find a lot about it on the web. Here is a picture of the power spectrum for three stars. One is cooler than the Sun and one is hotter. Discussion available at The Solar Spectrum (University of Tennessee). Notice how in each case, the spectrum rises rapidly to the peak, and then tails off into the longer wavelengths. Some light is at a longer wavelength than the peak, and some is at a shorter wavelength. The shape of a blackbody spectrum means than more power is emitted at wavelengths longer than the peak than at wavelengths less than the peak.Now as has been explained, the Sun has the peak in the visible spectrum, and so the long tail of the spectrum goes off into infrared. This means that the peak is visible light, but there is more power at longer wavelengths than at shorter wavelengths, which is what your link is saying.Your link provides an image of the Sun at 1083 nm, which is about 10000 Angstroms. This is only one infrared wavelength; not the whole spectrum; and it is about twice the wavelength of visible light. It is, in fact, just a little longer than the peak wavelength the cooler star Antares. It is a peak for a body at about 2400 Kelvin; half the Sun’s temperature.This is still significantly less infrared than the far infrared which the link you gave a few posts ago singled out as thermal radiation. What we normally think of as thermal has a wavelength more like 5000 nm. So ANY light can be heat radiation.Now if we apply Planck’s equation (ref Blackbody Radiation at Rutgers University) to the wavelength of 1083 nm, and using the solar temperature of 5800 Kelvin, we get a power magnitude B_λ of 9*10¹². But the peak wavelength is about 500 nm (which is visible yellow) and this has a power magnitude of 2.7*10¹³; which is three times greater.Sure; most of the power is infrared. But it is spread out over a large range of the spectrum. For good vision focus, you want to single out a small part of the spectrum, and to get the most power in a small range you should go to the peak.All of this is simply to explain why your link is correct, and why this does not conflict with the other information people have been explaining. And as an object gets hotter than the Sun, you soon get to a point where most power output is ultraviolet, with a peak in the far ultraviolet. Have a look at the spectrum for the hot star Spica. The radiates nearly all its heat in the ultraviolet.Cheers -- Sylas

That’s not bad! It is closer to being correct than the definition of heat as kinetic energy; at least by modern usage.From heat at hyperphysics;

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Heat may be defined as energy in transit from a high temperature object to a lower temperature object. An object does not possess "heat"; the appropriate term for the microscopic energy in an object is internal energy. The internal energy may be increased by transferring energy to the object from a higher temperature (hotter) object - this is properly called heating.

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Thus heat is energy in transit; not the kinetic energy itself. The one quibble I have with your description is where you say heat is always light; there are other ways in which energy flows from hot objects to cooler ones, and they are heat as well.Your conclusion: "Kinetic energy produces heat. Heat produces kinetic energy." seems pretty good to me at capturing the distinction between heat and kinetic energy of molecules.Cheers -- Sylas

At hyperphysics, actually.I want to give credit where credit is due; and this is a point TheLiteralist got correct. He deserves to have this acknowledged.Layman's definitions are bound to be corrected in a forum like this. The kinetic energy of molecules establishes the temperature of an object, or how hot it is. But in physics "heat" is reserved for energy flows. Now consider a case of two blackbodies of different temperatures in an enclosed perfectly reflected box. The heat in this system is the light, not the kinetic energy.TheLiteralist may even have read something like this in his own studies; such examples can be used to try and explain how heat is defined in modern physics.However, I endorse most of what Holmes and others are saying. Light is not heat; except when it part of the radiative transfer of energy from hot objects to cold ones. The simple association of all light with heat is therefore wrong; this is not an implication of the example, and it is not what I have been saying. I’ve just pointed out that heat can be encompassed by light at any frequency. It does not follow that that all light is heat.Also heat can flow by conduction; without involving light at all, as Holmes has been explaining.Heat seems to be a macro-level phenomenon; really only defined in terms of large systems; not individual molecules or individual photons.Cheers -- Sylas

Percy, I'm the wrong guy to arbitrate this. I'm not a physicist. I can only point out that some good references back up the convention that the word "heat" is linked with flows; and that the energy bound up in a molecular kinetic energy of a hot object is more correctly called "internal energy".Hyperphysics cites an article, which I will look up this coming weekend if you like: Anything I quote from this are extracts available fromheat at hyperphysics (which is an excellent general reference, BTW).Zemansky has a little jingle: More from hyperphysics:

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To describe the energy that a high temperature object has, it is not a correct use of the word heat to say that the object "possesses heat" - it is better to say that it possesses internal energy as a result of its molecular motion. The word heat is better reserved to describe the process of transfer of energy from a high temperature object to a lower temperature one. Surely you can take an object at low internal energy and raise it to higher internal energy by heating it. But you can also increase its internal energy by doing work on it, and since the internal energy of a high temperature object resides in random motion of the molecules, you can't tell which mechanism was used to give it that energy.

