I An interesting question from Veritasium on YouTube

[hutchphd]

I'm supposed to guess what the switch is connected to?

The description is complete. (Yikes! get a grip)

In this case the bulb will receive a very small signal through the air immediately but will heat up more than one second later, depending upon the coax propogation speed.

 

[A.T.]

- Inner coax conductor connects: battery positve to light bulb terminal 1
- Outer coax conductor connects: battery negative to light bulb terminal 2
- The battery is 1m away from the light bulb
- The coax cable is 1 light-second long, laid out in two straight parallel segments of 1/2 light-second each, 1m apart. At the far end (1/2 lightsecond from the battery & bulb) the coax cable simply bends around.
- The switch at the battery interrupts both coax conductors.

How long will it take for the light bulb to receive any power after the switch is closed?

In this case the bulb will receive a very small signal through the air immediately but will heat up more than one second later, depending upon the coax propogation speed.

Thanks for the reply. Is the initial very small signal through the air because the fields outside a coax cable are not completely zero in the transient phase, when the current builds up?

 

[Baluncore]

No, just how long the signal takes, without additional constraints on how it gets there.

1/c. The parallel external coaxial braids form a separate external two wire transmission line, shorted at the far end, where the line loops. Call that say Zo = 600 ohms, until the short circuit appears after about 0.7 seconds, because the coaxial external velocity factor will be greater than the inner. That circuit is closed on the centre conductor side by stray inductive and capacitive coupling between battery and bulb, terminals and wires.

Exactly 1 sec, when the signal on the inside of the coax gets to the globe.

Power is the rate of flow of energy. It is not energy.

 

[Baluncore]

Is the initial very small signal through the air because the fields outside a coax cable are not completely zero in the transient phase, when the current builds up?

No. It is capacitive and inductive coupling of the local wires. The outside of the folded coaxial cable is a quite separate transmission line to the inside of the same coaxial line.

 

[cmb]

The description is complete. (Yikes! get a grip)

So it is a quadruple pole switch, that is on both sides of the battery at the same time?

Somehow?

Really, a diagram would have clarified this.

OK, it's over my head now then!... I'm out.

 

[artis]

Well someone already mentioned this here that the bulb will light up later if the wires to the bulb have insulation around them. This I think is the simplest proof that the energy travels as fields and current is just the result of those fields. Insulation attenuates the field, it's propagation velocity decreases and boom there is your proof.

As for the transmission line and transients if the wire was not superconducting then the bulb would never light up I think simply due to the huge resistance, but a transient would still reach the bulb? I think this because the wires closer to the battery would have a higher voltage than the wire further away and since this is a transmission line the capacitance is equally spaced across the length so there would still be some capacitance in the parts of thew wire that are close to battery between those that are opposite to them and close to bulb.
So the transient would not light up the bulb but a sensitive voltmeter should be able to pick it up across the bulb?

 

[hutchphd]

So it is a quadruple pole switch, that is on both sides of the battery at the same time?

The technical term would be double pole single throw (DPST) switch.

May I also recommend the Wikipedia article:
https://en.wikipedia.org/wiki/Transmission_line
Really nice succinct rendition (at least for my old brain)

.

 

[cmb]

The technical term would be double pole single throw (DPST) switch.

I took "The switch interrupts/connects both coax conductors simultaneously" to mean it interrupts both coaxial lines.

I can accept the ambiguity may have been intended to imply a double pole on one coax only.

I feel that my proposition for a diagram to aid understanding was very reasonable.

On the subject of a double pole switch, in practice this would create an interesting outcome in that as both could not be timed perfectly, this may also have a consequence.

 

[Baluncore]

The coax cable is 1 light-second long, laid out in two straight parallel segments of 1/2 light-second each, 1m apart. At the far end (1/2 lightsecond from the battery & bulb) the coax cable simply bends around.

