Author Topic: "Veritasium" (YT) - "The Big Misconception About Electricity" ?  (Read 210538 times)

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Offline Kalvin

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #400 on: November 30, 2021, 10:11:33 am »
Anybody that thinks that the circuit will only show the characteristics of a transmission line is missing something. There are parallel wires in your street delivering loads of amps to all the houses. They don't behave like there's a few hundred ohms of impedance stopping the the flow of current. There isn't a huge drop of voltage because of reactance. They behave like low resistance wires with a tiny, tiny hint of a transmission line.

So there's no use in quoting transmission line formulae here. They really don't apply.

The frequency of the signal in power lines is 50/60Hz, which means that the short power lines you are referring to cannot be modeled as long transmission lines. You are comparing apples to oranges here.

Wikipedia article on power-lines as transmission lines: https://en.wikipedia.org/wiki/Overhead_power_line

Quote
The overhead transmission line is generally categorized into three classes,[3] depending on the length of the line:

The transmission lines which have a length less than 50 km are generally referred to as short transmission lines.
The transmission line having its effective length more than 50 km but less than 150 km is generally referred to as a medium transmission line.
A transmission line having a length more than 150 km is considered as a long transmission line.
This categorization is mainly done for the ease of performance analysis of transmission lines, by power engineers.

You are referring to short power lines (apples). I am referring to long power lines (oranges) as transmission lines.

There are more details in the following Wikipedia article:
https://en.wikipedia.org/wiki/Performance_and_modelling_of_AC_transmission

The same telegrapher's equations will appear when modeling the [electrically] long power lines, indicating that the [electrically] long power lines need to be modeled as transmission lines as well.
« Last Edit: November 30, 2021, 10:38:51 am by Kalvin »
 

Offline Vtile

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #401 on: November 30, 2021, 10:48:12 am »
Anybody that thinks that the circuit will only show the characteristics of a transmission line is missing something. There are parallel wires in your street delivering loads of amps to all the houses. They don't behave like there's a few hundred ohms of impedance stopping the the flow of current. There isn't a huge drop of voltage because of reactance. They behave like low resistance wires with a tiny, tiny hint of a transmission line.

So there's no use in quoting transmission line formulae here. They really don't apply.
Like Kalvin said, this is dependent of frequency and more precisely wave lenght (approx. λ=c/f) to component size relation (eg. local distribution network), although same transients are there in switching, load variations (eg. high-impedance load separations) and high-frequency switching (eg. VFDs without filters and chokes). For 50Hz the nominal lenght is 6000km while eg. 50kHz (high-audio) wave length is approx. 6 kilometers. That is also the reason we can calculate 50/60Hz distribution lines so conveniently with extended DC equations as they were DC (..and common practice is to forgot transients ... until they matter).  And there is transients, but that is usually measured as "Light bulb flicker" ... very scientific, indeed. Again some convention inherited from ages of tube radios and other atomic age apparatuses.

Also when in this Veritasium example the switch is closed there is Heaviside step function applied to network, with that step the change of rate is instant, so the frequency is infinite and wavelength is zero (should we say differential, so nobody will not take it as insult how math is broken) and energy needed for the change is infinite.  This applies also to real world operation, but with more sensible frequencies (still "extremely high")

What people are calculated here like 6mA is not even above noise floor in distribution lines (I really do not want to start to calculate or simulate this).

Edit. Corrected wave lenght for 50kHz, dropped some zeros... oops.
« Last Edit: November 30, 2021, 02:57:38 pm by Vtile »
 

Offline MIS42N

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #402 on: November 30, 2021, 12:38:53 pm »
Quote from a stack exchange article "Thus a wire becomes a transmission line when the cycle time of the signal energy is shorter than the propagation delay." [best I could find] Since this problem is a direct current problem, there is no cycle time. My comparison of street wiring with propagation delay shorter than 50Hz wavelength and a 300,000km wire with propagation delay of > 1 second and a zero frequency supply looks OK - in both cases the cycle time is longer than the propagation delay.

I tried to calculate the rate at which the current ramps up when the switch is closed. My intuitive idea of thousands of amps is flawed. Assuming an inductance of 1nH/m for a single wire (it appears to be around that according to articles), then 300,000km of wire has an inductance of around 0.3H, the circuit is 4 times that (1.2H) so if 12V is applied for any length of time with no light bulb then the current would ramp up - delta A = 10A/s. However, current doesn't increase from zero everywhere when the switch is closed, it travels at the speed of light from the switch. I don't know how to model that.
 

Offline Vtile

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #403 on: November 30, 2021, 01:02:48 pm »
Quote from a stack exchange article "Thus a wire becomes a transmission line when the cycle time of the signal energy is shorter than the propagation delay." [best I could find] Since this problem is a direct current problem, there is no cycle time. My comparison of street wiring with propagation delay shorter than 50Hz wavelength and a 300,000km wire with propagation delay of > 1 second and a zero frequency supply looks OK - in both cases the cycle time is longer than the propagation delay.

I tried to calculate the rate at which the current ramps up when the switch is closed. My intuitive idea of thousands of amps is flawed. Assuming an inductance of 1nH/m for a single wire (it appears to be around that according to articles), then 300,000km of wire has an inductance of around 0.3H, the circuit is 4 times that (1.2H) so if 12V is applied for any length of time with no light bulb then the current would ramp up - delta A = 10A/s. However, current doesn't increase from zero everywhere when the switch is closed, it travels at the speed of light from the switch. I don't know how to model that.
I would need to blow dust (a thick layer) from my handbooks, so I can not give you help about the calculation. However, your assumption is wrong that there is no "frequency" involved, any rate of change is actually Ac (altenating current) component. If you do not believe measure how capacitor conduct, when you vary the DC voltage from your laboratory power supply to it, that is DC, isn't it or is it?

Edit:
Quote from a stack exchange article "Thus a wire becomes a transmission line when the cycle time of the signal energy is shorter than the propagation delay." [best I could find]
....
It is pretty much the same as wavelenght vs physical size, both can be expressed IIRC by function permeability and permittivity and c. The reason this matters is because the current is at different (unknown)phase on both ends of object (eg. transistor), so the potential difference is also different what lumped model predicts, which leads to that you can not apply eg. kirchoffs or ohm's laws directly to it. You need a new (mathematical) model to apply these laws and it is called transmission line. This is how I have internalized it to my self and can remember at the moment, I do hope it is not too far off, so take this with grain of salt.
« Last Edit: November 30, 2021, 11:42:31 pm by Vtile »
 

Offline Kalvin

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #404 on: November 30, 2021, 02:01:51 pm »
I tried to calculate the rate at which the current ramps up when the switch is closed. My intuitive idea of thousands of amps is flawed. Assuming an inductance of 1nH/m for a single wire (it appears to be around that according to articles), then 300,000km of wire has an inductance of around 0.3H, the circuit is 4 times that (1.2H) so if 12V is applied for any length of time with no light bulb then the current would ramp up - delta A = 10A/s. However, current doesn't increase from zero everywhere when the switch is closed, it travels at the speed of light from the switch. I don't know how to model that.

