"Veritasium" (YT) - "The Big Misconception About Electricity" ?

[Psi]

[SiliconWizard]

Derek's video reminded me of the famous Dr. Walter Lewin experiment with two multimeters measuring different voltages at the same circuit points. I'm sure most of you know that lecture.

Well, the kind of topic yes, and this thread most definitely reminds us of this other thread about the Lewin vs. engineers debate. I wouldn't quite compare this one video to one of the Lewin's talks though. Not exactly the same caliber.

But as "useless" as the discussion may seem to most engineers, I don't think it is. It raises a number of interesting questions. Is it going to change your engineering practice? Probably not in the least. But it's still interesting to tackle more fundamental issues every once in a while, and realize ours models, however useful they are, are sometimes pretty dumb and limited.

The discussion following this video is certainly more interesting than the video itself.

[rs20]

It's really interesting to think about why we're happy to say that the power travels in the interior of a wave guide at xx GHz, yet we have such a visceral reaction against saying that the power travels outside the wires in a DC scenario. Yes, the way that we're taught how to think about wave guides advocates power travelling through the air; while the way that we're taught how to think about DC circuits is the much more simplified model, KCL and all that. This is fine and good and practical and exactly as it should be as already discussed. But, there is a perfect continuum from 45 GHz to 100 Hz to 0.01 Hz to DC, and Maxwell's equations apply equally the whole way. So at what point do we say that the power is flowing in the wires rather than the air? At the point where it becomes more convenient for practical analysis? On a philosophical level, it seems absurd to assert that the answer to the objective-sounding question "where is the power flowing" has an answer that depends on what technique the humans answering the question choose to use. With this in mind, I can perfectly foresee how a physicist looking at an engineer saying "the power doesn't flow through the air at DC/steady state" would think the engineer is utterly mired in closed-minded pragmatism-above-all-else.

Meanwhile, pragmatic engineers look at physicists making these observations, and struggle to understand to what extent this observation is "real" or meaningful.

Both sides might naively say that the other side is wrong, or disconnected from reality, or clinging too tightly to what they were taught at school. But really, they're both just two sides of the same coin, both views are correct in their own way, and it's just interesting to venture into each other's worlds once in a while. No need for accusations of "spherical cows" nor "denying Maxwell's equations". One of my pet peeves is people who say "my model is correct and your model is different to mine therefore your model is incorrect" -- no, very different models can both be correct!

[ Sorry for the reposted content; I had written this in an edit to a post that was 10 posts in the past by the time I had completed the edit... ]

[eti]

It always makes me chuckle when these YooToobaz “debunk” one another back and forth like a game of intellectual tennis. It rarely gets anywhere, “proves” little, and is usually the product of ego.

[Quarky]

I don't think Derek is trying to say this is useful for practical engineers... It's a physics question and we should approach it from that angle.

I think all the commenters and controversy is playing right into his hands. Everyone is now doing a double take on their own understanding.

Check out one of his very early talks about "The key to effective educational science videos".
(Clear and concise videos actually hinder learning for most.)



Anyway, let's not forget that even Maxwell's equations are simplified models and this rabbit hole goes deeep...

You can even think about there being no EM waves at all and the energy is be transported by photons (since after all they're the same thing) between the battery and light (and along the surface of the wires).

[bdunham7]

"my model is correct and your model is different to mine therefore your model is incorrect" -- no, very different models can both be correct!

Actually no model is completely correct.  And while there are cases where you can get the same result using different models, most of the time models make assumptions about the negligibility of certain parameters that only apply in a certain domain.  Sometimes a model from a different domain can be used to model secondary perturbations of a system in a different domain, but using the models and mathematics relevant to a microwave waveguide to design a battery cable for a car doesn't work from any perspective--engineering or science. 

As I explained earlier, the issue with power and frequency is EM radiation--it is zero with DC and then increases to a nuisance level at moderate frequencies, then a significant factor at higher frequencies and then the dominant model at even higher frequencies.  Of course transients and step functions can span quite a range of bandwidth, thus the current discussion.