Your circuit has internal and external transmission lines. Firstly there is the line inside the coax, say 70 ohm. Then there is the two wire line formed by the outside of the parallel braids, say 300 ohm.
Since you are relying on stray capacitance to complete the circuit during the first second, you might copy the Veritasium method by duplicating the TLs on both sides of the circuit.
Notes. DC bias is needed for the model to run, that is applied through nodes a, b, c and d.
The circuit is crossed over at the top to fix the polarity of the two paths so the bulb is driven by both sides in parallel, hence a 35 ohm bulb.
The first energy to reach the globe travels by the Zo of the external braid parallel line, but has high Zo.
Then the reflection of it's short circuit arrives later.
The signal by the internal coax path with the lower velocity arrives at one second. That line is modeled as two halves, joined at the bend. Some of the external braid energy reaching the bulb, heads backwards, down the internal transmission line.

 

[OmCheeto]

How Wrong Is VERITASIUM? A Lamp and Power Line Story
51,759 views
Dec 8, 2021
ElectroBOOM
4.85M subscribers

Yay! I thought I was losing my mind, but Mehdi came to the same conclusion that I did.

 

[Drakkith]

Someone did an actual experiment to settle this. The short answer is that there is a small amount of current through the light bulb almost immediately upon flipping the switch, followed by the circuit ramping up to full current once the signal has time to propagate along the wire in both directions.

 

[nsaspook]

Someone did an actual experiment to settle this. The short answer is that there is a small amount of current through the light bulb almost immediately upon flipping the switch, followed by the circuit ramping up to full current once the signal has time to propagate along the wire in both directions.

OK, Youtube should be happy now.

 

[hutchphd]

Yay! I thought I was losing my mind, but Mehdi came to the same conclusion that I did.

Which as near as I can tell differed very little (except for the hype) from Veritasium. Please tell me the substantive difference.

.

 

[Baluncore]

The short answer is that there is a small amount of current through the light bulb almost immediately upon flipping the switch, followed by the circuit ramping up to full current once the signal has time to propagate along the wire in both directions.

Those who understand transmission lines will agree with that analysis. There is no need for an experiment.

Those who do not understand the principles of transmission lines will argue that only their preferred part of TL theory needs to be considered.

 

[OmCheeto]

Which as near as I can tell differed very little (except for the hype) from Veritasium. Please tell me the substantive difference.

.

Derek showed the light coming on instantly. He said it was a lamp that came on regardless of how little current flowed through it. But real lamps don't work like that. According to his definition, his magical lamp should always be on.

 

[nsaspook]

Derek showed the light coming on instantly. He said it was a lamp that came on regardless of how little current flowed through it. But real lamps don't work like that. According to his definition, his magical lamp should always be on.

That's not really what was implied by Derek's statement because it defies engineering logic. It's easy to design systems that can detect a few nA of current from normal background noise or a hand clap with acoustic signals. No, that statement of the lamp being always on is bogus.

Pinned by ElectroBOOM

​

Veritasium
10 days ago
Thank you for making this video! While in hindsight I can clearly see that I should have gone into more detail with the explanation, I have really enjoyed watching all the response videos. For the record I was not suggesting the lightbulb lights at ANY current value but at some small but significant current value. I tested my LED bulb rated for 12V and found it turns on dimly when I apply 2V. There may yet be a follow up video coming. So thank you for this commentary - I'll incorporate it into any further work I do on this topic.

 

[OmCheeto]

That's not really what was implied by Derek's statement because it defies engineering logic. It's easy to design systems that can detect a few nA of current from normal background noise or a hand clap with acoustic signals. No, that statement of the lamp being always on is bogus.

So why did his lamp come on full brightness? He says one thing and displays something which defies engineering logic. Do you not find that bogus?

Btw, this is starting to sound like science by litigation. Waiting for someone to point out the inflection in Derek's voice as an argument in their favor.

 

[hutchphd]

Do you not find that bogus?

