| General > General Technical Chat |
| Sick of ridiculous KVL infighting |
| (1/15) > >> |
| instrumental:
40 pages and a few months on, some nerds don't seem to know when to quit arguing. The video Mehdi made about a "principled disagreement" with Lewin's lectures is rough as well, and now this has spiralled into numerous YouTube videos trying and failing to explain EM theory. I'm surprised to see so much discord in a community of professional electronics engineers. I'm quite tired of seeing this, so I am hoping to provide a fairly definitive explanation of: * Why KVL doesn't always work (e.g. why we don't use it for RF circuits or optics), * Why KVL can be used to analyse low-frequency circuits (power, audio, etc), and finally, * Where the gap is. So, here we go. Voltages measure electrostatic potential; the electric field is the gradient (slope) of the voltage, and the voltage is the potential function of the electric field. For a potential function to be defined on the field, the field has to be conservative. For a field to be conservative, it has to be path-independent (you can calculate voltage between two points by taking any path through the field; the voltage will only depend on the endpoints). For a field to be path-independent, it has to be curl-free. Imagine a toy boat circling a drain in the bathtub. We can divide the flow field into divergence-only (flow towards the drain) and curl-only (swirling around the drain) flows. Ignoring the divergence for now, imagine the toy boat is initially stationary. The flow will take the boat with it, and it will begin to pick up speed as work is done by the flow on the boat. Once the boat has made a full revolution of the drain, it is in the same initial place, but it is now in motion; it has acquired energy. This should illustrate to you that a circulating field is not a conservative field; we can't define potential energy between two points because different paths will do different quantities of work. Now, we might ask, what is the curl of the electric field? This is given by Faraday's law of induction -- it's proportional to the derivative of the magnetic field with respect to time. If we have a changing magnetic field, we have a non-conservative field. When do we have a changing magnetic field? Well, magnetic fields are made by currents; changing electric fields also create magnetic fields, which gives rise to EM wave propagation. So, any time we have a changing field or current (e.g. AC at any frequency), we can no longer define voltage. There are two ways out of this. Physicists often work with 4-potentials in EM theory. A vector potential can be defined for the divergence-free field; a scalar potential (voltage) can be defined for the curl-free field, and the two together produce the full field. This magnetic vector potential is significant: the Aharonov-Bohm effect illustrates this nicely. Unfortunately, this doesn't mean we get KVL. More commonly, approximations can be made which allow the re-introduction of voltages. If dB/dt is sufficiently small that it can be ignored, we can approximate the field as conservative. This is manifestly the case for power lines (50-60Hz) and holds (as a rule of thumb) roughly until the scale of the circuit under consideration is close to one-tenth the wavelength of the signal (e.g. a 1MHz signal has a wavelength in the tens of meters; you don't worry about MHz routing on a small PCB, but you run into RF effects with larger-scale antennae. By 1GHz, you're dealing with centimeters of wavelength and you're squarely in RF territory). TL;DR field theory explains phenomena from DC, to RF, up to the highest-energy gamma rays, with visible light in between. You wouldn't apply KVL to RF or optics, would you? |
| PlainName:
--- Quote ---I'm quite tired of seeing this, so I am hoping to provide a fairly definitive explanation of: --- End quote --- Looks to me like you think your explanation is the authoritative explanation and, in order that no-one gets sidetracked by lesser explanations, you've started a new thread just for this. Why couldn't you just put it at the end of the current long thread? Imagine if every participant there started a new thread just to highlight their own authoritative explanation - the entire forum would consist of root posts about this stuff. If you really want to make it count, delete this thread and just argue the merits in the proper place. Or just sigh and walk off knowing you are right and everyone else can wallow in their mistakes without further bothering you. |
| Siwastaja:
--- Quote from: dunkemhigh on January 13, 2022, 02:20:43 pm ---you've started a new thread just for this. Why couldn't you just put it at the end of the current long thread? --- End quote --- While I generally agree that creating multiple threads about the same matter is a bad idea - also against forum rules -, in this special case that particular thread moves so fast, driven by what basically is a semiautomated spambot - which is also against forum rules - copypasting same responses and same dummy "questions" weeks and months straight. Any sensible comment gets lost in the noise in a matter of a day, which is probably exactly the reason for such smoke screen. In reality, the fact is there is no large dispute. The few that run it make it look like large. Their capacity of even generating unique messages is exceeded long ago, which is why jesuscf just basically copypastes. Quite frankly, it's nothing but playing games and throwing the toys and sand around, by the few remaining (is there more than 2 anymore?) people who can't admit their mistake or shortcomings, like many, including me, could (for me, it sure did take time). This is why I would like to give a pass to this opening post which, despite being outside of the sandstorm thread, tries to actually be helpful and explain the issue. Electromagnetism is non-trivial, and for "practical engineers", it can be damn difficult to admit our math is lacking. But I also can't stand intellectual dishonesty. But really, the game is soon over. The quality of the "opposition" has gone down significantly, from the serious attempts of building usable equivalent circuits in SPICE, that model the experiment, to just rigging physical experiments with careful layout (which never was part of the experiment; this was the eye-opener for me; I can see the difference between circuit layout and good probing), shouting, and thinking that adding more exclamation marks makes KVL hold better. And why I call it intellectual dishonesty? The answer is simple: because the people who built these Youtube experiments, who constructed the layouts that give "expected" multimeter number on photographs or videos, themselves witnessed the path dependency during tweaking of the layout, to get the error within the expected <2% or so. Think about it. It sure seems more like flat-earth stuff every day. |
| PlainName:
--- Quote ---Any sensible comment gets lost in the noise in a matter of a day, which is probably exactly the reason for such smoke screen. --- End quote --- What will prevent this thread also suffering the same fate once it's been discovered? If someone is disrupting a discussion through foul means then surely the answer is to ask a mod to deal with them. I don't see that giving them more threads to disrupt achieves much. Of course, perhaps the mods will be too busy, fail to see your point or otherwise let things run, but you don't know unless you ask. But, on the whole, I am not averse per se with skimming the crust off into another smaller and newer thread. It was just the "Listen, my explanation is the right one, so I'm a special case" which poked me. |
| nctnico:
--- Quote from: Siwastaja on January 13, 2022, 05:01:23 pm --- --- Quote from: dunkemhigh on January 13, 2022, 02:20:43 pm ---you've started a new thread just for this. Why couldn't you just put it at the end of the current long thread? --- End quote --- While I generally agree that creating multiple threads about the same matter is a bad idea - also against forum rules -, in this special case that particular thread moves so fast, driven by what basically is a semiautomated spambot - which is also against forum rules - copypasting same responses and same dummy "questions" weeks and months straight. Any sensible comment gets lost in the noise in a matter of a day, which is probably exactly the reason for such smoke screen. In reality, the fact is there is no large dispute. The few that run it make it look like large. Their capacity of even generating unique messages is exceeded long ago, which is why jesuscf just basically copypastes. Quite frankly, it's nothing but playing games and throwing the toys and sand around, by the few remaining (is there more than 2 anymore?) people who can't admit their mistake or shortcomings, like many, including me, could (for me, it sure did take time). This is why I would like to give a pass to this opening post which, despite being outside of the sandstorm thread, tries to actually be helpful and explain the issue. Electromagnetism is non-trivial, and for "practical engineers", it can be damn difficult to admit our math is lacking. --- End quote --- Agreed. IMHO the whole point is to understand when a simplification works and when not. Showing electrical phenomena using simple multimeters also raises my eyebrows; I've seen that go wrong too many times. When skimming through the other thread an interesting question popped into my mind: where does the electricity go that is being fed into an antenna (*). >:D * Yes, this is formulated in a very poor way on purpose. |
| Navigation |
| Message Index |
| Next page |