Veritasium's Big Misconception About Electricity video and the point about Poynting

We discuss Veritasium's video "The Big Misconception About Electricity". In the video Veritasium asks the question of when the light bulb in a extremely long circuit will actually light.

This point is well discussed already in several videos, see this one in particular for a good explanation of the quiz.

We focus more on the strange nature of Poynting's vector.

Griffith and Feynman's standard textbooks:

Alternative theories

Poynting plots

#veritasium #electricity #misconceptions

[hankdewit]

I enjoyed your discussion one the Poynting vector. I have a few comments to add.

At 8:36 you go through Feynman's discussion of the stationary charge in the field of a stationary magnet. I realize he says this example will convince you the "theory is obviously nuts" he is actually just leading you along. If you read right to the end of the chapter you will see that he demonstrates that the momentum density of the field g is 1/c^2 * S, so that energy flow and momentum of the EM field are proportional. So 'he says' the strange rotating energy flow in this example can then be equated as angular momentum. He revisits an apparent paradox in 17-4 of the book, he say's "about a solenoid and some charges mounted on a disc? It seemed that when the current turned off, the whole disc should start to turn. The puzzle was: Where did the angular momentum come from? The answer is that if you have a magnetic field and some charges, there will be some angular momentum in the field. It must have been put there when the field was built up. When the field is turned off, the angular momentum is given back. So the disc in the paradox would start rotating. This mystic circulating flow of energy, which at first seemed so ridiculous, is absolutely necessary. There is really a momentum flow. It is needed to maintain the conservation of angular momentum in the whole world." So Feynman does take the Poynting vector seriously.

Later in the discussion ( 13:42 ) there is speculation about whether it might just be the divergence of the Poynting vector which might make physical sense as this does locate the real sources and sinks. But you are forgetting the rationale that Feynman used when opening Chapter 27. For the EM theory to conserve energy in a relativisticly consistent way the energy must be locally conserved and effectively track-able from source to sink. So the energy flow must be real, whatever energy really is.

[maddhopps]

Love that the science creators across YouTube have this type of interaction and that it’s always upfront, civil, and with underlying modesty. And when you see the creators hopping into each other’s comments to acknowledge those corrections or add nuance, then that’s even better because it shows the collegial, cooperative spirit of challenging one another that we’ve always loved about science.

[chuckbritton]

My problem with the original video is that he mixes transient phenomena with steady state. When the switch closes at t=0, a step voltage is launched down both left and right lines. The full circuit PVs don't exist immediately because the electric and magnetic fields develop as the wavefront propagates. That means the PV from battery to light is time variant just like the rest of them around the circuit. He makes it as if it's fully developed immediately in a steady state condition..

[jdbrinton]

Hi Tim, these videos are super informative to the larger public. But it sometimes is a bit hard to understand with the noise in the background. If you could mute people who aren't talking it may help.

[jonathanedelson]

A related area of discussion is trying to understand the difference between the 'near field' and the 'far field' of an antenna.

Clearly antennas radiate EM energy, and in the far distance from the antenna (where no charges are present or moving) the Poynting vector is critical to describing energy flow.

But _near_ the antenna you do have charges moving, and often it makes more sense to think of the charges moving and the current. Using the Poynting vector near the wires of a circuit is a description that is weird and difficult to work with. But the maths do work.

Jon

[thomas]

The Poynting vector describes a stead state, this is a dynamic system due to the switch. If you notice there is no time variable.

Consider if the switch was on the end of the line dangling out in space, a half light second from the lamp and battery. If you flip the switch would the light illuminate in 1m/c s seconds despite the fact that the switch is half a second away? That would mean breaking the speed of light so I doubt it. 🙂

[sarentascyan]

This is the only content I could find so far on this topic, which rigorously examines the Veritasium's video. All other explanations seem to try to match science to the result presented in the video and not really understand it. I think the truth lies somewhere in between and this video is really addressing this.

this is the best explanation video i watched about that video Derek made

[xSyph]

I really dislike hearing background noise from someone that isn't talking, have these guys never used a mute button? Great content otherwise.

[camilojcastillo]

Thank you very much for the video, it was illustrating to me to revisit the concept of Poynting vector.
The first point I'd like to emphasize is that the diagram of the electric field in Derek's graph is badly drawn. IMHO, that error is intentional. He tries to show that the Poynting vector is more or less the same in the space inside the circuit, regardless that both magnetic and electric fields are much stronger around the wires. That leaves to the misleading idea that the energy is transmitted in the vacuum and not in the wires, but that is not true. Inside the wire, as you've shown in the Griffiths example, the Poynting vector is pointing outwards, but outside the wire, as the electric field E tends to be perpendicular to the wire, so Poynting vector will be more or less parallel to the wire. Then when you close the circuit, even when a little current will be induced in the lamp, it will never light up. Poynting vector will travel along the wire just like the current, at almost the speed of light. That is way the lamp will turn on in 1 second, approximately, although a little EM pulse will arrive the lamp in 1/c seconds.
Anyway, Poynting vector does not deny that the current drives energy. Poynting's theorem says that the rate of energy transfer (per unit volume) from a region of space equals the rate of work done on a charge distribution plus the energy flux leaving that region. I.e.

du/dt=∇•S+ J•E

And this energy flux is the divergence of the Poynting vector. In the case of a battery powering a lamp, that divergence is zero, so all the energy is in the form of work done (J•E), that leads to the known Joule's law (P=I.V). In DC circuits there is no divergence of the Poynting vector, so there is no energy loses in the form of electromagnetic waves.
Of course there is a transient EM wave associated with closing or opening DC circuits, but they at most will interfere with your radio device, never turn on a lamp.