His main point was that it is the fields that carry the power.
At no point did I think he actually had wires $10^8$ meters long attached to his apparatus. If you thought that he did then in fact it was deceptive (and I have bridge you might be interested in...)
Otherwise give me a break.

 

[pbuk]

"some small but significant current value ... I tested my LED bulb rated for 12V and found it turns on dimly when I apply 2V"

Right, so we are supposed to give this ElectroBOOM guy how much credibility?

 

[nsaspook]

So why did his lamp come on full brightness? He says one thing and displays something which defies engineering logic. Do you not find that bogus?

Btw, this is starting to sound like science by litigation. Waiting for someone to point out the inflection in Derek's voice as an argument in their favor.

Because logically (in the eyes of a engineer) he was talking about a small current detector like the one in a garage door opener that can turn on a light or open a heavy door from a tiny detection of current from a remote source. What's bogus is using this lamp detail as a sleight of hand from the facts of the video, "it is the fields that carry the power".

 

[nsaspook]

Right, so we are supposed to give this ElectroBOOM guy how much credibility?

No, the "some small but significant current value" is a response from Derek to the bogus
lamp is always on comment from the ElectroBOOM guy.

 

[cjl]

Alternatively, it's a handwavey after the fact justification for his misleading answer.

Nowhere in his video does he ever mention that it could take a full light travel time or longer to deliver full power to the load. His entire video leads to the misleading conclusion that the light receives full power effectively instantly, and that conclusion not only is wrong, but it actively leads people away from a correct understanding of electricity and circuits. Derek's video is an excellent example of exactly what modern science educational content shouldn't be like.

 

[artis]

I think we should all just realize that Derek aka Veritasium is a youtube presenter, irrespective of his background or education and understanding he stands to make money from his youtube persona. Youtube due to it's algorithms which are tailored to the the preferences of the majority of viewers is not the best place for real science because in order to succeed it demands one to use speculative and sensational titles and similar video presentation. It is no secret that the channels that lately earn the most from that platform and are among the most popular do just that.

This being said I still find many Derek's videos entertaining and interesting and his science backing is still some of the most on point although it seems he himself deviates from that once in a while and this lamp video just happened to be one of them.

From a physics perspective there is no miracle here, i think @Baluncore and others were right saying that due to transmission line theory the bulb will indeed receive some minor ever increasing current almost instantly while the real current necessary to light a real practical bulb will get there only when there is full load current established all around the length of the wires and this will happen at c along the wire in vacuum or less than c in other mediums.I think Derek was a bit tricky and deceptive on purpose making the wires long in one direction while making the separation short in the other direction which happens to be the direction/plane in which the voltage source/ switch and load is located.
If the battery was located at one of the far sides of the rectangular loop while the load at the other then the answer would be completely different and in fact then the bulb would receive it's full current at the time the first fields also reach it.
In a sense he made this demonstration somewhat similar to a card trick whereby the details of the setup determine the outcome not so much the physics behind it.

 

[hutchphd]

Let me say again:

His main point was that it is the fields that carry the power.

Dr Derek Muller presented his circuit as a clever entree into this fact. I think, self evidently, that he succeeded. His was not a proposal it for Tesla -like power distribution.

Would that there were more educators with his knowledge and skill.

.

 

[nsaspook]

I think we should all just realize that Derek aka Veritasium is a youtube presenter, irrespective of his background or education and understanding he stands to make money from his youtube persona. Youtube due to it's algorithms which are tailored to the the preferences of the majority of viewers is not the best place for real science because in order to succeed it demands one to use speculative and sensational titles and similar video presentation. It is no secret that the channels that lately earn the most from that platform and are among the most popular do just that.

This being said I still find many Derek's videos entertaining and interesting and his science backing is still some of the most on point although it seems he himself deviates from that once in a while and this lamp video just happened to be one of them.