If you have a long transmission line (electrically long parallel lines), you cannot model that as a simple lumped circuit consisting of only an inductor and a capacitor. That is fundamentally wrong, will give you wrong answer, and will lead you into wrong conclusion what happens in the circuit as the switch will be closed.

You need to model a lossless long transmission line as distributed network of inductance and capacitance as shown in figure below:



Notice how the model for the long transmission line contains multiple L and C sections back-to-back, which represent the cable's distributed inductance and capacitance through out the cable's length. The more the model has those L and C sections, the more accurate the model will become.

Now, it is possible to calculate the characteristic impedance of the transmission line if the values of the distributed L and C are known. Alternatively, one can measure the physical distance between the wires, wire diameter, and plug those numbers into a impedance calculator giving an estimate for the wiring's characteristic impedance. And it is that characteristic impedance which will limit the initial current surge flowing in the circuit.

For completeness, the lossy transmission line model shown below contains some resistance and admittance in order to model the losses. I need to note here that those losses will not have any significant effect to the initial surge current as the switch is closed, because the cable manufacturer wants to provide as good power-line cable as possible.



Here is a lumped model for a short cable:



Here is a lumped model for a medium long cable:



Notice how the short and medium models are very different from the model of the long transmission line shown in the first figure. The long transmission line model contains multiple cascaded L and C sections back-to-back. It is not really possible to reduce those cascaded L and C sections of the long transmission line model into a single lumped inductor and lumped capacitor values shown in the short and medium cable models.
 
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Offline Kalvin

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #405 on: November 30, 2021, 02:32:29 pm »
I tried to calculate the rate at which the current ramps up when the switch is closed. My intuitive idea of thousands of amps is flawed. Assuming an inductance of 1nH/m for a single wire (it appears to be around that according to articles), then 300,000km of wire has an inductance of around 0.3H, the circuit is 4 times that (1.2H)

Ah, you forgot that EM-field cannot travel through the cable instantly. As we know, speed of EM-field is limited ultimately by the speed of light c.

Due to the the speed limit of the EM-field imposed by nature, you are able to calculate inductance and capacitance only for a very short cable section of the long cable at a time. This same speed-limit will also lead to the long transmission line model and its distributed and cascaded L and C sections, because the traveling EM-field doesn't have any idea what lies ahead.

Yes, you can measure the capacitance and the inductance of a long cable using capacitance and inductance meters, and those values are really just lumped values for that cable. Those values are only useful for very slowly changing signals when the cable can be considered as electrically short or medium.

Closing a switch is an event creating a fast change in the EM-field, thus you need to model the cable as an [electrically] long transmission line with distributed L and C sections.
 

Offline antenna

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #406 on: November 30, 2021, 08:27:27 pm »
With the rise time of typical scopes, one surely would not need 300,000,000m of wire to test this. 

Why not just run 100' of wire in the same configuration with a dual channel scope - one channel across battery and switch, the other channel across the light bulb?  Then, stretch the loop out into a circle and repeat.  There was a response from RSD Academy on youtube to Veritasium's video that mentions three mechanisms, capacitive coupling between wires, inductive coupling between wires, and, the wires acting like antennas ~ all three ignoring the short circuit at the ends with the assumption all three occur before the signal hits the end of the path where it folds back.  It may be a 1:1 turns ratio in the transformer sense and a dead short from the capacitor point of view, but increasing the gap should limit the current and change the rise time on the load so I would imagine the effects of separating that 1m gapped loop should result in changes between the two scope channel readings...

I read a different theory long ago. Take a pipe (conductor) filled with golf balls (electrons) and add another ball. Sure, you couldn't get the same ball through the pipe fast, but adding a golf ball on one end simultaneously pushed one out of the other end because it was already full of golf balls.  The electron that does the work does not necessarily need to be the electron you shoved into the wire.  I think physical experimentation is the next step because if it is EM fields rather than electron transfer as in the golf ball analogy, we should see the ballast effect of the air-coupled path (and an expected time difference supporting one theory over another).
« Last Edit: November 30, 2021, 08:30:07 pm by antenna »
 

Offline adx

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #407 on: December 01, 2021, 03:14:37 pm »
Late to the party, I skimmed the Veritasium video some days back - somewhat intriguing and a bit odd, but I didn't see anything really wrong with it (other than the fun but tangentially twisted injection of the transatlantic cable). As for the clickbait, all the better for it ("thought provoking" in marketing speak). I mean, what other purpose is there for YT 'science' content if not to hit people with a "what the..." learning opportunity at every possible turn? If I were a famous vlogger then I should hope I would always choose to do so.

But speaking of all that, I happened upon Dave's video a few days later, and got more dragged in, had a look at the web.

Two things confuse me:

(1)  Saying power "flows" through the space outside the conductor.

(2) What *is* electricity? I should know, maybe I once did, but I strongly suspect it all came about from being told stuff, and believing it with varying levels of reluctance. The balls in a pipe analogy is nice, but it swims amongst charges, fields, fanciful claims (like 1), and div and curl operators which leave you wondering about your misspent education, and its effect on your future (fortunately, none, I'm pretty sure it's never come up since - until now).

Pretty sure I never knew that a magnetic field was an electric field in the charge carrier's perspective, nor that it had "simple relativity" at its core. Why did no one say? Perhaps my educators either didn't really understand it themselves, or were too academic to see beyond established models (and therefore didn't really understand it themselves). Or I was absent that day. Or it just doesn't matter.

So is this quasi-classical description fairly right?: Electrons (and protons) have charge (excepting exotic matter), and are responsible for (all?) electric fields. Solid matter has electrons and protons, generally held together by electric fields at an atomic scale (not a nuclear scale). Some electrons are mobile in a metal, but otherwise follow the same rules which pack them in at a nominal spacing, so they are a barely compressible fluid (effectively in fixed sized piping). It takes mechanical force (for example an acoustic wave) to squeeze or stretch such materials, whether that be the bound nuclei or free electrons. The former does not alter the bulk charge, but does alter the size. The latter is the reverse. This force is the level of compression of the material, better described as its pressure.

Other than that, pressure is less of a real physical artefact relating to the matter, and more an external philosophical measure of the potential to do work - the so-called potential energy. Force (converted from pascals for atoms and volts for electrons) in newtons, times distance (in metres) is the actual energy (joules).

Current flowing in a DC circuit is a feature of charge, in a hydraulic circuit electrons and protons (and neutrons) flow, in an electrical circuit only the electrons move. Both happen as a result of the mechanical pressure, which supplies the potential energy, but it is the duration of this pressure which transfers real energy (power). In a lossless circuit (superconductor / superfluid) no pressure is required to keep the fluid flowing, so it takes no energy. In an open circuit pressure might be as high as you like, but nothing moves, so that takes no energy.

