Yeah. I find it really odd that Derek felt the need to dispel a simplifying “lie” that I had never even heard of, and yet totally ignore the really useful simplifying analogue that I was under the impression almost everyone started with.
The usual misconception that needs dispelling is that electrons move at c in the conductor. He doesn’t even start to address this. Indeed, even if you knew about the drift velocity of electrons, his video seems to be implying that that is wrong as well.
I rather feel he didn’t have a good model in his head about what his target audience was, and what he could assume they knew, and he had something of a guilt complex about something he used to mis-teach, and wanted to correct it ^{\dagger}. But all he ended up doing was creating a mess.
{\dagger} Does anyone here remember being taught that electrons carry a potential, and that flow is what carries energy? Maybe there is a pedagogy that tries this to explain electricity simply, but I have never come across it. How you explain even something as simple as Ohm’s Law is difficult to imagine.
I think once one hits on this, it’s clear that what he’s describing just plain isn’t what most people mean by an ‘electrical circuit’, because the reason that the lamp lights has nothing to do with the whole assembly being a ‘circuit’—nothing ‘goes around’. I think this should’ve at least been mentioned—and when I watched the video, I was sure he’d eventually get around to doing so, it seemed like he’d laid the foundation with mentioning that most circuits, e.g. those containing a transformer, aren’t actually closed loops. But as it is, I think the viewer’s left with a mistaken impression.
If we replace the DC source with a (sufficiently powerful) AC source, I’d expect the light would stay on even with the breaks in the far ends. The wires, assuming they are laid out literally as the video shows, form a huge-ass capacitor, which should block the DC but let the AC pass through.
Well, I’ve been taught that as a misconception. And I’ve seen that as a common misconception as well. Well, not that electrons carry “potential”, but that they are little bundles of energy which are delivered from source to drain. The electronic equivalent of photons, I suppose.
However, it’s also not a misconception that’s held beyond the first day in any class on the subject. And even those that hold it know that there’s something wrong with it, since circuits only function when they form, well, a circuit. The hydraulic analogy does a good job in showing that the electrons are necessarily to deliver energy without actually carrying energy themselves.
I think the biggest problem with this whole debacle is that the critique of the video is coming from people whose knowledge of the subject is far beyond that of the intended audience.
Errors notwithstanding, it seems to me the intent of the video is not so much to comprehensively (or even correctly) teach a thing, but rather to simply shatter a common layman’s misconception.
I think veritasiums video is straight-up wrong on many levels. Note, I am am experimental physicist that studies atmospheric electricity. I deal with these thing on a daily basis.
The above posters touch on the major issues.
Applying pointing vector in electrostatics is so egregiously ridiculous, I don’t even know how to respond.
In short, sure, maybe a tiny portion of energy will be transmitted via radio waves. But this amount will be so tiny (and only apply to an extremely short period of time after the switch is close) that it can (and should be) ignored.
The vast majority of energy is carried by the current. Is it carried in the electrons like people carrying buckets of water? No, this is silly. Carried by current, not by individual electrons. It’s like waves in the ocean, the water waves move and carry energy, but the individual water particles don’t really move (except up and down).
I do this exact type of experiment (on a smaller scale) regularly. It is very important in my field to measure time-delay due to cable lengths, and the time-delay is ALWAYS the wave-speed in the cable (smaller than C) times by length of the cable, and has nothing to do with how close the pulse-emitter and measurement take place (as generally these two sit in the same spot). (You measure time delay because it is easy, and maybe the wave-speed or length isn’t known accurately enough).
I don’t like veritasium. This isn’t the first thing he has got wrong, but it is the most egregious I’ve seen so far. I really hope ElectoBoom puts up a video on this soon.
Well of course you can’t apply the Poynting vector in electrostatics (or at least, can’t make any meaningful use of it). But a current-carrying wire is not static.
And I hadn’t noticed that this was a Veritasium video. I will say that, in general, there’s an extremely low signal-to-nonsense ratio on that channel.
The video takes a weird middle ground between a layman and expert explanation. It contains too many errors and hidden assumptions for an expert audience. But anything containing Poynting vectors is not appropriate for a layman. It fails even at a “lies for children” level, for which the hydraulic analogy is generally quite good.
If one is extremely charitable about everything he said, then you could say that he isn’t completely wrong in his analysis. But he presents it as the way current is carried and not one interpretation of how a small amount of energy is transferred via the EM field in one particular transient situation. In my book, that’s even more misleading than the conventional approach.
I thought the video from RSD that leahcim linked to was good. It makes clear that even considering the EM approach, there are at least three different ways of looking at the energy transfer (capacitive coupling, inductive coupling, and as an antenna).
You’re, of course, technically correct. However, many physicists, including myself, tend to treat DC current to be part of electrostatics since DC current doesn’t emit EM waves. i.e. many of the ideas and techniques that apply to electrostatics tend to still work with minor adjustment to DC current.
I mean, cynical hat on for a moment here. These videos are intended to be a open (and in parts, staged) debate between YouTubers, with one of the motives being to promote the discussion of scientific topics, but with another of the motives being to garner viewer engagement. As such, they have to start off with some people being partly or wholly wrong, or there is no debate.
