Science desk
< November 21	<< Oct \| November \| Dec >>	November 23 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is a transcluded archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

November 22

Two questions regarding electrical & thermal conductivities in metals and incandescence

a. Wiedemann-Franz law is an empirical law indicating the linear relation between electrical & thermal conductivities. What's the underlying microscopic-quantun explanation ? After all, thermal electronic motions are random in all directions, but electrical conductance is directional (opposing the electrical field).

b. What's the physical mechanism explaining the continuous spectrum emitted when an electric current flows through it (e.g., in an incandescent lamp or a oven) ? again, the question focuses on the fundamental description, on the basis of the electronic band structure in metals. Thanks, בנצי (talk) 20:58, 22 November 2022 (UTC)[reply]

a. The W–F law applies to metals, in which some electrons are not bound to atoms but can move around, the higher the electrical conductivity, the more freely. These electrons carry not only a charge, giving rise to an electrical current, but also thermal energy, giving rise to a flow of heat. Consider that the motion of the particles of a gas is also random in all directions. What gets transferred as heat are not the particles themselves, in this case electrons, but their kinetic energy.

b. In a sufficiently hot material, hot to the point of incandescence, the atoms and therefore the electrons and protons are violently jostled around, with velocities that can be modelled as a three-dimensional continuous Gaussian distribution. Since they are charged, their motion produces electromagnetic radiation, which in this situation is called thermal radiation.

--Lambiam 00:24, 23 November 2022 (UTC)[reply]

a. Your comments here repeat known facts, already included & implied in the question. Hence, I'll pinpoint & emphasize it. 1st, let me make a comment on your last phrases: what is transferred isn't just Ek, since the process of collisions between electrons & (metallic) ions on the line of the electric field is a very slow one (drift velocity). So the picture of conductance is more complicated - the energy is transferred like an EM wave. So it's not the mere charge motion. Moreover, this phenomenon depends on the existance of phonons etc. These are the background of my question. To conclude the focus of the 1st question, why electric & thermal are linearly dependent ?

A relevant remark: pay attention to fact that thermal energy transfer is slow, while the electric one is (much) fast(er).

b. Essentially, your answer here is based on the classical model of accelerating or decelerating charges, while in the question the QM picure was addressed - typical metallic energy band structure of the electrons.

c. Following the above discussion, how these two models (classical & QM) correspond to each other ?

@בנצי: Your harsh dismissal of Lambiam’s answer was unwarranted. You did link to the Wiedemann-Franz law, but you called it an "empirical law" and asked for a "microscopic-quantun [sic] explanation". It seemed reasonable to infer that you had not read or understood that article fully, as the section Wiedemann–Franz law#Derivation (the first section past the lead, with a fairly explicit name) explains that the linear relationship follows easily if you assume both transport phenomena follow the Drude model. Lambiam’s level of explanation was perfectly reasonable in light of these assumptions.

I do not know if the problem is poor communication, or a "knowledge exceeds wisdom" issue. In either case the problem was squarely on your end; but even if it had not been, you should still remain civil. Tigraan^{Click here for my talk page ("private" contact)} 10:53, 23 November 2022 (UTC)[reply]

Sharpness & focusing aren't harshness, and your comments don't contribute to the discussion. The original question was very clear, and I don't see why you find saying "empirical" a problem. Well, it's not a fundamental law but an empirical one, and was quoted to indicate the linear relation between the two phenomena. I was sorry to find & read this response of yours, let alone postulating on a "perfectly reasonable" with no substantiation on physics grounds, and apologize for having a professional standpoint, and even critical. Raising a question should not lead to inferiority, let alone comments regarding the extent of civil. My intentions were completely honest & genuine, trying to get better insight/s into the mentioned topic. The answers led to none.

Maybe "poor communication" or maybe not. Many thoughts & efforts were made to phrase the questions clearly, but they seem being diminuated, which is insulting enough, let alone taking the position of a judge, in the personal comments like "wisdom", "squarely" blaming me, and the "civil". Thank you very much indeed.

Eventually, I'll find out about what were asked elsewhere, and maybe add it here for the benefit & completion of the discussion, for all. בנצי (talk) 11:56, 23 November 2022 (UTC)[reply]

I have no doubt that you have genuine intentions to learn, and I think you were not uncivil enough to warrant sanctions.