So back to point (1) above, it's like dragging a brick along the ground with a string (in my very TBD vlog I go off on a tangent and precisely grind a V-notch around it to stop the string getting abraded, thereby getting all sorts of street appeal for being "interesting"): Once the superfluous setup montage ends, I am pulling the string in at a constant rate, performing what some would loosely call work, while getting all sorts of strange looks (I prefer to call them views). Undoubtedly transmitting power to the brick scraping surfaces (remember there are two) - but how? There is a negative pressure in the string, it is moving. Is the energy flowing "through" it? With respect to me as a stationary observer? I think it is. But I am putting the ground in compression (I am kicking it under the brick), it is moving relative to the string. So is the energy "really" flowing through the space between string and ground, on account of it enjoying both pressure difference and relative motion? With respect to the system on the physical scale of energy transfer (the current loop)? I think so. But its exact path (see Feynman lecture 27-4, handily provided in this thread), like the "potential" energy, are philosophical constructs designed to imagine something that doesn't seem to physically exist, not in this realm anyway. So it's a pretty tough call to make a statement that one particular location is "right" and another "wrong".
 

Offline nicolap

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #408 on: December 01, 2021, 05:16:14 pm »
The main context here is not phisics or electrical engineering: this is a multiple choises question!
Here if a question "seems" to be right but has an error inside it's wrong!
So, because the respose D contains a dimensional error (missing "m") it's wrong!
And the right answer is E :-)
 

Offline T3sl4co1l

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #409 on: December 01, 2021, 06:22:47 pm »
(2) What *is* electricity? I should know, maybe I once did, but I strongly suspect it all came about from being told stuff, and believing it with varying levels of reluctance. The balls in a pipe analogy is nice, but it swims amongst charges, fields, fanciful claims (like 1), and div and curl operators which leave you wondering about your misspent education, and its effect on your future (fortunately, none, I'm pretty sure it's never come up since - until now).

Digressing from answering your question for a bit, to ponder this subject in greater detail:
It's a good analogy for DC, and still kind of works for AC, but the boundary conditions are very different.  Also, it's very rare that acoustic power is transmitted over any kind of distance, it's mostly a nuisance (water hammer etc.), so we have essentially no need to understand its wave nature anyway.  Warning signs? :P

To be precise, the water (or chain of balls, or whatever general fluidic medium) pushes on itself, building pressure, which causes velocity, and so on alternately, thus we have waves.  The pressure is confined by the pipe, which itself is not incompressible either -- for waves to exist at all, it's necessary that the media have finite nonzero stiffness, as well as density; together, these determine the speed of sound and mechanical impedance.  Just as Zo = sqrt(mu_0 / e_0) and c_0 = 1 / sqrt(mu_0 e_0) for E&M.  So, we have a system where the pipe acts to confine pressure waves within it, and which has total internal reflection (except for a small amount that leaks out, due to its expansion (stress in response to internal pressure), which is very small as the metal's stiffness is a great many times that of air).  It's an acoustic waveguide.  At least, to longitudinal waves; transverse waves, or longitudinal going around a bend, of course transfer momentum to the pipe, hence the system shakes and emits audible sound in those cases.  (Or at ends, which arguably are some combination of a bend or restriction, so the impedance mismatch causes reflected waves; in either case, momentum is transmitted to the discontinuity.)

The big break, then, between water waves and E&M waves -- if we had the intuition about water waves to begin with -- is that they are confined (almost entirely) within pipes, whereas E&M fields are largely in the space between wires.  We can have an electromagnetic waveguide, but because there is no longitudinal mode, they cannot transmit at DC, but have some lower cutoff, where transmission is a decaying exponential with distance (tunneling, as we would call it in QM).  A central conductor is needed to take over at DC, so that we have a coax (TEM00) line, rather than a waveguide.

And this is not a terribly intuitive difference, it's a rather fundamental aspect of the medium -- E&M has transverse waves, liquids have longitudinal as well, and solids have shear as well -- three whole unique, interacting modes!  The curse, to be a mechanical engineer, can you imagine? ;D

Which leaves the last hope for intuition, as understanding that a wave medium has some combination of characteristics, which give rise to what modes they handle; which is a pretty big leap as far as processing all that, so I would guess not a lot of people intuit waves this generally, if they intuit any kind of waves* at all?

*To a useful level I mean, like, predictively, if not quantitatively as well.  So, something more than seeing ripples on water and saying, "ooh shiny". :P

Anyway, digression made...


Quote
Pretty sure I never knew that a magnetic field was an electric field in the charge carrier's perspective, nor that it had "simple relativity" at its core. Why did no one say? Perhaps my educators either didn't really understand it themselves, or were too academic to see beyond established models (and therefore didn't really understand it themselves). Or I was absent that day. Or it just doesn't matter.

Yeah, this is, I guess embarrassing to say, pretty fundamental to E&M and Relativity -- but it might still be no accident that you missed this particular fact -- whether through absence, or not having made the inference.  And, insights like these being what they are, it's no shame to miss such a thing -- we honor the names of those few who discover them, after all!

So:

Special Relativity follows almost directly from E&M relativity, which is where the Lorentz transformation was derived: E and M fields are aspects of the more general unified EM field, plus relative motion, motion being governed by that transformation.

It wasn't much of a stretch to take that finding, and the fact that c remains constant for any inertial observer, tack on a few more assumptions for good measure (conservation of momentum, etc.), and now you have Relativity.

Not to diminish Einstein's work of course -- it was no small feat bringing together these perhaps dubious assumptions, making a rigorous mathematical description, and then a bit later, formulating the full 4-D spacetime equations (General Relativity).  More to say that he was building on things that were already well known at the time, but no one had yet made the insight to say spacetime itself behaves that way.

So, the student is most likely to learn this, kind of incidental rather than explicitly, through the Lorentz force for example, and the usual induction topics.  (But yeah, just go infer all of physics from a handful of equations, what's so hard about that, right? ::) )  So, PHYS 102 or something like that, or the early EE fields courses, but again maybe not stated so explicitly.  Otherwise, the introduction to Modern Physics (SR and QM) should make it clear.

Engineering curricula being what they are (hastily jam-packed with tools, thin on theorems and proofs), Modern might not be seen at all, or skimmed over -- I forget.  (I have the fortune to have a physics degree as well, so I appreciate this may be a LOT less obvious to those without!)

I'm not actually sure if I remember hearing about this exact statement (E and M being relative), or if I read it much more recently, honestly.  It's been a long time since I had Modern...

On the upside, I'm not sure that it makes all that much of a difference -- it's a rather rare occasion indeed that we need to deal with electric or magnetic induction at relativistic velocities, ;D and other than that, the usual (static) induction relations are sufficient.

Or, for another thought experiment -- consider spinning a magnet fast enough that it emits significant electromagnetic radiation -- i.e. by itself, without surrounding antenna structures.  It simply has to spin so many orders of magnitude faster than any material can bear.  (To be a properly resonant dipole, it needs a tangential velocity very near c, after all.)

