3 ms·

> It depends what level of detail you want to explain... To be clear, the issue we are seeking to explain is the first second or so of the thought experiment.

> It depends what level of detail you want to explain... To be clear, the issue we are seeking to explain is the first second or so of the thought experiment. [1] > The chain model explains pretty well what happens in the steady state of the circuit, as long as we take the chain links to represent EM waves and not actual electrons. But that isn't the chain model that you are complaining Verisatium is deprecating! You are implicitly agreeing with him by adding the caveat that I have emphasized! > It also suggests that the motor starting to move the chain will cause some amount of motion to happen at the middle point by radiating mechanical waves through the air/ground. This is getting increasingly Rube Goldberg with every iteration! > This explanation is actually equivalent to the amount of detail that can be gleaned from Veritassium's video... This is not an explanation, it is at best an attempt at an analogy. > ...since his explanation also doesn't give (a) any quantitative description nor (b) any idea of what kind of motion/energy is being transmitted. As hand-wavy as his explanation is, it does actually talk about things that can be found in EM theory. On the other hand, while you have hinted at a new, improved chain model that incorporates EM waves, you have not actually rolled it out to explain what happens in that first second in those terms, and I have no idea how your rattling-chains analogy is supposed to get the job done. >[more of the same, but with pipes and loudspeakers!] > In fact, this sound-wave-in-water-pipe model would also predict the right phenomenon if you had two close parallel unconnected pipes, and were sending a sound wave through one of them... It does? Please explain, because I have no idea how you think that can be achieved. In my previous post, I acknowledged your point that Veritasium could have been clearer, but I do not think this is a step in that direction. [1] More generally, wherever the ratio of the characteristic distances to the characteristic times is multiples of c, which is commonplace in RF.