You seem to think that your post is entirely clear, and that Lambiam’s answer was useless. Fine, here’s a simple solution: let’s wait and see if someone else answers. If you don’t get any further replies, it must be because nobody on the RefDesk is qualified in those topics. In that case, make sure to post it on https://physics.stackexchange.com/ and give us a link so we can read the answers and be enlightened. Tigraan^{Click here for my talk page ("private" contact)} 13:00, 23 November 2022 (UTC)[reply]

I found neither harsh nor uncivil exressions in the OP's response to the first responder.

WF law has two aspects: the empirical one, and the theoretical one the OP asks their question about. Please note that this law really has the empirical aspect, in the sense that this law can be experimentally verified (at least in the specific cases checked out in the experiments), as opposed to some physical arguments which (hypothetically) can only be refuted yet not be verified (like many well known astrophysical theories, and even many mathematical hypotheses, e.g. "this random infinite sequence of digits contains no zeros", and the like, while WF law is different because it can also be experimentally verified). Anyway, the OP only claimed that this law, having its empirical aspect, must (probably) also have its quantum theoretical background, so what is it?

I won't be surprised if it turns out that the OP was one of the main contributors to our article about WF law. Anyway, I couldn't find out how your comment about "knowledge exceeds wisdom" could help.

As far as I could understand, the OP's first question is clear: An electric flow is measured as a vector, while heat is measured as a scalar, so how can this vector and this scalar be linearly related to each other, from a quantum theoretical viewpoint? How does Drude model explain this linear connection?

A last comment, but to the OP this time: Maybe you should have introduced yourself, before you asked your question on this reference desk, beacuse the previous editor could see no userpage of yours. סמי20 (talk) 18:39, 23 November 2022 (UTC)[reply]

I am not convinced by the remark that "thermal energy transfer is slow, while the electric one is (much) fast(er)" is an essential fact because a practical measurement of either must be affected by a finite thermal or electrical capacitance respectively. Philvoids (talk) 20:02, 23 November 2022 (UTC)[reply]

"Not convinced" in what ? what claim is not convincing ? Hold an iron bar in its end, while the other end is put in flame. You must be knowing you don't have to remove your hand too immediately. Hoever, making a call to you on the phone intercontinentally for example, or sending this reply via Wiki interface, require ~microsec. בנצי (talk) 11:23, 24 November 2022 (UTC)[reply]

The iron bar has an isobaric heat capacity of 25.09 J⋅mol−1⋅K−1. That has to be taken into account in your estimate of "time to say 'ouch' ". Philvoids (talk) 14:33, 24 November 2022 (UTC)[reply]

Yes, electric flow is measured as a vector, while heat is measured as a scalar. Conductivity, however, whether electrical or thermal, is neither. It measures the effect of applying a difference (voltage or temperature). One might just as well offer the objection that charge is measured as a scalar, while heat flow is measured as a vector. It is equally irrelevant. --Lambiam 20:04, 23 November 2022 (UTC)[reply]

Current is not vector, and it's a mistake said previously. — Preceding unsigned comment added by בנצי (talk • contribs) 11:31, 24 November 2022 (UTC)[reply]

The current density is definitely in essence a vector quantity.

In many useful applications, electricity can be assumed to flow along a single axis (because the length of a wire is much greater than its diameter). Then one can reason in of scalar properties (current, resistance), use Ohm’s law in scalar form, etc. Similarly, when looking at a one-dimensional heat transfer problem, one can use scalar values: for instance, if a thin insulating layer is placed on a large section of wall, the (vector) heat flux is mostly perpendicular to the surface, and one can reason as if temperature / flux varied only along that axis.

However, when looking at a problem where multi-dimensional effects are important, one needs to consider the vector formulation of current density, electric field etc.