So, to be sure, don't beat yourself up about it. :)


Quote
So is this quasi-classical description fairly right?: Electrons (and protons) have charge (excepting exotic matter), and are responsible for (all?) electric fields. Solid matter has electrons and protons, generally held together by electric fields at an atomic scale (not a nuclear scale). Some electrons are mobile in a metal, but otherwise follow the same rules which pack them in at a nominal spacing, so they are a barely compressible fluid (effectively in fixed sized piping). It takes mechanical force (for example an acoustic wave) to squeeze or stretch such materials, whether that be the bound nuclei or free electrons. The former does not alter the bulk charge, but does alter the size. The latter is the reverse. This force is the level of compression of the material, better described as its pressure.

Yes, that's more or less correct -- electrons are bound to nuclei by the Coulomb force and organized as quantum wave functions, because of course this is all very small stuff, and electrons are relatively large and poofy in comparison.  So instead of classical orbits we get probability clouds, and instead of orbital periods we get photons corresponding to transitions between energy levels.  We can ignore that quantum stuff, to the extent that we allow that matter simply clicks together however it does, and gives us these bulk properties that we can work with -- such as conductivity, rigidity, etc.

For example, mechanical rigidity is given by interatomic attraction on one hand -- mediated by ionic charge when applicable, polarization (var der Waals forces, etc.), atomic orbitals (molecular bonding), etc., and repulsion on the other -- mediated by the Pauli exclusion principle of what would otherwise be overlapping atomic orbitals.

Noteworthy I guess, that there are some neutral sources of EM waves -- for example the \$\pi^0\$ meson decays into a pair of gamma rays, despite having no (overall) electric charge; or the neutron into a proton and positron.  Both are composite particles under current understanding (QCD, with the quarks and all that), there's internal structure there -- so it's kind of cheating to assert this.  The only neutral, truly elementary (apparently structureless) particles are neutrinos and some bosons, and we might exclude the bosons as force carriers, leaving neutrinos as actual matter... if you can call them that.  Absolutely, for sure, 100.00..% of familiar fields are driven by the displacement of electrons (and occasionally of free protons or other nuclei). :-+

Note that the electron gas in a solid, doesn't have much for wave properties, in terms of what we'd think about with ordinary neutral gasses. The mass is so minuscule compared to the charge, inertial effects are almost imperceptible and EM waves dominate.  Even at atomic scales where you might hardly think of magnetic fields, they're relevant.  So, your usual bulk effects dominate -- any electron wave is just EM waves, maybe dragged down a bit, so, having some dielectric constant and loss tangent.

(Which conversely is why MHD (magnetohydrodynamics) is such a brainf**k: the inertial, propagating mechanical-wave and EM-wave, and dissipative (resistance or turbulence) modes, are so complex and interdependent that about all we can do is simulate them numerically or experimentally.  And on top of that, the ionization/recombination and other chemistry of physical plasmas.  So, MHD is a very challenging subject, and a big reason why fusion has taken so long to research.  Besides the very low funding level, I mean.)


Quote
Other than that, pressure is less of a real physical artefact relating to the matter, and more an external philosophical measure of the potential to do work - the so-called potential energy. Force (converted from pascals for atoms and volts for electrons) in newtons, times distance (in metres) is the actual energy (joules).

Physics tends to work more in energy, and energy seems to be more fundamental to quantum processes.  For classical problems, it tends to be a shortcut -- who cares how something gets there, just figure out where the energy goes -- but we might just as well imagine we're being smart using forces and trajectories to solve a problem, when the energy truly is more fundamental, while the trajectory is irrelevant, or even a fiction.

And yes, in any case, energy is an abstract quantity; it's not easy to teach I think, and coming up with analogies and explanations, may range from the philosophical to metaphysical...


Quote
Current flowing in a DC circuit is a feature of charge, in a hydraulic circuit electrons and protons (and neutrons) flow, in an electrical circuit only the electrons move. Both happen as a result of the mechanical pressure, which supplies the potential energy, but it is the duration of this pressure which transfers real energy (power). In a lossless circuit (superconductor / superfluid) no pressure is required to keep the fluid flowing, so it takes no energy. In an open circuit pressure might be as high as you like, but nothing moves, so that takes no energy.

So back to point (1) above, it's like dragging a brick along the ground with a string (in my very TBD vlog I go off on a tangent and precisely grind a V-notch around it to stop the string getting abraded, thereby getting all sorts of street appeal for being "interesting"): Once the superfluous setup montage ends, I am pulling the string in at a constant rate, performing what some would loosely call work, while getting all sorts of strange looks (I prefer to call them views). Undoubtedly transmitting power to the brick scraping surfaces (remember there are two) - but how? There is a negative pressure in the string, it is moving. Is the energy flowing "through" it? With respect to me as a stationary observer? I think it is. But I am putting the ground in compression (I am kicking it under the brick), it is moving relative to the string. So is the energy "really" flowing through the space between string and ground, on account of it enjoying both pressure difference and relative motion? With respect to the system on the physical scale of energy transfer (the current loop)? I think so. But its exact path (see Feynman lecture 27-4, handily provided in this thread), like the "potential" energy, are philosophical constructs designed to imagine something that doesn't seem to physically exist, not in this realm anyway. So it's a pretty tough call to make a statement that one particular location is "right" and another "wrong".

Well, two things:

1. Energy is relative, just as E and M are relative to velocity -- the most immediate admonition this suggests is, we must be careful and consistent with what frame of reference we are working in.

2a. The string is a different kind of wave medium -- in this case a solid one, so you can transmit three kinds of waves (with them all depending on the string being under tension, to propagate correctly, and even then at a dependent velocity).  And of those, really just the longitudinal mode (static tension) is doing anything, I mean you can shake the string and, what's going to happen, maybe the brick slides microscopically off-center when it does, sometimes; nothing on average.

2b. The tension doing work is DC, so we really don't have any insight into the wave mechanics, and where the energy is flowing (or not!).

Note that the tension (in the mental sense, hah) between "DC flows in wires" and "AC flows around wires" doesn't need to reference a different medium, to still be relevant.  We can happily ignore the fields around a DC battery, or most AC mains cables, and yet still have RF energy for example, very noticeably travelling in the space between wires, even in the same sorts of cables, at the same time.  I think the quibble is just that: we don't particularly care about the fields at DC, and at AC we have other phenomenon more directly relevant, like skin effect and "path of least impedance" (image currents under signal traces, etc.) which are more descriptive.  I think if anything, the confusion is really just this old dichotomy between DC and AC current flow paths, with some fields thrown in for spice.

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Offline Howardlong

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #410 on: December 02, 2021, 09:04:01 pm »
This evening I set up some 450 ohm ladder line to an old HP 54121T 20GHz sampling scope. It's a bit fiddly to do because of impedance mismatches and that I didn't use any baluns, but the gist of what's achievable is there.

For those who are unaware, old sampling scopes like the 54181T also have a TDR feature to trigger and fire off a fast rise time pulse, so the switch, battery and measurement aspect is baked into the instrument.

450 ohm ladder line has about 24mm separation. I used a 120cm length, using the centre points to make measurements and inject the pulse.

