|
Register | Sign In |
|
QuickSearch
Thread ▼ Details |
|
Thread Info
|
|
|
Author | Topic: A science question | |||||||||||||||||||||||
Silent H Member (Idle past 5820 days) Posts: 7405 From: satellite of love Joined: |
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. I think Sylas is backed up by the info at wiki, but I am in agreement with you that it becomes open to misinterpretation, and so the kinetic definition (which you call the layman's definition) is more convenient to use. holmes "...what a fool believes he sees, no wise man has the power to reason away.."(D. Bros) "...don't believe I'm taken in by stories I have heard, I just read the Daily News and swear by every word.."(Steely Dan)
|
|||||||||||||||||||||||
sfs Member (Idle past 2534 days) Posts: 464 From: Cambridge, MA USA Joined: |
quote:It's not just that solar radiation peaks around visible frequencies: our atmosphere also has a (fairly) narrow window of transparency in the same region.
|
|||||||||||||||||||||||
crashfrog Member (Idle past 1467 days) Posts: 19762 From: Silver Spring, MD Joined: |
Either I'm wrong or you guys are confusing kinetic energy with heat energy. There is no such thing as heat energy, to my understanding. For instance, one atom has no temperature. Heat is the epiphenomenon of the motion of many, many atoms in matter. This is the conclusion of the kinetic theory of gases, it's why hot things expand, etc. "Heat" is simply a label we apply to the level of random atomic movement in a system. That energy can be transmitted via collision (conduction/convection) or via radiation, when photons are emitted or absorbed.
The two forms of energy, while extremely interrelated and interdependant, are completely different forms of energy...I thought. If they're so interdependant and interrelated that you can't tell the difference, then no, they're not "completely different forms of energy." "Heat" is simply a word we use to model what is actually a consequence of kinetic energy in a system. This message has been edited by crashfrog, 03-07-2005 12:37 AM
|
|||||||||||||||||||||||
crashfrog Member (Idle past 1467 days) Posts: 19762 From: Silver Spring, MD Joined: |
I think (emphasize: THINK) it is only an illusion that heat is conducted. I believe the kinetic energy is conducted, and the heat results from the increased kinetic energy. Similarly, irradiation increases the kinetic energy--not the light energy--of the absorbing system. However, increased heat results from the increased kinetic energy. Perfect premises; exactly wrong conclusion. Only kinetic energy is conducted, only photons are radiated; the end result is the movement of kinetic energy. "Heat" is not produced as a result, "heat" is merely the word we use to describe this system of kinetic energy. But there is only kinetic energy, there is no such thing as heat.
|
|||||||||||||||||||||||
Percy Member Posts: 22394 From: New Hampshire Joined: Member Rating: 5.2 |
At a practical level I agree that heat is the aggregate motion of many atoms/molecules, but about this:
Crash writes: For instance, one atom has no temperature. 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
|
|||||||||||||||||||||||
TheLiteralist Inactive Member |
CF,
Thanks. If you are correct, then I think I finally "got it." Once again, I had a half-way decent concept of WHAT was happening, but had trouble with the terminology (you may recall a similar problem with "random mutations"...heh). Thanks everybody for your patience...--TheLit AbE:The illusion was that there was a type of energy called heat energy, eh? Haw! This message has been edited by TheLiteralist, 03-07-2005 14:40 AM This message has been edited by TheLiteralist, 03-07-2005 14:42 AM
|
|||||||||||||||||||||||
Loudmouth Inactive Member |
quote: I could be wrong, but I think microwaves and IR heat objects through different mechanisms. IR is directly absorbed and transfered into kinetic energy. Microwaves cause polar molecules to flip in synch with the EM waves causing friction between molecules which is then transfered into kinetic energy. This is why paper does not heat up but water, because it is polar, does heat up. Does this qualify as separate mechanisms, or am I smoking out of the wrong pipe again?
|
|||||||||||||||||||||||
crashfrog Member (Idle past 1467 days) Posts: 19762 From: Silver Spring, MD Joined: |
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? I'm no physicist, but I don't understand why that would be a useful model. How would you describe its linear velocity in degrees? We can say that my speed in my car, barrelling down the highway is a measure of my heat as well, but what would be the point? We'd still wind up with a number expressed in miles per hour. Its velocity is a measure of its velocity. You could call that "heat", but why bother? This message has been edited by crashfrog, 03-07-2005 14:55 AM
|
|||||||||||||||||||||||
Sylas Member (Idle past 5261 days) Posts: 766 From: Newcastle, Australia Joined: |
I think Sylas is backed up by the info at wiki, but I am in agreement with you that it becomes open to misinterpretation, and so the kinetic definition (which you call the layman's definition) is more convenient to use. 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
|
|||||||||||||||||||||||
Loudmouth Inactive Member |
quote: Increased velocity is increased kinetic energy. Temperature is the average kinetic energy of a body. For temperature or the transfer of heat you need at least two atoms. I would expect that without two atoms you would not be able to measure the "heat" content of a single atom. Kind of a Maxwell's Demon sort of exercise. ABE:Sylas snuck this tidbit in as well. We seem to agree at some level: "Heat seems to be a macro-level phenomenon; really only defined in terms of large systems; not individual molecules or individual photons." This message has been edited by Loudmouth, 03-07-2005 15:20 AM
|
|||||||||||||||||||||||
Percy Member Posts: 22394 From: New Hampshire Joined: Member Rating: 5.2 |
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.
Sylas writes: The kinetic energy of molecules establishes the temperature of an object, or how hot it is...Heat seems to be a macro-level phenomenon; really only defined in terms of large systems; not individual molecules or individual photons. 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.
Light is not heat; except when it's part of the radiative transfer of energy from hot objects to cold ones. 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
|
|||||||||||||||||||||||
Loudmouth Inactive Member |
Implication under debate: "Light is not heat; except when it's part of the radiative transfer of energy from hot objects to cold ones."
quote: I think the implication is wrong as well. I do agree that light is photons which have the capability of heating an object. However, light itself is not heat. In your example the colder object is still heating the hotter object through EMR, just at a slower rate than the hotter object's heat loss, in keeping with the Laws of Thermo. If heat transfer is through EMR I think Sylas was incorrect when he stated "from hot objects to cold ones". If he removes that part of the statement I think he is correct. This message has been edited by Loudmouth, 03-07-2005 16:08 AM
|
|||||||||||||||||||||||
Sylas Member (Idle past 5261 days) Posts: 766 From: Newcastle, Australia Joined: |
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:
"The Use and Misuse of the Word 'Heat' in Physics Teaching"
Anything I quote from this are extracts available fromheat at hyperphysics (which is an excellent general reference, BTW).by Mark Zemansky, in Physics Teacher, Sep 1970 p295 Zemansky has a little jingle:
Teaching thermal physics Is as easy as a song: You think you make it simpler When you make it slightly wrong More from hyperphysics:
quote: and
quote: 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
|
|||||||||||||||||||||||
Percy Member Posts: 22394 From: New Hampshire Joined: Member Rating: 5.2 |
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
|
|||||||||||||||||||||||
Percy Member Posts: 22394 From: New Hampshire Joined: Member Rating: 5.2 |
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
|
|
|
Do Nothing Button
Copyright 2001-2023 by EvC Forum, All Rights Reserved
Version 4.2
Innovative software from Qwixotic © 2024