From a physics perspective there is no miracle here, i think @Baluncore and others were right saying that due to transmission line theory the bulb will indeed receive some minor ever increasing current almost instantly while the real current necessary to light a real practical bulb will get there only when there is full load current established all around the length of the wires and this will happen at c along the wire in vacuum or less than c in other mediums.I think Derek was a bit tricky and deceptive on purpose making the wires long in one direction while making the separation short in the other direction which happens to be the direction/plane in which the voltage source/ switch and load is located.
If the battery was located at one of the far sides of the rectangular loop while the load at the other then the answer would be completely different and in fact then the bulb would receive it's full current at the time the first fields also reach it.
In a sense he made this demonstration somewhat similar to a card trick whereby the details of the setup determine the outcome not so much the physics behind it.

Yes, his video was a bit tricky and deceptive on purpose. For some reason that reminds me of many exam questions and life in general. That's what makes it a delicious generator of Youtube comments of all types. Those that understand the trick say, so what, old news but those that don't just might take the time to investigate the science beyond the level of a Youtube video in an effort to disprove Derek or at least have an open mind to the possibility he's right by looking at other videos of a similar nature like this from years ago.

 

[cjl]

AlphaPhoenix on youtube did an excellent practical experiment with an oscilloscope to show how this behaves in the real world:

 

[artis]

I think I just remembered a nice analogy for anyone having a hard time visualizing how the EM field can move across a wire countless times faster than actual current itself. This is also something which Derek tried to explain in his original video and which the youtuber "AlphaPhoenix" tried in his video above my post.

I remember seeing how large freight trains start up from a full stop, in the railcar couplers there is some slack, some free movement. There are times when upon start the locomotive pulls on the first attached car and if there is some slack there then the abrupt pull travels down each rail car down the whole train creating a traveling sound/wave effect. The actual cars themselves move very slowly if at all much like real electrons but the slack pull force and sound travels down the whole train at a very fast rate , reaching the end of he train in a matter of seconds.
See the video below,

 

[Tommie361]

None of the above for several reasons, as the speed of electrons swinging in a metal (prima facie assumed setting according to diagram with switch & bulb) as real material is much less than idealized c;
secondly, if light, as in in lightwave guides (e.g. glassfibers) is meant, then correct polarization could cancel out all light arriving at 'bulb'. If LIGHT goes straight line to the bulb, then the (TWO) lightwaves should run in quite in parallel and very adjacent (LASER?) OR bent à la Einstein by a huge mass, then the distance would be longer (this idea should be checked, as not sure). A sub-question is how the premise 'either way, left or right is '1/2 lightyear long' has been established to begin with. Hence , my first guess is a) >>> 1/2 year (longer) or b) NOT at all. Up to here it was theoretical, if a real, material setting is contemplated, then my answer is 'highly probable not measurable' at the end of line for many disturbing effects (magnetic, eddy currents, heat , heat -> R, more I >> more R >> melting of metals (or we consider super-conducts) to begin with. Anyway, much longer than 1/2 year (my 2 cents as a physics layman)

 

[Drakkith]

Up to here it was theoretical, if a real, material setting is contemplated, then my answer is 'highly probable not measurable' at the end of line for many disturbing effects (magnetic, eddy currents, heat , heat -> R, more I >> more R >> melting of metals (or we consider super-conducts) to begin with.

It is easily measurable even on a much smaller scale. See post #111.

 

[Nugatory]

None of the above for several reasons, as the speed of electrons swinging in a metal (prima facie assumed setting according to diagram with switch & bulb) as real material is much less than idealized c;

What matters is not the speed of the electrons, but the speed at which the change in the electric field propagates to the next electron when one electron is slightly displaced. This speed is much closer to $c$ than to (absurdly small) drift velocity of the electrons. As well as post #111 referenced by @Drakkith, the train video posted by @artis in #127 helps make this more intuitive.

 

[DaveE]

Derek released a really good update video. He's even more right this time.