The end result is yes, as expected, you do get a response on the bulb side about 80ps after the pulse (24mm / c = 80ps), but it's significantly attenuated (about 15% in the voltage domain, so ~2% in the power domain), no doubt at least in large part due to the many mismatches and unbalanced feeds.

A far bigger response occurs once we're effectively in DC land.

If Derek wants to do an experiment in the desert, he should, although I strongly suspect he'll have to compromise on his setup: 1m spacing is, shall we say, extravagant.
« Last Edit: December 03, 2021, 01:32:48 pm by Howardlong »
 
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Offline HendriXML

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #411 on: December 02, 2021, 09:53:45 pm »
The main context here is not phisics or electrical engineering: this is a multiple choises question!
Here if a question "seems" to be right but has an error inside it's wrong!
So, because the respose D contains a dimensional error (missing "m") it's wrong!
And the right answer is E :-)
In that case the "s" should be left out as well  :-+, so 2 errors.

c = 299 792 458 m/s.

1 m / c = 1 m /  299 792 458 m/s = ... s

Vs:

1 m / c s = ... s^2

I got this in my recommended video list:
Veritasium is wrong! https://youtu.be/-jJB8dyOJIw

(The 2 s answer was my first choice as well.)
« Last Edit: December 03, 2021, 10:53:30 am by HendriXML »
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Offline ledtester

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #412 on: December 03, 2021, 06:56:34 am »
Ok, I've got a question about how Veritasium is setting up the question...

Let's have the switch, battery and the person operating the switch all close to each other and the light bulb is 1 meter away. At time t=0 the operator closes the switch. Is Veritasium saying the operator will see light from the light bulb at 1m/c seconds or 2m/c seconds?

To me it seems the answer is 2m/c seconds -- 1m/c seconds for the E-M wave to travel to the light bulb and another 1m/c seconds for a photon from the bulb to reach the operator. Having the operator detect photons from the bulb at t=1m/c seconds would seem to defy causality.

 

Online bdunham7

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #413 on: December 03, 2021, 07:02:18 am »
To me it seems the answer is 2m/c seconds -- 1m/c seconds for the E-M wave to travel to the light bulb and another 1m/c seconds for a photon from the bulb to reach the operator. Having the operator detect photons from the bulb at t=1m/c seconds would seem to defy causality.

He's slaying 'misconceptions' and 'blowing minds', not sweating the details.  Now if you were observing the switch operator from the location of the light bulb, the two events would appear to be simultaneous, even though you 'know' that one occurred 'before' the other. 
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline MIS42N

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #414 on: December 03, 2021, 10:02:08 am »
I decided to look at the video again, his grasp of things is a bit astray. He says the wires stretch 300,000km in each direction, which the diagram labels as half a light second. But he says 1 light second. Next he says - half way to the moon. Unless it moved 200,000km since I last looked, think again. The wires are lying on the ground. Ground has some conductivity. While flux establishes currents in the ground it will propagate at less than the speed of light. I like the idea of making the ground have zero resistance (if he can have wires with no resistance, I can have ground with no resistance) which blows his argument right out of the window. Or make the switched wire a coax with grounded shield.

It's been an interesting discussion. I worked out the current due to capacitive coupling of 12V between 2 wires 1mm diameter and 1 meter apart in space, it is 32pA (or less). Is that enough to satisfy his unrealistic "the light bulb has to turn on immediately current passes through it". Is it larger than the Johnson noise current?. Wires lying on the ground will not interact as strongly. I am having trouble with current due to shared flux - or even if there is shared flux because for zero resistance wires one needs a superconductor, and superconductors expel flux.

Still thinking on it.
 

Offline T3sl4co1l

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #415 on: December 03, 2021, 03:25:18 pm »
The total propagation delay of the reflection is twice the line length or 1 second.

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Offline HendriXML

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #416 on: December 03, 2021, 03:43:54 pm »
I think this newly recommended video shows a better application of Poynting vectors in the thought experiment.
https://youtu.be/IDHxu3GxVRU
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Offline T3sl4co1l

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #417 on: December 03, 2021, 06:58:20 pm »
Unfortunately he takes the steady state condition, which obviously, you have to wait many seconds for, so of course you can calculate a delay in that case.  Really, delay is meaningless, it could be undefined at DC for all that matters -- DC means for infinite time.  It's not even a good motivation...

If he simply does the transient analysis he will find E and B in the local space where the wave has propagated (within some distance as a function of time), and nothing beyond, and so there is an ~instantaneous energy flow.

Unfortunate I guess that he decided to post that... alas, we all make mistakes sometimes.

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Offline adx

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #418 on: December 04, 2021, 12:51:01 pm »
Great, I was maybe not so far off the mark.

Pretty sure I never knew that a magnetic field was an electric field in the charge carrier's perspective, nor that it had "simple relativity" at its core. Why did no one say? Perhaps my educators either didn't really understand it themselves, or were too academic to see beyond established models (and therefore didn't really understand it themselves). Or I was absent that day. Or it just doesn't matter.

Yeah, this is, I guess embarrassing to say, pretty fundamental to E&M and Relativity -- but it might still be no accident that you missed this particular fact -- whether through absence, or not having made the inference.  And, insights like these being what they are, it's no shame to miss such a thing -- we honor the names of those few who discover them, after all!

Interesting turn of phrase - I would never make the inference myself, as students we were essentially not permitted to. Yes there was the whole "it's not like being taught at school, you learn how to learn" schtick, but ultimately, if it was not taught, it was not to be learnt. Engineering was a contradiction not wrapped in any sort of enigma or riddle. Our job was to transcribe and somehow learn the material as presented, think about it critically and with free and inquiring mind, but not question its Ultimate Truth. That only seemed to get me into trouble. While we revelled in the fact we were being trained as units of production with some begrudging allowance for original thought - it was simply so we could turn up at work with our brains switched on, not to ask questions. I can't see medicine or law being any different. Maybe physics is!

Great big chunks of it really never made a lot of sense. I can't have been very good. I never did any 1xx courses (somewhat lamentably now I think about it, that means no physics courses at university level). Can't remember exactly why not, but I was supposedly too good (also makes not a lot of sense). I can't imagine a year of "intermediate" (I think is the word) giving the time to ask the questions of the universe. Suffice it to say, if I didn't understand it, I wasn't interested. I read Einstein's "Relativity for Dummies" (not its actual name) some years later and for my own edification rather than education, but perhaps as expected for bedtime reading most parts of GR seemed too much for one sitting so I put off till later. Everything I learned on EM of any practical consequence (which is a lot, considering what I do) beyond learn as I go, was from asking the cleverest engineer at work what EM radiation "is". 2 minutes later I had the answer, and it's not as if something "finally clicked" from my years of toil and indifference, it was just the first time I heard an explanation that made sense. To this day I seriously question the purpose of university, not the environment which was very beneficial, but the academic process as it does its 'work' on each unit of production.

So what I'm trying to do here, is peel back the layers of instilled belief, for a better (but possibly incorrect or incomplete, I'm fine with that) grasp on what electricity "is".

