A science question

The rate at which a body can radiate heat is a function of its surface area, and the rate at which a body can cool is a function of the ratio of surface area to volume. A sphere is the most efficient shape for conserving heat, which means spheres cool more slowly than any other object.But I think your issue is actually one of scale. You mention the time it takes a ceramic to cool from red hot, but a ceramic has a very high surface area to volume because it is so small, even if you're making marbles. For example, a one inch radius clay sphere has a surface area of 4πr² or 12.6 in², and a volume of 4πr³/3 or 4.2 in³, for a surface area to volume ratio of 3. The earth has a surface area of 8.1x10¹⁷ in² and a volume of 6.8x10²⁵ in³, for a surface area to volume ratio of 1.2x10^-8, which is orders of magnitude smaller than for your clay sphere. Naturally it will take much longer to cool.But Lord Kelvin calculated that it would take the earth only about 20 million years to cool from a molton state to its current state (he took the increasing temperature with depth into account by obtaining measurements from deep mines), and while I can offer no precise figure for how long Kelvin might have thought it would have cooled completely, certainly it wouldn't have been on the order of billions of years. The reason the earth hasn't yet fully cooled after 4.5 billion years is due to the contributions others have mentioned, radioactivity primary among them. Radioactivity was unknown to Kelvin, though it's contribution was uncovered not long before his death.Here's an example from the real world. When you take a tour of the Hoover Dam you learn quite a bit about its construction. Given the size and thickness of the dam, had they just layed the concrete and allowed it to cool naturally it would have taken a couple centuries. Not wanting to wait that long, they included water pipes to circulate cold water in every concrete segment. In other words, big objects take a long time to cool.--PercyThis message has been edited by Percy, 01-26-2005 11:30 AM

There are two ways that heat can be transmitted:

1. Conductively. There must be a physical connection to conduct heat. Grab one end of a metal rod and hold the other end over a fire and in a little while the rod will be too hot to hold. Heat was conducted up the rod from the fire end to our end.
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   Since a vacuum is the absence of matter, heat conduction cannot take place through a vacuum, and therefore a vacuum is a perfect insulator.
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2. EMR (Electromagnetic Radiation - light and radio waves are examples). The most well known form that transmits heat is IR (Infrared Radiation), which is just a particular frequency of EMR. Matter is very responsive to IR - molecules of matter readily accept energy of EMR at this frequency and begin moving more energetically (heat is the rate of motion of molecules - in solids they merely vibrate in place). Matter is also a good transmitter of IR, as long as it isn't too cold. That's why infrared goggles work so well at night - the warmer an object (a person), the brighter it appears in infrared goggles.
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   Much of the electromagnetic spectrum passes right through matter with little effect. For example, AM radio waves weaken only slightly when passing through matter. Your house presents little obstacle to AM radio waves. Massive concrete structures like bridges are another matter, which is why AM radio stations fade out under bridges. FM radio, at a higher and more energetic frequency, is less susceptible to absorption by matter, but even FM fades out when you go through tunnels. Any electromagnetic radiation that is absorbed by matter becomes heat.
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   Microwave radiation represents a good compromise of energy, transmission and absorption that makes it very useful for cooking. Like IR it is absorbed by matter (not all matter - microwaves pass through plastic and paper with little effect, but not water and many types of glass which absorb them readily), but unlike IR it also passes through much of matter. The additional energy of microwaves enables a fair percentage of them to penetrate to the interior of food and heat it faster.
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   Naturally a vacuum presents no obstacle to EMR. Heat (actually, energy which only becomes heat when absorbed at the other end) can easily be transmitted across a vacuum using EMR.

Holme's Message 13 that you were responding to actually already has good answers in the two immediately following messages by Coragyps and JonF, Message 14 and Message 15. They pretty much said what I just said, only more briefly and clearly.So, the short answer to the issue of how the earth radiates heat into space is through EMR, probably mostly in the infrared.--Percy

