A science question

There's no necessity that we agree on the best way to explain heat. Your ability at explaining complex topics is better than most (certainly the best I've seen outside published books) and perhaps justifies your confidence that it can be explained sensibly to non-science types.--Percy

I think we can perhaps avoid the terminology issue by referring to temperature instead of heat. As the temperature of an object rises its molecules move faster and faster. It's really that simple.The requirements of a consistent conceptual framework for thermodynamics make impossible the equating of any formal definition of heat with the kinetic energy of molecules. The digression into the definition of heat was primarily about the importance of maintaining formal terminology at a lay level. I don't think there is any disagreement at all about what actually happens to molecules as they are heated.My concern throughout this discussion has been that you might only learn the proper words to recite without understanding what is actually happening to molecules of matter when they're heated. Without that understanding it is only too easy to confuse light and heat, something which in my view the formal thermodynamic definition actually encourages for people without a formal background.--Percy

I think you might have misunderstood what I was saying. I didn't mention the word "heat". I wasn't equating temperature to heat. I was abandoning the term "heat" altogether in an attempt to avoid confusion between its lay and formal definitions, and instead used the term temperature. Nope, and this is why I didn't want to get into formal definitions. According to Hyperphysics on Temperature, "Temperature is not directly proportional to internal energy since temperature measures only the kinetic energy part of the internal energy, so two objects with the same temperature do not in general have the same internal energy."To see why this is true you can read Hyperphysics on Internal Energy.A little knowledge is a dangerous thing. I find it wonderful that you're willing and able to examine and understand this stuff, just get used to making lots of wrong turns along the way. I make mistakes all the time, and I often find that I have misconceptions buried deep in areas that I thought I understood. If you enjoy learning for learning's sake then this is great for both of us, and Sylas deserves a lot of the credit. If you only wanted to know whether the earth is getting hotter or colder then this might be frustrating. I think you're reading more into this than I was saying. As Sylas has correctly pointed out, heat is not the kinetic energy of molecules. I was agreeing, and at the same time stating that the requirements of a consistent framework for thermodynamics require that heat not be viewed in this way.About the rest of the paragraph, along with snippets from here and there: What a "Through the Looking Glass" world we have now entered! Things were so much simpler and more understandable when heat could just be considered the kinetic energy of molecules. Now that we're so much more knowledgable we can confidently make statements like, "The object is heated but gains no heat."Sorry for the sarcasm, but we started out just trying to figure out whether the Earth gains or loses heat, and this whole heat digression, while extremely educational, isn't necessary.Light is not heat. Within a thermodynamic context light can be the means of conveyance of heat flow, but light is not heat.Consider an analogy. If I carry a glowing hot ingot from my fireplace to the anvil in my workshop, there has been heat flow between these two points. I was the means of conveyance of that heat, but I am not heat.What actually happens with infrared radiation is that some of the internal energy of a molecule is converted to a photon (EMR) which travels through space and strikes another molecule, adding its energy to that molecule's internal energy. Heat has flowed from one point to another by means of infrared radiation, which is just a frequency band of EMR, but infrared radiation is not heat. It is EMR.Another way to understand that EMR is not heat is to consider microwave radiation. Probably the microwave frequencies in your kitchen microwave are different from those used in microwave communication links, but I'm sure they're close enough that what I say here is accurate. A microwave photon strikes food in your kitchen microwave and adds to the food's internal energy, leading one to perhaps conclude that microwaves are heat. But a microwave photon from a microwave transmission strikes a receiving dish and is reflected to the central antennae where it excites electrons and causes microcurrents, not heat. So now microwaves are not heat. Then a storm blows a tree down into the path between the microwave transmitter and receiver, and the microwaves no longer reach the receiver but are aborbed by the tree and add (slightly) to the tree's internal energy. So now microwaves are heat again.But microwaves are not heat. Light is not heat. Infrared radiation is not heat. EMR is not heat. It's photons. According to Hyperphysics on Heat Radiation, "Radiation is heat transfer by the emission of electromagnetic waves which carry energy away from the emitting object."I long for a simpler time! --Percy