tsimionescu 5y ago
I thought I had explained this in an earlier post as well, but let's go into more detail with the chain analogy. First of all, our experiment looks like this: we have a length of chain inside a tube arranged into a rectangle with 2 short 1m sides and two much longer sides (hundreds of km?). The longer sides of the rectangle are placed top and bottom. At the mid-point of the bottom side we have a motor that can pull the chain. At the midpoint of the other end, we have a piezoelectric generator standing very close to the chain. Once we turn the motor on, it will immediately start moving the chain at the near end, but it will take some long amount of time until the links near the piezoelectric start getting pulled. However, the movement of the links near the motor will create sound waves that will start traveling radially outwards from each point, starting with the position of the motor itself. The piezoelectric generator will start being rattled by these sound waves and producing a very small current after a duration of 1m / speed of sound in connecting medium (equivalent to 1/c s). After length of chain / speed of chain seconds pass (equivalent to 1s), the piezoelectric generator will start being rattled by the movement of the chain links near it as well, which has a much higher value. This experiment models (a) how the engine transmits energy to a distant system regardless of how the chain is moving, and (b) what happens in the initial "1s" before the steady state. If we switched from one-way movement of the chain (DC) to alternative movement (AC), the setup also predicts (c) that the energy keeps flowing from engine to piezoelectric generator, even if the chain links are no longer moving along the whole length of the chain. It also shows that (d) energy is not in the chain links themselves, but in the sound waves they give off. Note that I used a piezoelectric generator as the first thing that came to mind that does useful work while rattling in any direction, but the electrical component is not really important. > > In fact, this sound-wave-in-water-pipe model would also predict the right phenomenon if you had two close parallel unconnected pipes, and were sending a sound wave through one of them... > It does? Please explain, because I have no idea how you think that can be achieved. If a water pipe A is carrying a sound wave, that sound wave will not be confined to the pipe itself, it will also propagate outwards radially from each segment of pipe. If there is another pipe, the sound waves from the original pipe will start vibrating the water in the second pipe, which will in turn cause other molecules of water to start vibrating and thus carrying the sound wave along the second pipe as well. Of course, in practice the effect will be significantly dampened, but it should happen. The one thing that these mechanical wave models are completely incapable of explaining, and the thing that Maxwell was able to, is why and how these effects keep happening with electricity even in a perfect vacuum. Modelling EM waves as mechanical waves actually works pretty well until you remove the medium and they keep propagating.
- mannykannot 5y ago
  Firstly, I want to apologize for the tone of my previous reply; I could, and should, have made the main point - that this is no longer the chain model Veritasium was speaking of - in one short paragraph. Beyond that, I think there is an issue here in the relationship between analogy and explanation. If we draw an analogy between two different phenomena, we have not, at that point, explained either. If the analogy is between one phenomenon we understand, and one that we do not, it does not follow that the explanation for the former also explains the latter; that is an additional premise that has to be justified independently. It may be that the analogy points us towards a justification, but we cannot assume that. Analogies can also be helpful as what Daniel Dennett calls 'intuition pumps' - a point of view that may help us form intuitions about what is going on (they can also pump incorrect intuitions, however, so we have to be careful.) With regard to electricity, there was a time when it just seemed impossible to me that AC could send power to a distant lamp or motor, and the chain analogy helped me get around that block (I sometimes wonder if Edison, who held a strong and largely irrational antipathy towards AC, could have benefitted from it!) This analogy gives useful intuitions with regard to simple DC and low-frequency AC circuitry, but it rapidly breaks down if we try to push it too far. The simple analogy fails as soon as we get to transformers, for example. At this point, we might be tempted to patch it up this way: imagine that the chain drives a sprocket, which in turn drives a second sprocket with a different number of teeth, which is moving a second chain. There are several problems with this, such as there being no obvious reason for conflating the transformer turns ratio with the sprockets' teeth ratio, and that, taken as an explanation, it implies that transformers work for DC. Furthermore, it is just an ad-hoc patch to the chain-loop 'theory' of electricity. Maybe you can come up with something that avoids the second of these problems (e.g. use a lever instead of sprockets), and perhaps even the first (but if you go with the lever, now you have added the problem of why a lever but not gears), but the third will always be there - as it is in the increasingly complex models you have presented in this discussion. The fundamental problem is that there is no analog, in the mechanics of matter (including acoustics), to the interaction between electric and magnetic fields in the vacuum. There is an interesting historical connection here: I am told that as Maxwell struggled with EM, he filled his notebooks with many mechanical models, having space filled with tiny gears meshing with one another, but they did not make it into the theory. They seemed to have served as intuition pumps, or as a Wittgenstein's ladder - a sort of scaffolding that gets you to where you are going, but which you can throw away once you are there - though physics did not completely abandon the luminiferous aether until Einstein's theory of special relativity showed it to be unnecessary. In this light, I take the argument in Veritasium's video to be structured this way: the chain model (i.e. the simple one, in which the links stand for electrons) breaks down for more complex situations, such as what happens in the first second of his proposed experiment. In EM theory, however, there is a construct - the Poynting vector - which shows the flow of energy and which is completely general, providing a single unified picture of what is going on in all cases. It works for the first second of the experiment as well as the cases where the chain analogy is an effective intuition pump, such as the steady state of the experiment. Thus I, personally, have no problem with Veritasium saying that it is a strictly better explanation than the chain model, and also than your ad-hoc additions and alternatives to it (though he did not, of course, take any position on those.) One other point: the transmission-line model itself contains an analogy, between the distributed properties of the line and a circuit of discrete components. In that case, however, the discrete model was developed, by Heaviside, rigorously from first principles, so we know it can be used in explaining what is going on. That is how you do analogies that will serve as explanations! Finally, here's an insightful essay by Freeman Dyson, entitled "Why is Maxwell's Theory so hard to understand?" https://www.clerkmaxwellfoundation.org/DysonFreemanArticle.pdf https://www.clerkmaxwellfoundation.org/DysonFreemanArticle.p...

tsimionescu 5y ago

I thought I had explained this in an earlier post as well, but let's go into more detail with the chain analogy. First of all, our experiment looks like this: we have a length of chain inside a tube arranged into a rectangle with 2 short 1m sides and two much longer sides (hundreds of km?). The longer sides of the rectangle are placed top and bottom. At the mid-point of the bottom side we have a motor that can pull the chain. At the midpoint of the other end, we have a piezoelectric generator standing very close to the chain. Once we turn the motor on, it will immediately start moving the chain at the near end, but it will take some long amount of time until the links near the piezoelectric start getting pulled. However, the movement of the links near the motor will create sound waves that will start traveling radially outwards from each point, starting with the position of the motor itself. The piezoelectric generator will start being rattled by these sound waves and producing a very small current after a duration of 1m / speed of sound in connecting medium (equivalent to 1/c s). After length of chain / speed of chain seconds pass (equivalent to 1s), the piezoelectric generator will start being rattled by the movement of the chain links near it as well, which has a much higher value. This experiment models (a) how the engine transmits energy to a distant system regardless of how the chain is moving, and (b) what happens in the initial "1s" before the steady state. If we switched from one-way movement of the chain (DC) to alternative movement (AC), the setup also predicts (c) that the energy keeps flowing from engine to piezoelectric generator, even if the chain links are no longer moving along the whole length of the chain. It also shows that (d) energy is not in the chain links themselves, but in the sound waves they give off. Note that I used a piezoelectric generator as the first thing that came to mind that does useful work while rattling in any direction, but the electrical component is not really important. > > In fact, this sound-wave-in-water-pipe model would also predict the right phenomenon if you had two close parallel unconnected pipes, and were sending a sound wave through one of them... > It does? Please explain, because I have no idea how you think that can be achieved. If a water pipe A is carrying a sound wave, that sound wave will not be confined to the pipe itself, it will also propagate outwards radially from each segment of pipe. If there is another pipe, the sound waves from the original pipe will start vibrating the water in the second pipe, which will in turn cause other molecules of water to start vibrating and thus carrying the sound wave along the second pipe as well. Of course, in practice the effect will be significantly dampened, but it should happen. The one thing that these mechanical wave models are completely incapable of explaining, and the thing that Maxwell was able to, is why and how these effects keep happening with electricity even in a perfect vacuum. Modelling EM waves as mechanical waves actually works pretty well until you remove the medium and they keep propagating.