[bdunham7]

You can even think about there being no EM waves at all and the energy is be transported by photons (since after all they're the same thing) between the battery and light (and along the surface of the wires).

Sure, look up the energy of a single photon for the DC case (frequency is zero).

[eti]

I don't think Derek is trying to say this is useful for practical engineers... It's a physics question and we should approach it from that angle.

I think all the commenters and controversy is playing right into his hands. Everyone is now doing a double take on their own understanding.

Check out one of his very early talks about "The key to effective educational science videos".
(Clear and concise videos actually hinder learning for most.)



Anyway, let's not forget that even Maxwell's equations are simplified models and this rabbit hole goes deeep...

You can even think about there being no EM waves at all and the energy is be transported by photons (since after all they're the same thing) between the battery and light (and along the surface of the wires).

I see this schtick coming a billion miles away / I don’t get dragged down rabbit holes like that. Besides, there ain’t enough hours in the day for these endless, (sometimes) fruitless “debates” online. I post things here because I hope others will find them interesting, and it does seem to have sparked a great debate here, but I truly wouldn’t care a hoot about being involved in it.

I suppose I am sort of “curating” things that I might feel others will enjoy. Derek is a nice enough chap, it appears, and he’s a “Jack of all trades”, and unless I truly needed to ponder over how he thinks current is transmitted, I’ll leave it here for those who care. How can he be an expert on so many subjects, or feign “interest” in them? Oh yeah… $$$$… how depressingly mundane a reason.

All this speculative “science” on YouTube,  loads of it I see in my feed, and that’s fine, but I look at it and think “Okay, so let’s say I waste an hour watching this arbitrary subject, and let’s then say I was the type who’d be dragged down a rabbit hole of speculation… which could maybe never bring a ‘right answer’ - so what?!!”

That’s why I have a select few channels I sub to, and even then I don’t watch EVERY video, as I tire of certain channels and have to have a few months break, and don’t like the idea of setting them up in one’s mind as “THE authority on X”  - not only does that significantly narrow one’s range of material, it’s just a matter of … “and? So what - how has this advanced me in any sense, now I know this arbitrary, detached piece of mental metadata?”

Any fool can speculate, and most of them do. I’m foolish at times too. 😁 - as Dave seems to be saying on this thread - this info Derek seems to believe, is all but useless to practical electronics engineers. Intellectuals like to think about the science behind the physical, practical materials that actual engineers just get on and DO.

[ejeffrey]

Look up "ladder line"

I know what it is, 300R flat antenna cables were common many years ago and I still have them roaming around my house.  But if I hook up a 9 volt battery to one end and a bulb to the other, I don't call that a transmission line.  And even if I tried to analyze the initial conditions of connection this way, it is still a 300R characteristic impedance while the resistance of the loop is likely to be much lower.

If it is 30000000 meters long and you are interested in the turn on transient behavior then it absolutely is a transmission line.

One way to work up to this is start with a circuit that has no transmission lines but to simply put 300 ohm resistors between the battery and the lamp.  Then what happens is obvious.  Now replace those resistors with 300 ohm transmission likes terminated by 300 ohms.  We know that in this case the behavior will be exactly the same as just the resistor no matter how long the transmission lines are. So make the transmission lines 3e8 meters long.  Finally replace the resistors with short circuits.  This changes the behavior but since they are a light-second away we don't see the effect until the reflected wave gets back.  The initial behavior will be as if fed by the characteristic impedance of the line then after a round trip time it will settle to the DC value.
 

[EEVblog]

It's really interesting to think about why we're happy to say that the power travels in the interior of a wave guide at xx GHz, yet we have such a visceral reaction against saying that the power travels outside the wires in a DC scenario. Yes, the way that we're taught how to think about wave guides advocates power travelling through the air; while the way that we're taught how to think about DC circuits is the much more simplified model, KCL and all that. This is fine and good and practical and exactly as it should be as already discussed. But, there is a perfect continuum from 45 GHz to 100 Hz to 0.01 Hz to DC, and Maxwell's equations apply equally the whole way. So at what point do we say that the power is flowing in the wires rather than the air? At the point where it becomes more convenient for practical analysis?