If you're really interested, G. Brent Dalrymple has an excellent account of the early attempts at establishing the age of the Earth and sun through thermodynamic analyses on pages 27-47 of his book The Age of the Earth. He concludes: In other words, it's sort of like predicting the weather. We understand the processes of weather very well, all the temperatures and air pressures and fronts and wind speeds and solar heating and so forth, but we can't measure everything and they interact in so complicated a fashion that we can only get a general idea of what's going to happen tomorrow. It's the same with the earth's thermodynamic behavior - we know all the factors involved, but there are so many factors and their interactions are so complicated and we can't measure everything, so while we know the earth is cooling, exact quantitative details aren't possible at this time.The early thermodynamic analyses relied upon making reasonable assumptions which nonetheless had such a wide range that one could get any answer for the age of the earth one wanted, from millions of years to billions of years (but not thousands of years). Lord Kelvin, the most prominent of those making age estimates based upon thermodynamics, argued for an age in the range of 20-100 million years, but this contradicted geological and evolutionary indications of a much greater age. The discovery of radioactivity as a source of heat showed that the greater ages predicted by geology and evolution were closer to the mark, and thermodynamic analysis became much less a viable possibility because of the addition of yet another not-well-understood (at that time) variable.You say that you don't believe the earth was ever completely molten, and that you don't believe the earth is losing heat faster than it gains it, but this is an inevitable conclusion from the data. The greater the depth the greater the temperature we measure. Naturally we can only dig so deep, but seismic analysis reveals much about deeper layers, and we know that below a certain depth the earth is molten right now. The temperature gradient of hot in the center to cool at the outside is what you get with a spherical body radiating heat into space. If we had a net acceptance of heat from space then the gradient would run in the opposite direction, with hot on the outside and cool in the center.--Percy

Oops! But physically moving a hot object to transmit heat seems so, well, primitive! --Percy

I agree with the various replies already posted. The key points:

1. Heat is the motion of atoms and molecules.
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2. Heat can be transmitted between atoms and molecules by contact. For example, a fast moving gas molecule colliding with a slow one will impart some of its kinetic energy, its heat, to the slower one. The slower one is now hotter, the hotter one is now cooler.
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3. Heat can also be transmitted by electromagnetic radiation, but EMR is not heat, it is energy. As Sylas says, matter at temperatures we're comfortable with emits EMR in the infrared region. When a molecule in the heating element of a heat lamp becomes hot enough it gives off a photon of energy at a frequency in the infrared region and temporarily becomes cooler (the electricity passing through the element quickly heats it again). This photon strikes a molecule of your skin and gives up its energy to that molecule, which now vibrates faster, and you experience this as heat.
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4. For completeness, and to avoid sending JonF into another coughing fit, heat can also be transmitted by convection, but this is not a transmission of heat between atoms and molecules, but merely the motion of atoms and molecules containing heat that are part of a viscous system and are moving because they are at a different temperature from other parts of the viscous system. Naturally, in a convecting system heat is also exchanged between atoms and molecules by collisions and EMR. (I suppose ancient catapults sending flaming tarballs over castle walls is another form of heat transmission. We could call this ballistic heat transmission. )

But we're getting off the subject. My original point, which is the same point others have also made, is that we can't measure all the contributions and losses of earth's heat to perform an accurate heat budget. But we *do* know that the earth is losing heat to space and not the other way around because the earth is hottest on the inside. If we were gaining heat from space we would be hottest on the outside. And we *do* know that the interior of the earth is hotter than the outside by direct measurements down a few miles, and by seismic studies revealing a molton interior.--Percy

I can see where your definition is more encompassing and descriptive, but I think it is vulnerable to easy misinterpretation by non-experts. Rather than elevating the discussion to a level where few can follow (including me), it might work better to stay at a layman's level.I think the layman's level understanding of heat is still pretty accurate, so maybe you can describe where you find fault with this. Heat is the motion of molecules, and the hotter an object, the more rapidly its molecules move. Moving molecules possess kinetic energy. A molecule can give up some of this kinetic energy by emitting photons (EMR), often at infrared frequencies. A molecule can increase its kinetic energy by receiving photons. EMR is not heat. A photon is not heat. While a photon is definitely "energy in transit", it is not heat, and this is where I thought your definition was most open to misinterpretation.--Percy

At a practical level I agree that heat is the aggregate motion of many atoms/molecules, but about this: Taking the simple case of an atom moving at a constant velocity in a vacuum, why isn't its velocity a measure of its heat? Why wouldn't increased velocity be interpreted as increased heat.--Percy

Hi Sylas,I'm not a well-known friend of ours, but I'm about to become utterly indistinguishable from him as I raise what are probably a string of red herrings. The first sentence seems contradicted by the second. A single molecule *does* have a kinetic energy. Whether it strikes another molecule or not, that kinetic energy is available for transfer to other molecules if a collision should happen. The exchange of kinetic energy is heat transfer through conduction. Following the implications of this, say we have a hot object and a cold object, both emitting EMR with a profile appropriate to temperature. Some proportion of this EMR from each strikes the other and is absorbed, but only the EMR from the hotter to the colder object is heat, according to your definition. This means the EMR from the colder to the hotter is...what? You've said it isn't heat, but then what is it? This seems contradictory.Maybe I'm dead wrong, but to me, EMR is not heat. EMR is photons, and photons are not heat. They are a means of conveying heat. Light is never heat. Light as photons is one type of energy. Heat as matter in molecular motion is another type of energy. You can convert back and forth between the two, but EMR and moving molecules are not the same thing.I don't pretend to understand the "heat as flow" paradigm, but perhaps mixing the laymen level understanding with elements of the more scientific level may be doomed to raise contradictions.While the heat (kinetic energy) of a single molecule might not be something measurable on any practical level, it still seems a very practical visual aide. I'd be the last person to deny physicists their preferred definition of heat, but that definition doesn't change the fact that when I heat something, its molecules move faster.--Percy