My point was to intentionally equate hydraulic and electric current as not just an analogy, but physically equivalent. So the balls in a pipe go in one end, and come out the other. A wire as an acoustic waveguide for electrons, forced in one end using real newtons, and exerting a force at the other in real newtons, propagating via longitudinal electric field waves, same as water does. Except the electrons being very light, can't really carry what we would call a "wave" (pressure on inertia causing velocity etc, as you say it needs density), so it still propagates but with infinite velocity (which is c).

This infinitely fast massless longitudinal wave (current) can't store or propagate energy on its own (ping pong balls with the same rigidity as steel bearings), all it can do is exert a force at the same instant it is applied, which is why the only mode possible is transverse pressure (pulling on string while pushing on ground, imagine if they were massless, again this is a real state of matter and the physics are not an analogy). And other things.

Perhaps through a trick of the light that is as yet undiscovered and relativistic effects, the pipe analogy is strengthened (water hammer) but pipe reality deteriorates, to the point educators forget that it wishes to transmit longitudinal modes. Then some lay commenter throws a "there are actual pipes called waveguides" soundbite into the analogy, despite having zero compressible charge carriers in the pipe to transmit any sort of longitudinal wave. The waveguide thus carries magic (and by inference, so do wires, a fact confirmed by expert video testimony that the energy really flows outside them). To the extent that it does (carry magic), fine, but students get stuck between mathematical hyperbole and a "we'll upgrade your understanding next year but for now this will do". In other words, establishing truths that are not real risks a belief in magic.

My confusion, instilled from an early-ish age, is with the physical reality of charge, pressure, current, and energy. Poynting might agree that energy is a concept, not something that anyone can take a picture of (drawing a diagram is not the same as taking a photo). Similar for time, and pressure. Consider a pipe with water at 1000 psi in it, versus a cylindrical hole in an infinite solid made of the same thing: Despite the mechanical configuration of water being the same (compressed to the same degree), the former has potential energy, for the latter it has something that does not exist. The maths is the same, the model is correct, the concept exists the same in both cases, but a seemingly irrelevant change makes it physically implausible in one case.


Other than that, pressure is less of a real physical artefact relating to the matter, and more an external philosophical measure of the potential to do work - the so-called potential energy. Force (converted from pascals for atoms and volts for electrons) in newtons, times distance (in metres) is the actual energy (joules).

Physics tends to work more in energy, and energy seems to be more fundamental to quantum processes.  For classical problems, it tends to be a shortcut -- who cares how something gets there, just figure out where the energy goes -- but we might just as well imagine we're being smart using forces and trajectories to solve a problem, when the energy truly is more fundamental, while the trajectory is irrelevant, or even a fiction.

And yes, in any case, energy is an abstract quantity; it's not easy to teach I think, and coming up with analogies and explanations, may range from the philosophical to metaphysical...

Some commenters (here or on YT - can't find now) have boiled the "energy flows outside the wires" down to nicely intuitive statements that basically go; the space between the conductors is where the potential difference exists, the conductors are where the charge carriers flow, so it has to be a combination of wire and space that "energy" traverses. (I was going to add the example of a PCB with power and ground planes, and say the only place you need go looking for power is in the gap - but that's kind of redundant.)

Except it's worse than that - a location for the potential difference isn't needed, nor its "field strength", it just has to exist. And that is clear from the language, the focus is on force and movement. No one seems to question why power in a chain drive flows "outside the chain" (which is the same kind of situation).

I'd go one step further though; and infer that because energy seems to take a path that occupies either all or no space, and seems to transmit as if there were nothing in its way, it seems not to flow in spacetime at all. There is just distance and time. Kind of like it goes in a straight line but without direction, and chooses where to go based on external constraints. Photons show this behaviour.


Or, for another thought experiment -- consider spinning a magnet fast enough that it emits significant electromagnetic radiation -- i.e. by itself, without surrounding antenna structures.  It simply has to spin so many orders of magnitude faster than any material can bear.  (To be a properly resonant dipole, it needs a tangential velocity very near c, after all.)

I have! I wanted to make one (not a lot). Put it in a ferrite rod (which I guess is the epitome of an antenna structure) and there's not much difference between that and an AM radio. It also becomes a lot more practical to spin at 60MRPM.
 

Offline T3sl4co1l

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #419 on: December 04, 2021, 06:35:39 pm »
Interesting turn of phrase - I would never make the inference myself, as students we were essentially not permitted to. Yes there was the whole "it's not like being taught at school, you learn how to learn" schtick, but ultimately, if it was not taught, it was not to be learnt. Engineering was a contradiction not wrapped in any sort of enigma or riddle. Our job was to transcribe and somehow learn the material as presented, think about it critically and with free and inquiring mind, but not question its Ultimate Truth. That only seemed to get me into trouble. While we revelled in the fact we were being trained as units of production with some begrudging allowance for original thought - it was simply so we could turn up at work with our brains switched on, not to ask questions. I can't see medicine or law being any different. Maybe physics is!

Yeah, school sucks sometimes.  Some of them, even all the time...

I don't know overall how .au is, if you got screwed by coincidence (poor teachers), or just didn't attend the right kinds of schools -- over here, curiosity is very much beat out of you in public school (grade to high school <18yo), then a bit better on average in college/uni.  I attended private colleges, which tend to be better, and one of which is "liberal arts" so tends to emphasize the use of critical thought.  Even there, there were occasional profs who just weren't very good, whether at teaching in general, or at the kind of teaching suited to the school.  They were usually tentative or visiting, so didn't stick around, but nonetheless left their "mark" on the students who took their classes (in contrast to the same classes taught by ordinary (assoc/tenured) faculty).

As for engineering school, the one I attended anyway, I think they were just trying to cram so much material into a curriculum that there was no time for proofs, but that leaves a lot of room for profs that prefer things as wrote and god-given rather than derived and connected.  As I understand it, education in non-western countries tends to bear even harder on wrote methods, such a shame.

Small schools too, so while there wasn't much of any research going on, there also wasn't any of that nonsense with hundreds of students jammed into a class being taught by TAs, while the tenured prof works away on pet projects.


Quote
My point was to intentionally equate hydraulic and electric current as not just an analogy, but physically equivalent. So the balls in a pipe go in one end, and come out the other. A wire as an acoustic waveguide for electrons, forced in one end using real newtons, and exerting a force at the other in real newtons, propagating via longitudinal electric field waves, same as water does. Except the electrons being very light, can't really carry what we would call a "wave" (pressure on inertia causing velocity etc, as you say it needs density), so it still propagates but with infinite velocity (which is c).

This infinitely fast massless longitudinal wave (current) can't store or propagate energy on its own (ping pong balls with the same rigidity as steel bearings), all it can do is exert a force at the same instant it is applied, which is why the only mode possible is transverse pressure (pulling on string while pushing on ground, imagine if they were massless, again this is a real state of matter and the physics are not an analogy). And other things.

Ahh, hmm.