Yes, exactly that.

[ejeffrey]

As I explained earlier, the issue with power and frequency is EM radiation--it is zero with DC and then increases to a nuisance level at moderate frequencies, then a significant factor at higher frequencies and then the dominant model at even higher frequencies.  Of course transients and step functions can span quite a range of bandwidth, thus the current discussion.

It's not EM radiation per se but even at DC the energy storage and power transfer are through the E&M fields, and generally energy dominated by the fields in free space and dielectrics.  It's all near field stuff of course there are no propagating waves hence no "EM radiation" but there are still fields.

[bdunham7]

If it is 30000000 meters long and you are interested in the turn on transient behavior then it absolutely is a transmission line.

One way to work up to this is start with a circuit that has no transmission lines but to simply put 300 ohm resistors between the battery and the lamp.  Then what happens is obvious.  Now replace those resistors with 300 ohm transmission likes terminated by 300 ohms.  We know that in this case the behavior will be exactly the same as just the resistor no matter how long the transmission lines are. So make the transmission lines 3e8 meters long.  Finally replace the resistors with short circuits.  This changes the behavior but since they are a light-second away we don't see the effect until the reflected wave gets back.  The initial behavior will be as if fed by the characteristic impedance of the line then after a round trip time it will settle to the DC value.

Yes, I think we've settled that part, although I have a sneaky feeling I/we have all missed something.  What I didn't see initially is that he had included zero resistance wires in his model.  That actually makes a huge difference, which I think threw my initial intuition off.  That and my mis-estimation of the characteristic impedance of a twin-line transmission line with a separation of a meter.  The simplified formula for characteristic impedance assumes you are in the transmission line domain re frequency, so you can't infer anything about LF/DC.  The complex formula will go to infinity at DC if there is conductor resistance but no dielectric conductance, and to a set value if the dielectric has some conductance.  If you take the resistance of the conductors and the conductance of the dielectric out of the equation (they go to zero) then you are left with the inductance and capacitance only, Z₀ = (Ldl/Cdl)^1/2.  Thus the impedance is constant down to DC as long as your line is infinite, as you've stated.

It will be amusing to see what he comes up with for a 'demonstration' of this effect.  Since zero-resistance wires aren't likely, he is probably going to need to scale the effect down to a few hundred nanoseconds or a few microseconds at best.  And with real wires, I think the apparent impedance of the lines (there are two, in series here) will rise pretty rapidly. 

[Quarky]

You can even think about there being no EM waves at all and the energy is be transported by photons (since after all they're the same thing) between the battery and light (and along the surface of the wires).

Sure, look up the energy of a single photon for the DC case (frequency is zero).

In the DC case (if we advance time infinitely into the future after the switch is closed), energy will be carried from electron to electron down the length of the wire via virtual photons (virtual photons have independent frequency and energy terms i.e. frequency can be zero, but it can have a non-zero energy). Virtual photons can "exist" due to the uncertainty principle and only live for a very short period of time so it does not have much range. Therefore the energy should propagate along the wires...

However, any change in the system at the source or load will immediately cause very real photons to be emitted from either the source or load and absorbed at the other side in 1/c seconds.

[Kalvin]

One caveat: If someone tries to explain this experiment setup and what is going on in the circuit, it may be tempting to simplify things a bit, and say that the two parallel wires that are 1m apart are forming a big, long capacitor. When the switch is closed, there will be a current flowing in the circuit as soon as the the change of the electrical field reaches the 1m distance, as the capacitor starts to discharge.

This is only partly true, because it doesn't explain why the lamp doesn't get the full power right after the switch is closed. If the capacitor model was a correct one, the lamp would get full power right after the switch was closed and the change in electrical field has propagated that 1m distance.