This is the new collab mode, and I think we’ll see more of it.
As a layman when it comes to the electrical engineering or physicist understanding of electricity, this video just left me confused. I’m not sure what the final point was.
I get that electricity doesn’t really work in a simple model of individual electrons making their way down a wire, like a chain in a pipe. That’s fine, nothing is ever simple, and analogy is analogy, not reality. But what does waving your fingers around have to do with a light switch?
Capacitors have gaps in them, and motors spin because of magnetic fields, not electrons pushing on a water wheel. I get all that, too. But if I have a circuit and I use a switch to break the circuit, the light turns off. If I use the switch to complete the circuit, the light turns on. Bigger gaps?
I’m not saying that I think anything he said is wrong (I really don’t have the knowledge to comment on that), rather that he left me without the information to understand what his point was in the first place.
Finally, I thought he was saying that the distance between the light and the power source will determine how quickly the light turns on, not the length of the wire. Was that what he was saying?
There’s a small amount of energy transfer that goes straight from the power source to the bulb at the speed of light. There are several equivalent ways of describing this, but regardless, it’s an infinitesimal amount (not enough to actually light the bulb), and transient in nature. The same transfer would happen even if you cut the wire at the endpoints (i.e., there is no circuit).
I’d say one of the flaws in the video is taking this transient behavior and assuming it applies in the steady-state. Also that his particular thought experiment is generally applicable. It may well be that in a setup like that, using AC, almost all the transfer is radiative. I’d have to model it to be sure. But that doesn’t mean it applies to circuits of more normal dimensions.
Based on experience I can guarantee extremely little energy is transmitted via EM waves in any practical AC circuit. The physical arrangement of the circuit of course will affect the amount of energy significantly, and the setup he describes (parallel wires close together) is close to optimal I think.
Think about it this way. We recently have wireless chargers, I.E. setups DESIGNED to transmit power wirelessly. They only function efficiently because they are very carefully designed, and the transmitter and receiver sit on top of each other so they act more like a transformer than really an antenna (i.e. I suspect that near-field components dominate over far-field, which means this isn’t transmitting power in the same mode a radio or microwave oven transmits power).
I assumed we could reasonably model the system as a pair of transmission lines. All the worry about capacitive and inductive coupling gets folded into this, and does so in a quantitative manner. It properly models the EM field behaviour in a lumped manner. That gets you an impedance of about 800Ω. Since the lines are in series, that gets you 1600Ω. For a transient turn on, and for the time it takes the transient to settle, that seems a reasonable approach. There is no possibility the bulb will light.
The transient will be overtaken and overwhelmed by the DC flow running around the wires, so there is never any meaningful periodic behaviour to worry about. The impedance of the transmission line is replaced by only the internal resistance of the battery in the experiment. From power too feeble to light the bulb, we transition to enough power to start a car.
A simple capacitor is just a couple of hunks of metal with a gap between them, and a switch, when turned off, is just a couple of hunks of metal with a gap between them. A switch is a very small capacitor, because the capacitor “plates” are small and far apart. It’s small enough that you can basically ignore its capacitance, but when you get down to it, there’s capacitance (along with resistance and inductance) even in the wires around your house.
When you turn off a switch, the switch contacts will actually draw an arc. The nice thing about AC is that the voltage drops to zero twice during the AC sine wave cycle, and when it drops to zero, the arc tends to naturally extinguish. Switches get a lot more complicated when you are switching high DC voltages and currents. High voltage DC will draw an arc a very long distance when you try to turn the switch off. Switches can be more complicated than you think.
The natural capacitance and inductance in wires also gets to be an issue when you start talking about power transmission lines. When you need to move a lot of power a long distance, a lot of it is done using high voltage DC transmission lines, because DC doesn’t have all of the capacitive and inductive losses. But then the switches and transformers on either end of the line get much more complicated and expensive, which is why we don’t use DC for shorter distances and lower power levels.
Your body has capacitance in it as well. Touch lamps work by sensing this capacitance.
When you design electronics and electrical stuff, you need to constantly consider that natural inductance and capacitance is everywhere.
That seems to make much more sense. The original video left me with the idea that wires aren’t really necessary.
Let me see if I understand this:
Electrons can jiggle a bit on a wire
the direction of this jiggling (DC) works one way
and causes electrons to push on their neighbors.
The pushing is the electromagnetic force
which travels in photons
that move at the speed of light.
The pushing is greater the closer the neighboring electrons
which propagate the pushing down the wire to their closer neighbors.
Some pushing is done on far away electrons, but not much
Because the switch and lightbulb are physically close, the light speed delay for the small push on the electrons near the lightbulb is less than the big push from the electrons that go around the wire
The small push doesn’t travel around the wire? (is that induction?)
What if instead of next to each other on the table, the switch was on one end of his loop, and the lightbulb was 500m away at the other. Would the small push and big push then arrive at essentially the same time?
Is another analogy to this something like, if I’m playing pool Newton tells me all I need to know. If I’m building a GPS satellite network, I’d better take Einstein into account. If I’m wiring a house, Ohm’s law is good enough, but for other electrical applications, I’d better take all this stuff into account? What sort of applications?