There’s another subtlety of intensive and extensive properties - for instance current, an extensive property, is the product of the (intensive) current density times the cross-section of the wire; but it’s not really relevant, one can define a scalar current density just fine. Tigraan^{Click here for my talk page ("private" contact)} 14:07, 24 November 2022 (UTC)[reply]

@20סמי:,

Thank you for your nice & relevant contribution to this discussion, and I'd like to make a few comments:

a. W-F law applies not to "special cases", but proved for metals due to their common structure & their consequent properties. בנצי (talk) 12:13, 24 November 2022 (UTC)[reply]

Oh, I didn't write it applies to special cases. Of course, it's a physical rule, so it must be universal (regarding metals), so it's supposed to apply to all cases. Maybe I had to add more words to my previous response, for making it clearer. I've only indicated that WF law (like every physical law) is experimentally verified at least in the specific cases checked out in the experiments. Like every physical law. Actually, no physical law can be experimentally verified for all cases, because the set of all cases is infinite. For exmaple, if I measure the mass of one electron, the result is valid for this electron only. For this result to be valid for a second electorn, one has to repeat the experiment for the second electron, and so forth. This principle is true for all physical laws, including WF law, and I'm sure you agree. סמי20 (talk) 16:59, 24 November 2022 (UTC)[reply]

b. Current isn't vector. בנצי (talk) 12:13, 24 November 2022 (UTC)[reply]

Oh, I didn't write current is a vector. I've only indicated it's measured (e.g. by it's density) as a vector. Anyway, I had to be more precise, because you were asking about the electrical conductance, i.e. the ratio of current to voltage. סמי20 (talk) 16:59, 24 November 2022 (UTC)[reply]

c. Drude model isn't the most update one, and is capable of explaining only some of metal properties. בנצי (talk) 12:13, 24 November 2022 (UTC)[reply]

@Tigraan, are you hearing? סמי20 (talk) 16:59, 24 November 2022 (UTC)[reply]

d. Self-introducing. Well, good idea, but better to be raised in the 'talk' page. בנצי (talk) 12:13, 24 November 2022 (UTC)[reply]

I'm sure if Tigraan had seen your userpage, they wouldn't have thought what they still think about you. Actually, you are more than a username: You are an individual, a person, a human being, and as such you may own several usernames on Wikipedia, so I still claim I won't be surprised if it turns out you are one of the main contributors to our article about WF law (not necessarily under your current username). סמי20 (talk) 16:59, 24 November 2022 (UTC)[reply]

I won't be surprised if it turns out that the OP was one of the main contributors to our article about WF law You know you can check that, right? (The answer is no.)

A last comment, but to the OP [...] Nobody is under any obligation to create a userpage or introduce themselves to edit Wikipedia, and the RefDesk is no exception. I do not modify the answers I give based on the identity of the person who asks (except maybe if the question is clearly susceptible of two interpretations, and I can guess which interpretation to take based on that, but that’s rare). If anyone is provably giving poor-quality answers to people they do not know or like, I support the usual escalation mechanism towards sanctions (friendly warning on talk page, if it fails stern warning, if it fails admin actions).

The OP’s command of English is poor (presumably because it is not their native language); there is no shame in that, but it does impede communication. There might also be a cultural barrier thing, where a perfectly acceptable sentence in one language becomes too harsh when translated in another language. Speaking for myself, I find Your comments here repeat known facts... pay attention to fact [sic] that... to be much harsher than what one would reasonably expect to find in a Q-and-A session.

Finally, I will elaborate on my "knowledge exceeds wisdom" line. I suspect that OP believes they have understood many things which in fact they have not mastered, moved on to much more complex topics, and as a result of that asked a poorly-formulated question (the abovementioned English difficulties compounding it). For example, suppose that someone asks why humans still exist, when our article Cretaceous–Paleogene extinction event says that no tetrapods weighing more than 25 kilograms (55 pounds) survived. Answering that question requires to make assumptions about what misconceptions the asker might have (for instance, do they believe humans coexisted with dinosaurs?); it might require counter-questions to find out where the misunderstanding is.

If you look at the original question, it is hard to tell if the OP would be best served by a basic explanation of how electrons conduct electricity, a hand-waving explanation of Drude model basics, or a technical discussion of the limitations of Drude and alternative models; or an analysis of experiment vs. models for the WF law; or something entirely different. Their following comments do not help much, either. Tigraan^{Click here for my talk page ("private" contact)} 14:45, 24 November 2022 (UTC)[reply]