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Zemansky points to the First Law of Thermodynamics as a clarifying relationship. The First Law identifies both heat and work as methods of energy transfer which can bring about a change in the internal energy of a system. After that, neither the words work or heat have any usefulness in describing the final state of the sytem - we can speak only of the internal energy of the system.

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I have no authority of my own to confirm this. To establish common usage, I'd simply go to a uni libary and check out half a dozen good thermo texts, and see how they use the terms. I have not done this yet.Cheers -- Sylas

Frankly, you and Holmes are most likely one up on me in thermodynamics. I happened to know a bit about the blackbody spectrum from my interest in cosmology; the CMBR is just about the most perfect known blackbody in real life; and stars are surprising close approximations to blackbody radiators as well. This example was able to show why infrared radiation does not have any special status in thermal physics.But in the rest of discussion, I'm exceeding the bounds of my ability.The one point I'd make here on alienation is that we can alienate a new contributor who is on the receiving end of a whole pile of people telling him he is "wrong" by failing to acknowledge bits he gets right.I'm not enough of a physicist to stand in judgement of your views; so I don't say you are wrong and indeed I could most likely stand to learn a fair bit from you guys.On the other hand, you might be glossing over some details that others are trying to get correct. Some aspects of the posts from TheLiteralist could be based on more technical definitions. There does seem to be some correspondence, in any case. If by glossing over details you also obscure the bits he got right, that can alienate as well.Just a thought... Cheers -- Sylas

This is also indirectly a reply to Percy in Message 99, who I am sure would have been disappointed had I not written this post. Looks good, but there are a few points to clarify.I'm not confident in dealing with this off the cuff, so I went and checked a few reference texts. They are all pretty consistent with each other (and inconsistent with Percy!) so on that basis I'll hazard some comments. I have close to hand An Introduction to Thermal Physics (by Daniel Schroeder, Addison Wesley 2000) and Elementary General Thermodynamics (by Martin Sussman, Addison Wesley 1972).On your first point, heat is a form of energy, but like work it is properly used only for transient energy; not for any stored energy. Like all energy, whether stored or transient, it is measured in Joules.From Sussman (p 11):

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Heat cannot be stored in a system. A system, no matter how hot, contains no heat. It contains accumulated internal energy. ...

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Heat is energy in transit between systems at different temperatures. Once it enters a system it becomes some other kind of energy. ...

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From Schroeder (p 18):

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Notice that both heat and work refer to energy in transit. You can talk about the total energy inside a system, but it would be meaningless to ask how much heat, or how much work, is in a system. ...

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Percy may have been trying to get a rise by being so emphatic about heat being very definitely the kinetic energy of molecules. This is sloppy terminology and would be marked wrong in an exam.Your second point in relation to light and heat is potentially awkward. Explaining this is going to take us into some advanced concepts.Certainly radiative transfer is one way that heat can flow. The problem is that light can also be work. Sussman describes heat and work as the two forms of transient energy, and then comments as follows on light (p11):

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It is sometimes convenient to include a third form, radiation. This relieves us of the need to decide whether a particular radiation is WORK (coherent), or HEAT (blackbody), or a mixture of the two. The need arises when we deal with laser and other very special radiation sources and receivers. At this point in our learning process, we shall consider radiation as a form of transitory energy and thermal radiation as equivalent to heat energy.