BTW, you're a real EE if your heart briefly stops at 18:28 when they lift a really nice scope up to the top of the ladder. I'd have a top rope and someone on belay if that was mine, LOL.

 

[cjl]

Eh. I'd say it's better than his first video, but he's still doing a lot of troubling doubling down on some things that, while technically not incorrect, are certainly at the very least heavily misleading.

This whole thing has really left me with a substantially reduced opinion of Veritasium, to be honest.

 

[hutchphd]

are certainly at the very least heavily misleading.

Specifics might be better than opinions...

 

[nsaspook]

Eh. I'd say it's better than his first video, but he's still doing a lot of troubling doubling down on some things that, while technically not incorrect, are certainly at the very least heavily misleading.

This whole thing has really left me with a substantially reduced opinion of Veritasium, to be honest.

Exactly what is misleading?

 

[DaveE]

Eh. I'd say it's better than his first video, but he's still doing a lot of troubling doubling down on some things that, while technically not incorrect, are certainly at the very least heavily misleading.

This whole thing has really left me with a substantially reduced opinion of Veritasium, to be honest.

I'd guess a perfect physics lecture hasn't ever happened. Maybe you could point us to a better one, or a nice treatment of the things you didn't like?

BTW, if you refer me to the Jackson Classical Electrodynamics text, you've missed the point.

 

[DaveE]

BTW, their load doesn't match the line like they calculated or they would have gotten a nice 50% step followed by 100% later without more reflections. Every EE that ever played with coax and a step generator has seen that. Much easier matching than copper pipes 12' above the olive walk. No ladders required with shielded cables.

 

[artis]

Just watched the Veritasium update video , I think it is good. I think the problem with his original video was focusing too much on the "hype" about how cool is it that you can get "action at a distance" in a way that defies common sense most have about circuits. Instead he should have done what he did now, keeping the fields explanation but then showing and applying how those fields interact within a transmission line due to capacitance and inductance between the wires.
Given his case is special as the two wires are identical and small distance apart all the way, as the contact closes the changes in fields travel along the wires charging up each successive capacitor along the way, the more "capacitors" get charged while the field expands the more current flows within his load.
And he is correct , even with such small capacitance as the first "capacitors" charge up there is enough current to light up a small LED.

Maybe to illustrate this even better he could make a similar circuit like he made but with wires that are made out of planar sheets of copper in an arrangement such that the flat planes of the wires all face inwards thereby the mutual capacitance between the long sections of the wires is greatly increased. I suspect with such higher capacitance the change in the E field due to switch action going outwards would greatly increase the current observed at the load.

 

[DaveE]

I suspect with such higher capacitance the change in the E field due to switch action going outwards would greatly increase the current observed at the load.

Yes, because the characteristic impedance of the transmission line ($Z_o = \sqrt{ \frac{L}{C} }$) would be lower. But then he would have changed his load impedance when he matched the line accordingly. In a matched line you'll get a 50% voltage step immediately. With lower impedance you would have more current everywhere. If he didn't choose to match the line, then it's more complicated with reflections and such. But his basic point remains the same - stuff happens right away.

Normalization is a useful concept in getting at the basic concepts here.

 

[vanhees71]

Just watched the Veritasium update video , I think it is good. I think the problem with his original video was focusing too much on the "hype" about how cool is it that you can get "action at a distance" in a way that defies common sense most have about circuits. Instead he should have done what he did now, keeping the fields explanation but then showing and applying how those fields interact within a transmission line due to capacitance and inductance between the wires.
Given his case is special as the two wires are identical and small distance apart all the way, as the contact closes the changes in fields travel along the wires charging up each successive capacitor along the way, the more "capacitors" get charged while the field expands the more current flows within his load.
And he is correct , even with such small capacitance as the first "capacitors" charge up there is enough current to light up a small LED.