Hi JustinC,Without getting into too much detail, I'll just say I don't feel entirely comfortable with the definitions of heat and related terms at HyperPhysics. My guess is that it's not because they're wrong, but because my background isn't sufficiently deep for me to make the proper interpretations of definitions and descriptions. Language is not a very precise means of communicating something as mathematical as heat. I have a feeling that after one plays with the equations and some example problems for a while that the proper concepts and their interrelationships become embedded in one's mind, and then the language descriptions make perfect sense. But when all you do is attempt to learn something by reading instead of by doing then you're left trying to make Talmudic distinctions between word definitions, and language just isn't that precise.Sylas's suggestion to improve our understanding by tackling an actual problem is a good one, but it's not for everyone. Some people understand things quickly, and this is approach is best for that type of person. Some people like me are slower on the uptake, and this approach can take some considerable time, so while I would like to do this, I won't be. Some people just aren't science and math people and are never going to reach the point where they can make sense of the math. And some people, while curious, aren't *that* curious - they just wanted to have an interesting discussion.From a lay perspective, the biggest single problem I see with the definition of heat at HyperPhysics is one you already noted: that heat is energy in transit. But thermodynamics says that heat can only flow from a hotter body to a colder body, and there is definitely energy in transit from the colder body to the hotter body, just not as much as in the reverse direction. Perhaps by heat they really only mean the net of all energy in transit, and that's why in one of my posts I said perhaps they're actually looking at the problem at a process level, i.e., at a higher level of abstraction. At the level of solving thermodynamic problems perhaps they do not care as much about the means of heat flow, though one might then ask why they're so careful to define the three different ways of conveying heat.Whatever the right answer, I feel very uncomfortable with a definition of heat that means sometimes light is heat and sometimes it isn't. Light is always photons or electromagnetic waves (wave/particle duality), and its nature is unchanged whether or not it becomes heat at its destination.--Percy

Hi TheLit,You were originally wondering how we know the earth as a whole is cooling. We know this because the temperature gradient within the earth goes from cool on the outside to hot on the inside. If the earth were getting hotter then the reverse would be true, with the outside being hotter than the inside.We could confirm that the earth is cooling through a rather complex mathematical exercise, assuming that sufficiently accurate data is available, but this would to tough even for scientists who specialize in the field. Suffice to say that scientific work on this area *has* been done (several people have posted links), and if any contradiction with expectations had been found it would be well publicized, especially in Creationist circles. People who enjoy proving things to themselves should not be discouraged from attempting this exercise, but it certainly isn't for everyone.Surprisingly, I was able to type this post without using the word "heat".--Percy

I guess there are really two questions:

1. Is the earth a net radiator or net receiver of heat?
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2. Is the earth growing hotter or colder?

Regarding the first question, does the earth radiate more heat than it receives, the answer is an unequivocal yes. Once we know the direction of the gradient, the thermodynamic laws are all we need to conclude whether the earth is a net radiator of heat. Heat can only flow from hot to cold. Considering only the solid Earth as our system, if the interior is hotter than the exterior, then heat is flowing from the interior to the exterior. No other conclusion is possible. Heat can't flow in the other directionOur atmospheric buffer between the Earth's surface and outer space may give the appearance of introducing complexity, but not really. If the heat from the interior of the earth were not being radiated out into space, either directly or first to the atmosphere and then out into space, then the atmosphere would simply get hotter and hotter. But it doesn't. This brings us to my second question, is the Earth itself growing hotter or colder? After all, the Earth could be radiating more heat than it receives from the sun, but its internal heat engine could be generating more heat than is radiated, and we could be getting hotter.There are a couple arguments against this possibility. One is the dominant theories of the origin of the solar system, which hold that the planets were much hotter shortly after their original formation. If the earth cooled from an original molten state but is no longer cooling, then what occurred to reverse the trend? One possibility is that perhaps the Earth has reached thermodynamic equilibrium and so is now radiating as much heat as it generates, which means the Earth is not cooling but is merely standing pat with regard to temperature. Growing hotter wouldn't be a possibility unless the heat engine increases its output over time.Another argument against the possibility that we're getting hotter is that I think we have measurements indicating that the solid inner core is slowly expanding, which could only happen if the earth were cooling.One potential source of confusion is that the temperature gradient does run in the opposite direction near the surface in many parts of the world. For instance, I know that the temperature of the earth 330 feet beneath my house is 52^oF because that is the temperature of the water my water pump brings up from the ground. In the winter the surface temperature is often colder than 52^oF, and in the summer it is often warmer. But these are mere boundary condition variations. The net must be that the earth is a net radiator of heat into space, because the temperature gradient of the other 4000 miles beneath the surface permits nothing else.--Percy

Great link - thanks! An aside: I loved his unique contribution to the technical jargon: "vanishingly dinky"!--Percy