Because the long wires has inductance, and the long wires are placed 1m apart from each other, the wires are forming a transmission line in which the inductance and capacitance is distributed along this long pair of wires.

Using a transmission line model and impedance will explain correctly why the lamp will receive only partial power after switch is closed, and why the lamp will get full power only after the circuit has reached the steady state. A simplified, lumped model will not provide correct answer and reasoning.

[EEVblog]

Well so much for my 10min video.
Just the simulation part turned out to be 8 minutes 
I could of course get that down if I re-recorded with some more suscint speech, but I'm not so good at that...
And that doesn't even include mention of the effect of moving the switch, or making the circuit a giant loop instead of 1m apart.
And mentioning that, the question is very deliberately set to 1m apart, because he knows damn well the capacitance will be practically zero so it's harder to justify his claim with that. When it's 1m apart you have some realistic numbers to work with.

[Kalvin]

Well so much for my 10min video.
Just the simulation part turned out to be 8 minutes 

Excellent circuit model and explanation.

[rs20]

It's really interesting to think about why we're happy to say that the power travels in the interior of a wave guide at xx GHz, yet we have such a visceral reaction against saying that the power travels outside the wires in a DC scenario. Yes, the way that we're taught how to think about wave guides advocates power travelling through the air; while the way that we're taught how to think about DC circuits is the much more simplified model, KCL and all that. This is fine and good and practical and exactly as it should be as already discussed. But, there is a perfect continuum from 45 GHz to 100 Hz to 0.01 Hz to DC, and Maxwell's equations apply equally the whole way. So at what point do we say that the power is flowing in the wires rather than the air? At the point where it becomes more convenient for practical analysis?

Yes, exactly that.

That's... not right. You're advocating that the answer to a perfectly cromulent scientific question ("how much power is flowing in this point in space") is not just a function of the circuit/situation, but also a function of what the engineers looking at the problem are thinking. I'm puzzled that you wouldn't see that as "pragmatism" taken way too far?

[EEVblog]

It's really interesting to think about why we're happy to say that the power travels in the interior of a wave guide at xx GHz, yet we have such a visceral reaction against saying that the power travels outside the wires in a DC scenario. Yes, the way that we're taught how to think about wave guides advocates power travelling through the air; while the way that we're taught how to think about DC circuits is the much more simplified model, KCL and all that. This is fine and good and practical and exactly as it should be as already discussed. But, there is a perfect continuum from 45 GHz to 100 Hz to 0.01 Hz to DC, and Maxwell's equations apply equally the whole way. So at what point do we say that the power is flowing in the wires rather than the air? At the point where it becomes more convenient for practical analysis?

Yes, exactly that.

That's... not right. You're advocating that the answer to a perfectly cromulent scientific question ("how much power is flowing in this point in space") is not just a function of the circuit/situation, but also a function of what the engineers looking at the problem are thinking. I'm puzzled that you wouldn't see that as "pragmatism" taken way too far?

Err, last I checked, engineering is an applied science.
It's why ohms law, Kirchhoff's laws, and countless other practical theorems were developed, so we didn't have to "go back to basics" and use Maxwell and Poynting for everything. Even conventional current flow is a thing for a reason.

As for "that's not right". You asked for my opinion at what point we say the power flows in the wires, I gave you a practical answer that's used by almost every practicing engineer.
Even Richard Feynman agrees:
https://www.feynmanlectures.caltech.edu/II_27.html

You no doubt begin to get the impression that the Poynting theory at least partially violates your intuition as to where energy is located in an electromagnetic field. You might believe that you must revamp all your intuitions, and, therefore have a lot of things to study here. But it seems really not necessary. You don’t need to feel that you will be in great trouble if you forget once in a while that the energy in a wire is flowing into the wire from the outside, rather than along the wire. It seems to be only rarely of value, when using the idea of energy conservation, to notice in detail what path the energy is taking. The circulation of energy around a magnet and a charge seems, in most circumstances, to be quite unimportant. It is not a vital detail, but it is clear that our ordinary intuitions are quite wrong.

