General > General Technical Chat
"Veritasium" (YT) - "The Big Misconception About Electricity" ?
adx:
--- Quote from: bsfeechannel on January 16, 2022, 04:26:49 am ---
--- Quote ---Conclusion: the statement "energy flows only in wires" is correct.
--- End quote ---
What you're doing is the same thing you're accusing Derek of having done. If the energy flows ONLY in the wires, the other theories are wrong.
--- End quote ---
I used to play chess. Wasn't awfully good at it, but good enough to see what just happened there.
adx:
Got it.
The Veritasium circuit (the one with the LED filament lamp, the plain DC or 50 / 60Hz one) is a transmission line. It gets 'loaded up' with a DC pulse at the rate of V^2/Z per light-second of line. All of this energy passes through all the Ls and Cs at the speed of light. Therefore all of the energy delivered to an impedance matched load travels via the fields.
How I came up with this insight: I was reading Sredni's post[1] properly with a view to a quick reply. But the conundrum I've addressed before came up: While a relatively small amount of energy is stored in the L and C of the wire, the bulk of the energy seems to be sucked by the resistor which creates (wrong word) a voltage drop at the end of the line (establishing a (longitudinal) pressure gradient all down the circuit, and same current, so it 'enjoys' the largest pressure drop but mostly nothing to do with the mechanical topology of the circuit). It seemed implausible that such a small amount of energy stored in the fields could be totally responsible for the power flow. Then I saw my mistake: The energy flows at the speed of light in electrical circuits, I was thinking more of charge carriers (in my defence also correct as power = force * velocity). Interesting but hardly a new revelation. Until I was having a chomp on some cheese a little later, and thought about my earlier post[2] about transmission lines and how they are charged with a moving but DC wave (and I meant perfectly DC, not the intentionally mixed metaphor above). I thought that seemed usefully convincing. It also helps place this "energy is in the fields" concept more firmly in the context of waves than mechanical flow. I assume it would be the same for a hydraulic circuit - if interested in how much energy is carried by an acoustic wavefront (pulse), you're not going to use the material flow rate in the calculation. Waves are strange.
Note I'm not trying to say or prove energy is actually in the fields. It is just a piece of the conceptual puzzle my brain needed to accept the Poynting vector is not a complete physical nonsense. If the kernel of my insight isn't clear enough (it's bordering on fading away already), it is that a transmission line is filled with energy, that is the energy's only purpose from when it goes in to when it comes out. Even if DC.
Also better say I use the terms "energy" and "speed of light" somewhat metaphorically.
Links more conveniently out of the way:
1. https://www.eevblog.com/forum/chat/veritasium-(yt)-the-big-misconception-about-electricity/msg3935938/#msg3935938
2. https://www.eevblog.com/forum/chat/veritasium-(yt)-the-big-misconception-about-electricity/msg3895310/#msg3895310
Naej:
--- Quote from: HuronKing on January 16, 2022, 02:57:43 am ---
--- Quote from: Naej on January 16, 2022, 01:49:34 am ---A charge here creates scalar (Lorenz) potential V everywhere.
A moving charge here creates vector (Lorenz) potential A everywhere.
Potentials propagate at the speed of light, and this gives the answer: when you close the switch an EM disturbance is created, it propagates to the light and "switch it on".
(It's all fields until you remove them. See Liénard-Wiechert potential, for example in an Atoms & Sporks video or in Wiki)
--- End quote ---
Maybe I'm an idiot but I really don't see how this interpretation removes any of the fields. And I looked at the Liénard-Wiechert potential - it's defined in terms of vector fields. We're still talking about the propagations of fields, through empty space.
--- End quote ---
Depends what you mean by fields, when people say "energy is in the fields" they mean in fact "energy is in vacuum", and with Carpenter's interpretation, V,A are mathematical fields but vacuum plays no role at all, it's just the place where they propagate.
Liénard-Wiechert showed that you could completely remove them. In Wiki the formulas give the potentials produced, because everyone thinks in terms of potentials/E,B, but you can just compute the forces exerted on particles by each other in this way and stop here.