[mb]

For people who are complaining about the noisy audio: Mute your speakers and turn on subtitles. The contents of what is being said is worth watching this video. One can even get the transcript by clicking the three dots menu.

[EvanZalys]

My favorite way to see this is that a transmission line is a waveguide whose pass band extends to DC. The free charges in the conductor result in fields that steer the poynting vector down the wires. I find it helpful to try and see the metal much in the same way we’ve learned to view dielectrics. We see dielectrics as polarizable bound charges that result in some net effect in the presence of fields. Conductors do a similar thing. Locally, a field can push electrons through a lightbulb but globally the fields are sort of continuously outrunning the charges and shifting them in such a way that they generate conjugate fields that guide the field along the conductor, where this effect can extend down to DC.

[KaliFissure]

The energy exists all over. The electrical potential in the wire DIRECTS it, perturbs it, in a certain orientation.

[mntbighker]

The chain in the plastic tube, and the reference to the under sea cable, to me buggered up the central point of the video. He made some good points, but they didn't help his case. A few issues with his conclusions I had were, if all that were true, why do wires melt trying to flow too much current if things are mostly external to the wire? And why do stranded wires act differently from solid core wires? Anyway, good discussion. In general the internet has gotten way deep in the weeds on what I think was a "simple" concept he was trying to convey. But it's probably not possible to simplify it the way he was attempting?

[randallgoldapp]

Changes in potential propagate at the speed of light. Think of the charges as low frequency radio waves and the conductive as transmission lines.

[bobwhite]

The 2004 paper is exactly right. Feynman's example of resistive wire is misleading at best. He is only showing the tangential E and ignores the normal E field. The true S vector even in Feynman's case would be tilted into the resistive wire - the component normal is the resistive loss, and the component tangential will flow along and decrease as it progresses. For a good conductor, the tangential E field is tiny - the normal field would typically be orders of magnitude larger. So S is almost fully parallel to a good conductor.

[DeusExAstra]

I think the Poynting vector here makes sense. I mean, it's just looking at the E and M fields in a certain area, which I think we'd all agree are real. And those are just measures of the photons being propagated through that space. So if there are photons moving through a space, there is energy passing through it. The problem, I think, is that the drawing makes us think most of the energy lighting up the bulb is coming through those fields into the bulb from outside the wires when the bulb lights up. I dont think that's the case. Most of the energy will come from inside the wires in the DC case. But when you have a spike in current, and then the capacitance that moves energy through space to the bulb, this is indeed moving along those Poynting vectors to the bulb, and that small bit of energy goes into the nearby wires and bulb metal and induces a current that might... maybe, dimly light the bulb.

[laurisuoranta]

This is OK discussion. However: 1) Capacitance is property of non-ideal conductors. It was clearly ignored by Veritasium and the analysis backing him, so I think one should assume conductors having no capacitance. 2) It seems that different variations of the setup are being analyzed without noticing the distinction. The original setup had one switch next to the positive terminal of the battery. So at t<0, the bulb and practically all the wire would be at equipotential with negative terminal of the battery. Anyhow, it's disappointing characteristic of the social media era that the original video, whose only merit is posing an interesting question, has 10M views and 377k likes. Valid discussion on actual physics of the problem receives practically no attention.

[ptitnhane]

Thanks for the video. People get so focused on the direction of the Poynting vector that they forget that it also has an amplitude. Yes, SOME energy is flowing through the air from the battery to the light bulb... But the amount of energy flowing like this is insignificant compared to the amount flowing along and close to the wires.
The problem is that all of us like to think intuitively, and vector direction is very easy for that, and are just too lazy to do the computation and see through which part of the Poynting field does MOST of the energy actually flows.

[jongeduard]

So the real question is: what ìs actually Energy? And does it actually even "flow" at all.
Questions like these exist for very long, and no one as ever found the final answer as far as I know.
I understood that the best known explanation is simply that energy is a configuration in which things are. In that way, it's very related to the word "information".
I have seen another video, in which pretty much the same idea is shown as on the left side at 10:35 in this video, where you see pointing vectors everywhere, all in perpendicular directions to the current.
This perfectly fits into the idea of a general Electric Field throughout the entire universe, that can fed with energy from anywhere, and from which energy can be extracted from anywhere.
In this Field it does not matter to much how energy flows, but the fact that it has a value, and that energy is always being conserved.