[rs20]

I guess it's just a difference in perspective. I quite enjoy making a distinction between science and engineering, and find it hard not to treat a question such as "how much power is flowing in this point in space" as a scientific question with an absolute definitive answer that is the same irrespective of what nearby humans are thinking. The idea that different people could have different answers to such a specific-looking question and both be considered right just because their answers "suit their own practical purposes" is bizarre to me. Like two people looking at a bucket full of water and one person (correctly) saying "that bucket is full of water" and another thirsty person noticing the bucket is full of salt water and saying "I need fresh water, all other water is irrelevant, that bucket is empty" (which is totally wrong unless the thirsty person inserts 'practically' into the sentence). Science: The bucket is full of water, no matter what anyone is thinking. Engineering: the bucket doesn't contain any drinkable water.

Whereas you seem to see "how much power is flowing in this point in space" merely as a potential means to an end, thinking about problem solving/engineering and applied, applied, applied. I'm not making a judgement call here, just trying to get the root of our subjective difference of opinion.

[snarkysparky]

In this example there is no steady state power flow in the space between the wires.   PERIOD.... 

Ver is completely wrong about this.

Poynting vector is for self propagating electromagnetic waves.   Not static waves.

The electric field is preserved along the wire by its low resistance and shows up at the load to push electrons through the load.  Work done is  E*J     ( * is vector dot product   E is electric field and J is current density vector )

[sandalcandal]

Well so much for my 10min video.
Just the simulation part turned out to be 8 minutes 

It was always going to be a challenge

Pretty nice video on that one aspect regardless. I suspect there will be confused and angry comments from physicists without prior knowledge of transmission lines but not too much you can do about that I suppose. I think it would be nice if you could show how (roughly) a transmission line model can be set up using the given problem parameters to produce the same 1/c delay predicted by the "physics", that would really help push the point that the transmission line based model is accurate and equivalent; similar to the slides provided to Derek by the professors but explained more eloquently.

[sandalcandal]

In this example there is no steady state power flow in the space between the wires.   PERIOD.... 

Ver is completely wrong about this.

Poynting vector is for self propagating electromagnetic waves.   Not static waves.

The electric field is preserved along the wire by its low resistance and shows up at the load to push electrons through the load.  Work done is  E*J     ( * is vector dot product   E is electric field and J is current density vector )

See rs20's previous post: https://www.eevblog.com/forum/chat/veritasium-(yt)-the-big-misconception-about-electricity/msg3829172/#msg3829172

[penfold]

With absolutely no discredit to Dave's response video, the transmission lines approach represents the length of the wires effectively and demonstrates the coupling from source to load is not relying on anything that cannot be modelled with lumped components and certainly doesn't require analysis of all the fields going on...

But... does it not highlight one weakness of the transmission line model in that it does not handle the lateral (1/c) delay properly? Only that of an infinitesimally thin structure that so happens to have the equivalent characteristics of something 1m wide? So it doesn't *actually* provide an answer to the multiple-choice, only that it's not 0.5, 1 or 2 seconds?

[bdunham7]

DC the energy storage and power transfer are through the E&M fields, and generally energy dominated by the fields in free space and dielectrics.

In the case of DC, can you show me how any of the energy transfer from one end of the wire to the other is due to any field external to the wire? 

[snarkysparky]

https://www.researchgate.net/post/What_is_the_significance_of_the_Poynting_vector_in_a_static_electromagnetic_field/2

https://www.researchgate.net/profile/Eric-Lord-2/post/What_is_the_significance_of_the_Poynting_vector_in_a_static_electromagnetic_field/attachment/5c4f2143cfe4a781a57aa3ef/AS%3A720071072706562%401548689730973/download/Field+Energy+and+Field+Momentum.pdf

I think it says that at a point in a static field the energy into the point and out of the point are equivalent.   So no actual energy flow.