mannykannot 5y ago
Firstly, I want to apologize for the tone of my previous reply; I could, and should, have made the main point - that this is no longer the chain model Veritasium was speaking of - in one short paragraph. Beyond that, I think there is an issue here in the relationship between analogy and explanation. If we draw an analogy between two different phenomena, we have not, at that point, explained either. If the analogy is between one phenomenon we understand, and one that we do not, it does not follow that the explanation for the former also explains the latter; that is an additional premise that has to be justified independently. It may be that the analogy points us towards a justification, but we cannot assume that. Analogies can also be helpful as what Daniel Dennett calls 'intuition pumps' - a point of view that may help us form intuitions about what is going on (they can also pump incorrect intuitions, however, so we have to be careful.) With regard to electricity, there was a time when it just seemed impossible to me that AC could send power to a distant lamp or motor, and the chain analogy helped me get around that block (I sometimes wonder if Edison, who held a strong and largely irrational antipathy towards AC, could have benefitted from it!) This analogy gives useful intuitions with regard to simple DC and low-frequency AC circuitry, but it rapidly breaks down if we try to push it too far. The simple analogy fails as soon as we get to transformers, for example. At this point, we might be tempted to patch it up this way: imagine that the chain drives a sprocket, which in turn drives a second sprocket with a different number of teeth, which is moving a second chain. There are several problems with this, such as there being no obvious reason for conflating the transformer turns ratio with the sprockets' teeth ratio, and that, taken as an explanation, it implies that transformers work for DC. Furthermore, it is just an ad-hoc patch to the chain-loop 'theory' of electricity. Maybe you can come up with something that avoids the second of these problems (e.g. use a lever instead of sprockets), and perhaps even the first (but if you go with the lever, now you have added the problem of why a lever but not gears), but the third will always be there - as it is in the increasingly complex models you have presented in this discussion. The fundamental problem is that there is no analog, in the mechanics of matter (including acoustics), to the interaction between electric and magnetic fields in the vacuum. There is an interesting historical connection here: I am told that as Maxwell struggled with EM, he filled his notebooks with many mechanical models, having space filled with tiny gears meshing with one another, but they did not make it into the theory. They seemed to have served as intuition pumps, or as a Wittgenstein's ladder - a sort of scaffolding that gets you to where you are going, but which you can throw away once you are there - though physics did not completely abandon the luminiferous aether until Einstein's theory of special relativity showed it to be unnecessary. In this light, I take the argument in Veritasium's video to be structured this way: the chain model (i.e. the simple one, in which the links stand for electrons) breaks down for more complex situations, such as what happens in the first second of his proposed experiment. In EM theory, however, there is a construct - the Poynting vector - which shows the flow of energy and which is completely general, providing a single unified picture of what is going on in all cases. It works for the first second of the experiment as well as the cases where the chain analogy is an effective intuition pump, such as the steady state of the experiment. Thus I, personally, have no problem with Veritasium saying that it is a strictly better explanation than the chain model, and also than your ad-hoc additions and alternatives to it (though he did not, of course, take any position on those.) One other point: the transmission-line model itself contains an analogy, between the distributed properties of the line and a circuit of discrete components. In that case, however, the discrete model was developed, by Heaviside, rigorously from first principles, so we know it can be used in explaining what is going on. That is how you do analogies that will serve as explanations! Finally, here's an insightful essay by Freeman Dyson, entitled "Why is Maxwell's Theory so hard to understand?" https://www.clerkmaxwellfoundation.org/DysonFreemanArticle.pdf https://www.clerkmaxwellfoundation.org/DysonFreemanArticle.p...