Well, electrons certainly convey force, as we understand it -- ordinary mechanical force is nothing more than the collective effect of the smallest interatomic forces.  We don't treat such [interatomic] problems in terms of forces, but we can derive force from the energy gradient: energy is force times distance, so if we differentiate energy with respect to distance, we get something force-like.  And while energy is a scalar (a single value at a given point in space -- relative to a consistent reference, of course), its derivative in this way (gradient) is a vector field, and so we have forces with direction, as we expect.

So the question is, what forces act between electrons?  If we aren't dealing with electrons bound to atoms as such, but electron gas in a crystal, then we won't have interatomic forces per se (which do carry a force, and propagate waves -- represented by the quantum quasi-particle, the phonon, characteristic of acoustic and heat energy).  As it happens, there are no other forces, and so, electrons in a metal are dominated by the electric force (AFAIK).

If you consider depositing a patch of charge on one side of a metal ball: intuitively, those charges are going to spread out quickly, over the surface, until equalizing the charge distribution with respect to external fields.  Some energy will be lost as radiation (the initial charge distribution has a strong dipole moment), some due to current flow in the surface.  Nothing need flow through the bulk (indeed, over short time scales, nothing can due to skin effect), and indeed nothing need even travel very far at all, as charges are readily available throughout the surface and very little motion is needed to present a given amount of charge.

The other thing that comes to mind is the Debye shielding distance, which relates to how far the field around a given particle extends; the shielding effect occurs because neighboring particles move in response to the particle's field (whether spatially as for free electrons/ions, or by polarization).  This looks like skin effect at optical frequencies, but is also a static effect, and AFAIK determines the layer thickness that counts for "surface" in the case of static surface charges.

Ahah, looks like my charge-distribution thought experiment is spot on:
https://en.wikipedia.org/wiki/Plasmon
They don't discuss dimensions very much here, though frequently referencing nanoparticles and other small structures; I think the point is, the propagation mode is strongly dissipative, so it's only noticeable as single-surface effects (like the color of certain metals/alloys), or on very small particles.

And that squares with expectations; the propagation must be strongly electromagnetic, because charge dominates over mass for the most part.  And the metal is highly conductive (but not perfectly so), so skin effect applies.

Incidentally, another thing that pops out of statistical mechanics, is the effective mass of the electron; being confined in a crystal, it seems, has quite a strong effect, with the typical effect being like 100 times more massive or something.  (I forget if that's typical of materials, or some particular material.)  This is analogous to the propagation properties of a waveguide, but in 3D (like how group velocity is slower, and phase velocity is higher; though I forget which way both parameters go in a crystal, but anyway it's determined by the boundary conditions, the periodic potential).  Likewise, don't pay too much mind to the parameter, it's an effective figure for the system and not really measurable in bulk external properties, just neat that it's there.

Statistical mechanics...yes...  These are all topics in stat mech, a notoriously difficult subject.  I certainly wouldn't expect you to go out and start working problems in it; it's probably dubious that I even bring up much of it, given that I can barely explain it anymore, let alone work problems...  But if nothing else, it's various points of interest, and, let's say, something to work up to.


Quote
To the extent that it does (carry magic), fine, but students get stuck between mathematical hyperbole and a "we'll upgrade your understanding next year but for now this will do". In other words, establishing truths that are not real risks a belief in magic.

I can't say I ever much appreciated the way that schools teach subjects.  "You can't divide by zero!" "You can't square root a negative!" "Here's these things called polynomials, enjoy! ??? "

Just to contradict themselves later, by showing you can take limits near a singularity, or integrate around it, and "around" might mean using sqrt(-1), or some vector space (where "numbers" are more than just numbers), or...

Or to teach something so utterly irrelevant and forgettable as polynomials, when the hell am I ever going to use this?  Turns out they're extremely useful -- but only in the few professions that actually use them.

It would be so much better to just leave a little pin in everything, just a few sentences if that, hinting that there's far more depth beyond here, but we will confine ourselves to this for now because we have to keep it simple.  And within the scope of what we're doing, these are the facts.  Or what kinds of applications motivate these relationships, or structures or objects.

Maybe that still isn't enough, I don't know.  I'm no teacher.  If you leave a mystery, while also trying to make rigorous what material you are covering in detail, does that just leave too many mental "outs" for a student?

They do usually go for a few worked examples, like electric and mechanical lumped-equivalent circuits in diff eq, maybe in a handful of calculus problems too.  Not that everyone necessarily gets it; intuition about any given subject is probably scattered at best, so you just end up with every student being equally frustrated overall... 😅

Or of polynomials, I don't recall if I touched those again until transfer functions or diff eq.  They're fine to introduce in high school, it's an algebra topic to be sure; but what good is it, if there's nothing to connect it to, y'know?


Quote
My confusion, instilled from an early-ish age, is with the physical reality of charge, pressure, current, and energy. Poynting might agree that energy is a concept, not something that anyone can take a picture of (drawing a diagram is not the same as taking a photo). Similar for time, and pressure. Consider a pipe with water at 1000 psi in it, versus a cylindrical hole in an infinite solid made of the same thing: Despite the mechanical configuration of water being the same (compressed to the same degree), the former has potential energy, for the latter it has something that does not exist. The maths is the same, the model is correct, the concept exists the same in both cases, but a seemingly irrelevant change makes it physically implausible in one case.

Ah but does it, really?  A pipe at only 1000 PSI is a vacuum compared to deep underground.  Energy is relative!

I'm not sure what you're getting at with the infinite solid?  Mind, it's not perfectly rigid, the hole expands somewhat under internal pressure, tangentially stretching at the inner surface while radially compressing the surrounding material (how much, is given by the elastic modulus).  In terms of the pipe's stretchiness adding to the compressibility of the fluid and thus affecting wave velocity/impedance, the two situations will be different, but the latter will certainly not be the same as an ideal (truly incompressible, perfectly rigid) pipe, there will always be some effect.


Quote
Some commenters (here or on YT - can't find now) have boiled the "energy flows outside the wires" down to nicely intuitive statements that basically go; the space between the conductors is where the potential difference exists, the conductors are where the charge carriers flow, so it has to be a combination of wire and space that "energy" traverses. (I was going to add the example of a PCB with power and ground planes, and say the only place you need go looking for power is in the gap - but that's kind of redundant.)

Except it's worse than that - a location for the potential difference isn't needed, nor its "field strength", it just has to exist. And that is clear from the language, the focus is on force and movement. No one seems to question why power in a chain drive flows "outside the chain" (which is the same kind of situation).

I'd go one step further though; and infer that because energy seems to take a path that occupies either all or no space, and seems to transmit as if there were nothing in its way, it seems not to flow in spacetime at all. There is just distance and time. Kind of like it goes in a straight line but without direction, and chooses where to go based on external constraints. Photons show this behaviour.

Well, hold on a moment.  Energy is certainly flowing in the chain -- it might not be obvious how much is there, from just looking at one side of the drive, but considering the complete chain, we can take its velocity (which will be, on average, equal for both up and down sides), and the total tension (i.e., the difference -- the total with respect to a consistent direction, as one side is pulling up, the other down), and there's the power.  Clearly the power is contained within the chain!