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Neither book goes into radiation much; but the comments by Sussman illustrate an important point. Whether light is corresponds sensibly to heat depends on the whole spectrum; not just one special frequency. Thermal radiation is another term for blackbody radiation. This is analogous to the internal energy of gas as follows.The internal energy of an ideal gas is roughly the kinetic energy of the particles. However, the particles don’t all have the same energy. For a gas in equilibrium (a very important concept in thermodynamics) the kinetic energy of constituent particles follows a certain distribution. The peak of this distribution is defined by the temperature; but some molecules will have less energy and others have more. If they all have exactly the same energy then the gas is not (I think) at equilibrium; and its temperature is not well defined.In the same way, light where all photons have the same energy does not really have an associated temperature. But if the distribution of photon energy in light is a blackbody spectrum, then we can sensibly speak of the temperature of the light.My guess is that there may be two ways of speaking about light. It we are regarding light as part of the state of the entire system, then it is not treated as heat, but as stored energy. But if we are speaking of radiative transfer of energy from one reservoir to another, then we can speak of heat flow. There are whole textbooks on radiative transfer; I didn’t borrow those.I ran into this some years ago in talk.origins with analysis of reactions of oxygen and ozone with ultraviolet light. I got tied up in knots trying to manage the thermodynamics of the reaction until I figured out how light is handled; and it can be handled either as a reagent to the reaction or as part of the energy budget. With the help of various savants I solved some of the questions. Here is a link to my post which describes some of the solution (probably badly); check out also other posts in the thread. It does not give a simple answer to whether light is heat, but illustrates something of how analysis might proceed.Another problem is that we can’t say that all energy flow from hot objects to cold objects is heat; only if the energy flow is spontaneous (another thermodynamic term) by virtue of some thermal contact between them. (I may have got this point wrong myself in earlier posts... sorry!)Say, for example, we have three objects, A, B and C, all at different temperatures. We use the temperature difference between A and B to drive a heat engine, which drives a generator, which sends an electrical current down to C, and heats up C. Whether C is hotter, or colder, than A or B, makes no difference. The energy flow is work, because it is not by thermal exchange.For the rest, I agree. It’s unfair how much easier it is to say I agree than to quibble about the points I disagree upon, and this can give the wrong impression. There is more agreement than disagreement. The disagreement is just more fun to write about.Cheers -- Sylas

Thermal contact includes the notion of a space over which associated blackbody radiation can transfer, so there is no problem here. This is explicit in the texts I am using. The notions of "contact" and "spontaneous transfer" are linked.I disagree that the standard notions used in physics are less useful or accurate. In fact, that is an absurd position to take. Real accuracy requires one to use terms according to the standard definitions; and this is what I am doing.In Message 70, you gave a link to a No webpage found at provided URL: wiki definition of Heat. The very next two sentences after the portion you quoted read as follows:

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It is a common misconception to confuse heat with internal energy, but there is a difference: heat is related with the change in internal energy and the work performed by the system. Understanding this difference is a necessary part of understanding the first law of thermodynamics.

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Quite so.Cheers -- SylasThis message has been edited by Sylas, 03-08-2005 05:18 AM

We're moving off topic here. I don't think we need to correct every instance of slightly improper usage when we see it; but I don't see a need to use it incorrectly ourselves. I'll prefer to say "hot", not "having heat"; and to let other usage slide in informal conversation. But when science or thermodynamics is the actual topic of discussion, then it is worth getting more pedantic.As for my background; nothing formal. I used to be outstanding in physics at high school, and I did one year of physics in a BSc, before being sucked into the dark side of computers. Sometimes I regret that switch; in any case I still read widely.Cheers -- SylasAdded in edit: I've just looked over the thread again; and I am not sure if you have a clear answer to your questions. The answer is that the nearly all energy entering and leaving the Earth is by EMR. Most of the input is from the Sun, with a hot blackbody spectrum. About 31% of this is reflected; and the rest is absorbed. The amount absorbed is balanced by the amount leaving as infrared radiation. There is slightly more radiation leaving the Earth than is ariving, by a factor of about 1.00003; the excess is due to heat from the Earth's own core. Percy is correct in pointing out that the center of the Earth being hotter means that the Earth is cooling, with a net flow of heat from from the center to the surface. But since the surface remains at roughly a constant temperature, there is a mean energy balance between what is radiated from the Earth and what is received from the Sun, and received from the Earth's core. This energy flow does indeed continue out into space by infrared radiation.Cheers -- SylasThis message has been edited by Sylas, 03-08-2005 07:01 AM

I understand this; but I don't believe it matches what happens in this thread. It was not me, but TheLiteralist who first said in Message 61 "Heat (or light) is NOT kinetic energy, but heat is very, very closely related to kinetic energy. Kinetic energy produces heat. Heat produces kinetic energy."MY point, which I ALSO insist upon, is that when someone actually introduces into discussion a correct point, we should ACKNOWLEDGE IT.OK? He was RIGHT. He scores a point. Given that he had been making a lot of other errors, which I was correcting, I also felt it was important to speak up and give credit where due for what bits of his posts were correct.Then you guys started jumping all over me, as if it was somehow vitally important to keep protecting the sloppy usage. What gives? It sure as eggs looks to me that TheLiteralist was correct on this point, and you chaps couldn't see it.Sure, when helping a student we don't want to get overly pedantic. But when THEY introduce a correct point, recognize it for heaven sake.Sheesh.SylasThis message has been edited by Sylas, 03-08-2005 09:30 AM