After a little more thinking, I'm beginning to wonder if the definition of heat that Sylas found at HyperPhysics is intended at the process level. In the process level view, heat is an entity in and of itself that can flow within a system. What's important in the process view is not the means of heat flow (i.e., radiative versus conductive versus convective versus macro-level motion), but the rates, direction and distribution of flow. By this definition a hot object heating a cool object is an example of heat flow, and the details of the heat flow are unimportant. The fact that the heat flow might actually be the net of EMR going in both directions can be ignored.But at a lower level of detail, if the heat flow was by way of EMR, then it must be understood that while EMR was the means of conveying heat, it is not itself heat. Faster molecules are heat. Sorry to beat on this so hard, but I see the diversion into the "heat as flow" definition as being very confusing to someone who was working very hard to understand heat. I don't claim to have the be-all-and-end-all best perspective on how to understand heat, but I think I can spot the potential for confusion pretty well.--Percy

Hi Sylas,I used Sears/Zemansky in high school, but you look a bit young to be using a 1970 textbook, and we didn't have that new an edition.I'm a patsy for simplicity. I like Zemansky's jingle, but ask if its practical here to expect the intellectual effort necessary to comprehension at the correct level. Even if I finally grok the proper physics definition of heat, I don't want to isolate myself from others not so lucky by demanding that they first understand it at my level. I want to be able to present what I've learned at their level. Glossing over details always risks being not quite right, but to not do so risks alienation through unintended obfuscation.Terminology is never a problem for me, but I don't see the necessity for a term like "internal energy" unless and until we reach the point where such fine distinctions became necessary. Insisting too early in the discussion on this switch to a less "visual" term might impede eventual understanding rather than enhance it. As HyperPhysics says, "It is better to say that it possesses internal energy as a result of its molecular motion." I understand the work versus heat argument, but this isn't an issue in the current discussion, so why not just stick with molecular motion?Most of us have played with molecular tinker toys in science class and have one visualization or another of what a molecule looks like. And we can add a visualization of these molecules moving and colliding like billiard balls on a pool table as an analog to molecules exchanging energy (heat) with one another. Or we can imagine molecules vibrating with heat in a crystal matrix that is gradually conducted through the matrix via the chemical bonds. Or we can imagine a molecule vibrating with heat in a crystal matrix that suddenly emits a photon and afterwards vibrates more slowly. But I have no good visualizations for "internal energy" (I get an image of a steam cooker with a pressure valve about to burst, but I can't think of a visual analog for transferring this steam in the cooker to other molecules).By the time we reach thermodynamic arguments about heat perhaps all our levels of understanding will be sufficiently elevated that we can switch neatly to a more appropriate definition, but doing so now wouldn't help the discussion.Please don't invest your free time with thermo textbooks on my account. I'm probably unsalvageable. I fear I'm as fixed in my views of heat as other people are in their views of space.--Percy

Don't worry - I by no means underestimate my ability to make a botch of it. Doesn't keep me from plowing ahead anyway! You're not usually reticent at challenging others views, so don't start now. When you detect a speck of inconsistency or error there's usually something to it. The way you're regarded here is not due to luck and happenstance.--Percy

Different people have described heat in different ways. Without getting into whether some ways of describing heat are more right than others, the different definitions can definitely be combined in ways that couldn't be called correct.Your post does an outstanding job of combining many of the definitions that have been offered, but it contains some contradictions. I would argue the following:

1. Heat is never light (electromagnetic radiation - EMR). EMR is photons and photons are not heat. Heat and EMR are different forms of energy. When a photon strikes matter it can be reflected or absorbed. If it is absorbed then the matter picks up the energy of the photon and starts moving or vibrating faster. The faster motion is increased heat. (Absorption of the photon isn't the only possibility, but it's the only one relevant to heat.)
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   Photons carry energy from one point to another. If the emitter of the photon emitted it due to heat, and if the receiver of the photon transformed the photon's energy into kinetic energy (heat), then you could argue that the photon transferred heat from one point to another, but the photon is not heat. You can use a bucket to carry water from one point to another, but the bucket is not water.
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2. Heat is very definitely the kinetic energy of molecules. No matter how complex and detailed the definition, no matter what particular terminology you use, there is no other way to look at it. (Somebody is bound to think of a context to illustrate that I've overstated the case, but ignore them and just accept that for any situation you'll ever encounter in your life on this earth, heat is just the kinetic energy of molecules.)
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3. I'm not sure about the definition of internal energy. It probably has a very definite context-dependent definition that I'm not going to bother looking up, but I don't think it is useful in a simple context. Even if we got the definition right today, it is certainly at a minimum vulnerable to being misremembered at a later date. For instance, say internal energy *is* just kinetic energy. Next month am I going to remember that internal energy doesn't include the energy of the chemical bonds of the molecule? Will I recall that it doesn't include the atomic energy? I don't trust my memory that much, so I prefer to stay with the familiar everyday concept of kinetic energy. If it were wrong to say heat is the kinetic energy of molecules then I'd make an effort to get it right, but heat *is* the kinetic energy of molecules.
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4. You say the term heat is very limited in scope of application. Perhaps for thermodynamic scientists this is true, I don't know. It isn't uncommon to hear a researcher say the more he studies the less he knows and the more mysterious things become. The more closely you study something the more the details seem to retreat into a gray fog of confusion. But we're not studying heat at this level of detail, and we don't want to or need to. Heat is the kinetic energy of molecules. Period.

The stuff about heat flow I view askance, but I won't challenge it. I don't find it terribly useful, either, but then I'm a rank amateur. I'd say so. You've sure done a far better job than I would ever have done on a new topic. Hats off! --PercyPS - Apologies to other participants in this thread if I seemed to run roughshod over your own preferred definitions. Regrets, etc...

Hi Sylas,I think Holmes and I are trying to make the same point, however unsuccessfully. If it were just you and me trying to have a discussion about heat, I'd switch to the formal definition in an instant. But we're trying to have discussions about heat with people with little background in science, and the more formal definitions you're pulling out of physics textbooks like An Introduction to Thermal Physics and Elementary General Thermodynamics are not accessible to this type of layperson. He has nothing in his experience upon which to base his understanding of these principles expressed at this level. The confusion is most evident where light is now being confused with heat.Sorry to be emphatic again, mostly it's just lack of time, didn't say everything I wanted, but gotta go.--Percy

Look at what else TheLiteralist said in Message 61:

• Heat is light--always.
• Kinetic energy produces light--always.
• I think (emphasize: THINK) it is only an illusion that heat is conducted.

Placed in context, the statement you quoted doesn't seem to reflect understanding. He made lots of statements about heat, and to me it seems that it is only by happenstance that one of them is reminiscent of the formal definition which draws a distinction between internal energy and molecular motion. But since internal energy of an object "is related to the random motion of their atoms or molecules" (Wikipedia's entry on Heat), the additional level of abstraction seems an impediment to clarity. Granted. Were I more familiar with the formal definition of heat I might have recognized his definition myself and acknowledged it. Certainly I have acknowledged TheLiteralist's success at making good progress despite our best efforts to confuse him. But I would probably have added that a discussion concerning whether the Earth is a net gainer or loser of heat doesn't need overly formal definitions of heat. Sorry it seemed that way. I tried to balance my posts. At one point I mentioned the high regard in which you're held here, at another point I likened my objections to being akin to Buzsaw in style, and at another acknowledging my potential for making a botch of it, all this trying to indicate that if my style seemed overly emphatic that I still understand my definition is "wrong" in a formal sense, but still strongly feel it more appropriate for this discussion.--PercyThis message has been edited by Percy, 03-08-2005 11:10 AM

Attempting to make the case for my simpler definition based upon web-available references...At Answers.com (http://www.answers.com/heat#Encyclopedia): At Wikipedia (Heat - Wikipedia): These sources are also unequivocal in defining heat as a "transfer of thermal energy" or as "nonmechanical energy in transit", and they are careful to distinguish between heat and internal energy, so you are correct, and I never meant to imply otherwise. When the discussion reaches the point where these distinctions are important then I'll advocate for the formal definitions right along side you.While the distinction between heat and internal energy is important to you, the distinction between moving molecules (heat, in my misbegotten terminology) and photons is important to me.At the level of detail of the discussion about whether the Earth is gaining or losing heat, it really only boils down to whether the Earth is getting hotter or colder, and temperature is a measure of the average kinetic energy of molecules. To make this point about the Earth I don't see the necessity, and see a lot of downside in the area of clarity, to saying that temperature is really only indirectly related to internal energy which is related to the motion of molecules and isn't really heat because heat is actually a flow. Sorry, now I'm tending toward sarcasm, but I'm just trying to be realistic. I can't say if I have an above or below average memory, but I guarantee that if I come back to this post next month and read what I just wrote that I will have little recollection of what I was talking about, and will have to follow the links to refresh my memory. But heat as kinetic energy of molecules seems like a reasonable and simple definition not only less likely to be forgotten, but also very easy for anyone to understand.--Percy