If you're saying that there cannot be heat flow between the mantle/core and atmosphere, then I don't think this is correct. You go on to say: The rate at which cheese conducts heat may be less than tomato sauce, I don't know, but it is not 0. There is no perfect insulator of conductive heat except a vacuum. If you pack a container of boiling water inside a sphere of styrofoam a mile thick, the water will still gradually cool by heat conduction through the styrofoam to the outside environment for as long as it is warmer than the outside environment.Thus, what you said in Message 127 about "A hard crust acts much like the crust on the top of a pot of cooking soup, trapping the heat below" isn't the way heat works. The hard crust conducts heat just like everything else made of matter. Some materials are better insulators of heat than others (the absence of all material is the best insulator of conductive heat, a vacuum again), but all materials conduct heat.Engineers designing the Hoover Dam calculated that the temperature of the concrete would rise 40 degrees while curing, and that the entire structure would take 125 years to cool naturally. Because the stresses that would develop during uneven cooling would crack the concrete, they laced the structure with 1" water pipes during construction through which they ran river water and later refrigerated water, and they monitored the exiting water temperature so as to know the proper temperature of water to send in so they could achieve even cooling as the cells of the structure were completed. Hmmm, didn't mean to get into that much irrelevant detail, but the point is that even very large structures made of materials you might think would impede heat flow conduct heat, even if slowly. And while I'm not going to bother to look up figures, my guess is that rock and concrete are pretty fair conductors of heat.Nature abhors a temperature gradient and will do its best to smooth it out - this is the 2nd law of thermodynamics. If you have a temperature gradient then there must be a heat flow, and there is no material in the universe that can stop it. On average, all the solar energy received from the sun is radiated back out into space. If it weren't then the atmospheric temperature would be increasing over time. And the temperature gradient within the earth requires that the earth itself is radiating some additional heat off into space, either directly into space or indirectly by conduction through the atmosphere, otherwise once again the atmospheric temperature would be increasing over time.--Percy

Edited for style. --PercyI think I've already made the points I can think of. Most of the details you mentioned don't seem directly relevant to whether the earth radiates more heat into space than it accepts. I can point out a few things that don't like quite right to me, so in case it helps I'll address just those points. No one thinks the atmosphere should be hotter than the core, and I can't think of anything I or Sylas said that would make you think we were arguing for that. If you can find the place where we said something that led to this misimpression then maybe I can clarify. Heat cannot be trapped. Short of a vacuum insulating layer, a temperature gradient is *always* indicative of conductive heat flow. If this indicates that you believe the heat flow is causing the different layers of the earth to change temperature, then I don't think this is correct. At the undetailed level appropriate to this issue, there is a temperature gradient from the hot core to the cool lithosphere. Heat flows along the gradient, but the temperature of the layers doesn't change.--PercyThis message has been edited by Percy, 03-12-2005 19:23 AM

It occurred to me recently that this probably isn't true. A molecule on the inside of a solid probably emits as much radiation as a molecule on the solid's outer surface. Radiation emitted from molecules within the solid are probably reabsorbed by nearby molecules. Such radiation must also play a role in heat conduction in a solid.I need a mental image for conveying heat through mechanical conduction in a solid. Does a molecule in a solid that is vibrating with "heat" transmit that vibrational energy to adjacent molecules through its bonds, i.e., through the interaction of its electrons in the outer energy levels? Or is it via some other means?Now that I think about it, my mental image for radiation being emitted and absorbed as a means of conveying heat isn't that good, either. When a photon strikes a molecule, I imagined it being absorbed by the electrons in the outer levels of the atoms of the molecule. The electrons have the option of jumping to a higher energy level, and they can re-emit a photon by falling back to a lower energy level, but what happens when the molecule absorbs the photon and increases its kinetic energy? How does that happen?I poked around on the web a bit, and one of the explanations I found said that collisions between atoms can cause an electron to rise to a higher energy level. When it drops back down a photon is emitted. But what is it about the mechanical collision between atoms that causes the electron to rise to a higher energy level. The higher energy level of the electron of one atom has to be matched by a decrease in energy level in some form but in the same amount in the other atom. What is it that happened to that other atom? Did one of its electrons drop to a lower energy level? Is the atom now moving more slowly? If this latter possibility is the case, then how did kinetic energy become transformed into the higher energy level of an electron?Of course, logic and common sense suggest that atoms are really just miniscule billiard balls and we can skip all this other complicated nonsense! --Percy

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