My guess is that somewhere TheLiteralist has read descriptions of heat as being different from internal energy, and that heat was only properly used for a transfer of this energy. His actual errors were failing to recognize conductive transfer, and limiting heat to a certain frequency range of EMR. But his association of heat with transfer rather than kinetic energy itself was correct.TheLiteralist... apologies for talking about you so much in the third person here. Making you the topic of discussion may be a bit off putting.So I had not seen this as being correct by happenstance; but more likely as something he did learn, while drawing some invalid implications. I felt it good pedagogy to acknowledge and build on what was correct; and to explicitly affirm that he got this right; especially when I spent so much more time on the bits that were wrong.I disagree that correct usage is an impediment to clarity, and I disagree that the level of discussion in this case was such that we should avoid being accurate. It was actually at quite a high level, and explicitly dealing with terms and definitions which deserved to be treated carefully.I agree that there are some cases in which the distinction between heat and internal energy can be glossed over; though I still believe it is possible even then for our input to use terms carefully (for example, by saying "hot" rather than "has heat").But once this distinction has been introduced, then the proper thing is to explain it accurately. This is actually fundamental; it shows up right at the start of the texts I was using, which were themselves at an introductory level.You've both picked up on "jumping all over me". That came across badly; no offense is taken. I do find it weird that when the distinction had been introduced you still wanted to maintain technically incorrect usage, even to the point of being so emphatic about heat "very definitely the kinetic energy of molecules". An explcit statement like that, in the context where definitions of heat and internal energy were starting to be used, can only be called wrong; just as "heat is light--always" is wrong.Cheers -- Sylas

I am leaving this thread for others at present; but I want to endorse the above. I'm not really as insistent on rigidly precise formal usage as it appears looking back on this thread; mainly I was standing by a comment that one thing TheLiteralist got right was a distinction between heat and kinetic energy of molecules.For thermodynamics, I'm just a novice who borrowed a couple of text books. One thing you guys might like to try is a few simplified calculated examples. Consider a planet with a homogenous surface, bathed in solar radiation from all directions (ignore night and day, winter and summer) put in some numbers for things like specific heat of the surface material, the albedo, the amount of absortion in the atmosphere at visible and at infrared wavelengths, and so on. See if you can get a simple model that allows a temperature for the surface to be calculated. This may clarify some of what is actually happening.Cheers -- Sylas

Actually, the greatest amount of energy by far for the Earth is coming from external sources, not internal ones, by a factor of about 300,000 or more.That energy radiates out into space again, rather than percolating down in towards the centre. We know this, because the temperature gradient really does mean that there is a flow of heat from the centre out to the surface. This contributes its tiny fraction to the net energy flux, the vast majority of which ends up radiated into space as infrared radiation. The net energy flux, for Earth, or Venus, or any other planet, is effectively zero. The surface temperature is governed by the way in which that energy flux is absorbed and transformed and passed on. For example, a thick atmosphere (a bit like a blanket) does not actually provide any additional energy; it just slows the transmission of energy, especially if it is opaque to infrared light. The amount actual radiated energy is going to balance the input insolation; but the less efficient transmission means a warmer surface is required to drive the balancing infrared emissions. The temperature is basically driven to the point where the energy flux is able to balance the books and bring the output infrared radiation to match the input insolation (plus the tiny little bit of extra energy from internal processes).When you say we don't have the energy budget worked out; what this really means that that we don't have nailed down all the various transport mechanisms by which energy transfers. That the end result balances inputs and outputs over time, however, is given. If there was any excess input, or excess output, this would drive a temperature change until they balance again; a nice example of a feedback loop.To get an idea of the complexity of all the processes that are involved in a full description, see this Lecture online at California State University.Cheers — Sylas

The energy budget on the surface of the Earth and in the atmosphere is very complicated. The link I gave previously can be summarized in this diagram: Temperature gradients within the atmosphere are quite complicated. The flow of heat by convention and conduction can move both up and down, and at the same time there is a flux of radiation passing through the atmosphere. But heat flow by conduction always goes along the gradient.Within the Earth the matter is simpler. There is no radiation; and the temperature gradient sufficies as a formal thermodynamic proof that heart is flowing outwards to the surface; not inwards from the surface. The source of this heat is radioactive decay.Cheers -- SylasThis message has been edited by Admin, 03-13-2005 08:31 AM