Or for a more mathematical treatment: say we slice the system in half, between pulleys.  One side of the chain flows into the cutting plane, the other side out.  Integrate the tension over the chain cross-section (well, it'll be pressure at this point), and multiply by velocity.  Now we don't need to look at chains or belts under tension, we can do it for any mass flow: the crack of a whip, or fluids in a pipe (or not, like a waterfall).  And, as long as our cutting surface is closed (an infinite plane can be seen as a facet of an infinite sphere, or we can make a smaller box around a source or load of interest), we'll always have the correct total; we'll never miss the return path of a hydraulic pump for example, or when fluid is spraying out onto the floor.  (Not that it's necessarily easy to account for such flows, like evaporation and ground-seepage of water in the environment -- just that, in principle, it will be in this way.)

And, voila, that's how you use a Gaussian surface, you look at the total flux in/out of the surface, and that corresponds to the total contained within.

Well, if we do the same thing with the circuit, we find a superposition of two things:
1. DC flow in the wire,
2. AC flow around the wire (and along its surface).

The Poynting vector is just the quantity we integrate when we want to find total power flow.  How it's distributed spatially, depends on which case we're checking; both are valid in general!


Quote
I have! I wanted to make one (not a lot). Put it in a ferrite rod (which I guess is the epitome of an antenna structure) and there's not much difference between that and an AM radio. It also becomes a lot more practical to spin at 60MRPM.

;D

Or the classical examples like the Alexanderson alternator -- a multi-pole and slotted (reluctance) machine, used to generate high power at up to 100kHz.  Here, core and windings are used to couple the variations in magnetic intensity to an antenna.

Basically, the sort-of-magnetostatic field of a spinning magnet, is equivalent to an extremely low impedance.  We need many windings, or quite long extensions (as in your example), to match it up to the impedance of free space.

Or likewise for a spinning battery, the impedance is quite high so a great amount of capacitive division is needed to match to free space, and a capacitive divider can only be done at high Q (meaning, the mechanical load on the rotor will be extremely small).  That, or we use a transformer (which is arguably magnetic, but it's also generally electromagnetic, and we're after EM waves, so it's surely fair game, eh?).

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #420 on: December 04, 2021, 07:46:41 pm »
The total propagation delay of the reflection is twice the line length or 1 second.

Tim
Or 2 seconds. He says the lines are 300,000km long. And since current propagates slower than light speed it will be greater.
 

Offline T3sl4co1l

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #421 on: December 04, 2021, 08:33:39 pm »
End to end:



Tim
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Offline MIS42N

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #422 on: December 05, 2021, 09:21:08 am »
what about this And how do you get pictures inline? I said put this inline but it didn't.
« Last Edit: December 05, 2021, 09:28:44 am by MIS42N »
 

Offline etiTopic starter

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #423 on: December 05, 2021, 09:56:38 am »
Oh me oh my. What on earth avalanche did I set in motion when starting this topic!

😂
 

Offline MIS42N

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Re: "Veritasium" (YT) - "The Big Misconception About Electricity" ?
« Reply #424 on: December 05, 2021, 10:42:40 am »
@adx - it took me a while to get past the 'electricity is like water in a pipe' concept. It works for DC but as soon as any charged particle is accelerated or decelerated the analogy doesn't hold. Water particles require force to accelerate them and gain momentum. Charged particles require a force to accelerate them and gain a magnetic field. The difference is momentum can only be transferred by smashing one particle into another. Charged particles can do it by exchanging magnetic fields at a distance. This gives rise to the idea that energy is in the field, not in the wire.

But it doesn't pay to get too invested in the 'all energy is in the field' concept. A piezo transducer works by electrostatics, charged particles being pushed around by other charged particles. One could consider it a field but it's not magnetic.

When it really matters to me, I try to think what is actually happening. I rushed at this topic without thinking it through. I have now done so and I think I have a good grasp of which equations need to be solved to put real figures on conjecture. This is the analysis for the first half second:

The problem can be considered as a wire of infinite length in free space, when the voltage at one end is switched from 0 to 12V.
Then place another parallel wire at a distance and see what interaction there is between the two wires. If the interaction is small then whatever happens in the second wire can be assumed to have negligible effect on the analysis of the first wire.

To raise the voltage in the switched wire from 0 to -12V, electrons must flow into the wire. This is because the electron cloud in the wire is slightly compressible. This compressibility is seen in a Van de Graaff generator, where electrons are physically moved to the target sphere until the electrical field is so high some electrons have to leave. If the electron cloud was incompressible, for every electron added one would have to leave resulting in a continuous small current.

When an electron tries to move across the switch to the wire it is accelerated from its normal state toward the wire, and as it is now a moving charge will create a magnetic field that opposes the movement. However it is being pushed from behind by other electrons so a bunch of electrons are moving from zero net velocity (ignoring they are being agitated by heat and have their own constrained orbits) to some small finite velocity. This appears to the battery as a resistance, the value of which I am unable to find good documentation. I believe it varies with wire diameter and material and is in the order of hundreds of ohms. There will be a wave front of electrons being accelerated, moving along the wire at near, but not quite, light speed.

Radiating out from the wire is a magnetic field that goes from zero to some value. Once the wave front has passed a point the field is constant.

So now to the effect on a parallel wire. There are two effects to consider, one is the current induced in the wire by the fluctuating magnetic flux created by the switched wire. The second is there is capacitance between the two wires. It is easy to calculate the current due to capacitance.

Capacitance - assume two wires 1mm diameter spaced 1 meter apart and 150,000km long. The capacitance is approximately 1.33pF and is charged to 12V in something more than half a second, but assume 1/2 a second. This requires a current of 32pA or less. And yes it starts in the non switched wire after 3.3ns, the time it takes a field to cross 1 meter.

Magnetic flux - I cannot find a formula that shows what fraction of the flux around one wire encloses two wires. Or at least I can't understand how to apply the ones I've seen. I think it will induce a larger current than due to capacitance. However there is a question mark over this because superconductors (and the problem requires them) behave oddly in magnetic fields and I don't know if the flux can cut through the second wire or all flux from the switched wire is constrained to the gap between the two wires. In which case the switched wire does not affect the parallel wire magnetically.

In the video, the two wires are shown lying on the ground. The ground can be conductive and will affect the calculation resulting in smaller values. If the ground has zero resistance then the second wire will be unaffected. If the problem poser can pull a stunt like 300,000km of zero resistance wire, I think I can have some zero resistance ground. Or posit that the switched wire is coax with the outer conductor grounded.

The unknowns are: resistance of an infinite wire as seen by the battery. Propagation speed of the wave front. Proportion of flux cutting a parallel wire. Effects of superconductivity. All of which will be in textbooks somewhere.

This only gets through the first half second. After that there is counter flow through the parallel wire and the analysis has to start again. None of this requires me to believe the energy is in the field, the wire, or someone's back pocket. If the video likes to say there is a misconception about electricity, who am I to say. I get the results and they are more useful.
 


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