Maybe to illustrate this even better he could make a similar circuit like he made but with wires that are made out of planar sheets of copper in an arrangement such that the flat planes of the wires all face inwards thereby the mutual capacitance between the long sections of the wires is greatly increased. I suspect with such higher capacitance the change in the E field due to switch action going outwards would greatly increase the current observed at the load.

Indeed, one should show this movie to all the "physics pedagogues" still advocating a kind of "water-pressure model" of electric circuits, which is completely wrong in many ways, particularly in the discussed issue with how energy is transported from the source ("battery"). It's not along the wire and not via the electron fluid but via the field, as nicely demonstrated by the simulations shown in the movie and also verified by measuring the signal of the transient state.

 

[vanhees71]

Yes, because the characteristic impedance of the transmission line ($Z_o = \sqrt{ \frac{L}{C} }$) would be lower. But then he would have changed his load impedance when he matched the line accordingly. In a matched line you'll get a 50% voltage step immediately. With lower impedance you would have more current everywhere. If he didn't choose to match the line, then it's more complicated with reflections and such. But his basic point remains the same - stuff happens right away.

Normalization is a useful concept in getting at the basic concepts here.

It happens "right away", if understood in the correct relativistic sense. Everything related to electromagnetism is correctly described only in a relativistic way (although quite often non-relativistic approximations can be well justified).

 

[tech99]

It's not along the wire and not via the electron fluid but via the field, as nicely demonstrated by the simulations shown in the movie and also verified by measuring the signal of the transient state.

But would you agree that the electrons have a longitudinal motion, oscillatory for AC, in response to the fields?

 

[vanhees71]

Sure, they make up the current.

 

[bob012345]

Indeed, one should show this movie to all the "physics pedagogues" still advocating a kind of "water-pressure model" of electric circuits, which is completely wrong in many ways, particularly in the discussed issue with how energy is transported from the source ("battery"). It's not along the wire and not via the electron fluid but via the field, as nicely demonstrated by the simulations shown in the movie and also verified by measuring the signal of the transient state.

I expect Veritasium to make a video saying the battery is not the source next. :)

 

[tech99]

I expect Veritasium to make a video saying the battery is not the source next. :)

The switch is the source of the wave in a circuit.

 

[anorlunda]

The switch is the source of the wave in a circuit.

Some people will quibble over the choice of words. The switch triggers the wave, but it is not the source of power. Closing a switch in a wire with no power source, produces nothing.

 

[tech99]

Some people will quibble over the choice of words. The switch triggers the wave, but it is not the source of power. Closing a switch in a wire with no power source, produces nothing.

Yes, battery at the exchange, dial at the subscriber premises.

 

[tech99]

Sure, they make up the current.

So can we say that the electrons execute a longitudinal wave? And can we also say that they do this in response to a longitudinal, rather than transverse, electric wave?

 

[berkeman]

Closing a switch in a wire with no power source, produces nothing.

Tree, forest, no people, ...

 

[Baluncore]

By changing the voltage sources, or arrangement of the conductors, resistors and insulators, we reconfigure the electric field. Those electric fields are guided by the wires and components of our circuits.

In response to the changing electric field, a current flow is required to redistribute the charge of the circuit capacitance. That current generates a magnetic field, guided by the currents in the wires.

Changes to the electric and magnetic fields constitute a redistribution of energy in the fields. The direction of that energy flow is the (cross) product of the electric and magnetic fields, we call it the Poynting vector.

Those EM fields fill the space between the circuit elements, where changes propagate at a speed close to the speed of light. It follows that electrical energy travels to the load through the space surrounding the circuit elements, guided by, but not through, the conductive wires.

 

[hutchphd]

It follows that electrical energy travels to the load through the space surrounding the circuit elements, guided by, but not through, the conductive wires.

That is the well stated answer as reiterated several different times recently I thought we had all figured this out.

So can we say that the electrons execute a longitudinal wave? And can we also say that they do this in response to a longitudinal, rather than transverse, electric wave?

Please let's not play twenty questions. If you have a point, please make it completely.