In this way, you get the dynamic version of Coulomb force (which is equivalent to Maxwell's equation), and where vacuum plays absolutely no role (except retarding forces), there is no need to define a potential in vacuum.
If you take this to be "the truth" then the concept of light is this: accelerated charges create forces on charges which slowly decrease with distance.
--- Quote from: HuronKing on January 16, 2022, 03:38:34 am ---
--- Quote from: SiliconWizard on January 16, 2022, 03:24:47 am ---The Casimir effect is a pretty peculiar beast. =)
But one thing to consider is that vacuum is not void.
--- End quote ---
Indeed. And I don't mean to be unfair by bringing it up. The Casimir Effect is not something predicted by Classical ED even if it has thematic similarities to aether ideas from Maxwell. I guess it just shows they had a shadow of vision of the future.
What I'm driving at is that in neither Classical ED, nor Einsteinian QM, nor in modern QFT is the vacuum considered to be a void that can't have energy in it. Thus, I don't really respond to philosophical arguments that suppose the vacuum can't have energy going through it 'just because' - whether it's energy-less photons or these "signals" (as Atom & Sporks says in the video recommended video by Naej) from one charge to another that are very totally not energy propagating in a field, just, 'remote action effects.' :-[
--- End quote ---
The concepts have to change with the theory you use, and the theory with what you're doing.
If you want the current conception of vacuum, then it's a (local?) minimum of energy, whose energy (named dark energy) is driving an exponential-like acceleration of the size of the universe. And Casimir effect is because you can go below the vacuum in energy density.
If it's only a local minimum then it can decay, and it's one popular theory of what happened at the initiation of the Big Bang.
bsfeechannel: so you reject a theorem with 19th century philosophy. Ok. |O
Also I said many times that both Poynting's and Carpenter's view are correct, so if you don't see a difference with what Derek said, then you don't see much.
adx: if you follow Poynting then copper wires are the low-frequency equivalent of light fiber, a transformer is impedance matching, and a resistor is a low-frequency black-body.
For acoustic waves, half the energy is in the pressure, and half in the velocity (much like in light, half is in E, half is in B).
rfeecs:
--- Quote from: HuronKing on January 16, 2022, 02:57:43 am ---Yes, I'm aware of Feynman's explanation of radiation pressure - that's why I question the statements Naej is making in their interpretation of Carpenter. After all, Feynman remarks a moment later,
--- Quote ---Therefore the force, the “pushing momentum,” that is delivered per second by the light, is equal to 1/c times the energy absorbed from the light per second! That is a general rule, since we did not say how strong the oscillator was, or whether some of the charges cancel out. In any circumstance where light is being absorbed, there is a pressure. The momentum that the light delivers is always equal to the energy that is absorbed, divided by c:
⟨F⟩=dW/dtc.(34.24)
That light carries energy we already know. We now understand that it also carries momentum, and further, that the momentum carried is always 1/c times the energy.
--- End quote ---
--- End quote ---
Carpenter denies there is any experimental evidence that light carries momentum:
--- Quote ---The most obvious is momentum. The idea of electromagnetic
radiation carrying with it a momentum, and
hence exerting a force on any absorbing surface, epitomises
the properties which are customarily taken as
direct experimental evidence of the existence of the field.
But, as is well recognised [4-9], these properties are
unsupported by any evidence which is independent of the
way in which they are defined.
--- End quote ---
I'm not buying it. I suppose he means that the way he defines things, it is charges acting on each other at a distance rather than fields acting on particles.
This would require reformulating lots of physics since Maxwell.
ogden:
--- Quote from: SiliconWizard on January 15, 2022, 10:13:54 pm ---That is because " Engineers are essentially dumbed-down "physicists". " :popcorn:
--- End quote ---
LOL :) Quite accurate. Engineers are taught to be effective - to *not* dig into deep details of underlying physics and calculations as long as simplified model gives results within target tolerance.
TLDR, sorry. I wonder - it was mentioned or not that such kind of transmission lines (twin-lead) are lossy? They radiate energy away as EM waves disregarding zero conduction losses and being in vacuum. It means that lamp may go off after initially lit, to wait for DC conduction to kick-in.
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