General > General Technical Chat
"Veritasium" (YT) - "The Big Misconception About Electricity" ?
SandyCox:
--- Quote from: Sredni on January 03, 2022, 11:01:33 am ---
--- Quote from: SandyCox on January 03, 2022, 10:54:41 am ---Ascribing meaning to the Poynting vector at a point leads us to the wrong conclusion, as shown by Fig. 11.3.1.
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I didn't see Haus and Melcher recant what they wrote
"Even with the fields perfectly stationary in time, the power is seen to flow through the open space to be absorbed in the volume where the dissipation takes place."
Did you?
--- Quote ---I’m not quite sure what you a trying to say by “glowing red hot”. Are you saying that energy is now transferred through thermal radiation?
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No, I'm saying that by making rod and washer of very different materials you can have one glow red hot while the other stays cool, and viceversa. There still will be Poynting field lines in the space inside the can and they will account for the difference between the total power delivered by the battery and the power absorbed by the rod.
In one case you will see a lot of lines coming out of a cool washer to impinge into a red hot rod.
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On the next page they say:
" we illustrate the danger of ascribing meaning to S evaluated at a point, rather than integrated over a closed surface."
I'm not sure why you are dragging thermal issues into the argument. We can also make your resistors glow, but why would we? Add a cooling system if you are worried about conductors glowing red.
The point is that there is no power being transferred from the washer to the rod. All the power is accounted for. Misinterpreting the meaning of the Poynting vector leads us to incorrectly believe that power is flowing from the washer to the rod. Please do the calculations. You have all the information you require.
adx:
--- Quote from: bdunham7 on January 01, 2022, 05:00:55 am ---... 'Millikan tongs' ... So in this special case, can we agree that the energy is 'flowing' in the succession of 1eV-potential electrons being carried by the tongs? It's hard for me to see it any other way.
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Hard to argue with that. If you pick up a charged battery, with separated + and - ions in it, and carry them in the same direction (the conventional approach to battery transport), you are transmitting power (briefly), and inducing (or via) an EM field. It's not steady state, but is sort of DC (not in the Fourier sense, but the non-accelerating A to B sense once launched), and there's just no easy way I can wrap my head around the potential fact that this is a radio transmission, despite there being moving E and H fields.
adx:
--- Quote from: Sredni on January 03, 2022, 02:49:18 am ---
--- Quote from: adx on January 03, 2022, 02:36:31 am ---
--- Quote from: Sredni on January 03, 2022, 02:12:48 am ---Enclose each resistor in a gauss surface and compute the net flux of S through that closed surface to find the power absorbed and you will see that each resistor is getting the power corresponding to V times I. ...
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Do the same procedure on some space inside the loops but not where wires or resistors are. Then tell me how the result in any way supports the concept that "power doesn't flow in the wires"?
(Yes I'm aware of what will happen if you include some wire. That's not the question.)
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I guess we can get to the conclusion that vacuum does not absorb energy?
(I wonder what a QFT theorist would say about that...)
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Then the wires become the question. The vacuum can have E-field without charge, the conductor can have charge without E-field. If neither can absorb energy, what gives one precedence over the other to be able to transmit it? Is it because a magnetic field can't exist in the conductor? What if it is a non-ideal conductor, which permits magnetic field everywhere at DC while absorbing a tiny amount of energy similar to what a 'semi-conductive vacuum' could?
(Not saying I know the answers, just pushing for a line of reasoning which might help untangle this.)
adx:
--- Quote from: HuronKing on January 03, 2022, 08:12:06 am ---
--- Quote from: adx on January 02, 2022, 07:22:32 am ---
--- Quote from: Sredni on January 01, 2022, 09:09:39 am ---...
Or, if you want to fly a bit lower, Kraus
John D. Kraus
Electromagnetics 2e
section 10.20 Circuit Applications of the Poynting Vector
p. 416
on p. 418, after considering a circuit with a battery (DC) and a resistors he writes:
--- Quote ---"In Fig. 10-19aflow lines of the Poynting vector (power flow lines) are shown. It is evident that the power flow is through the empty space surrounding the circuit, the conductors of the circuit acting as guiding elements. From the circuit point of view we usually think of the power as flowing through the wires but this is an oversimplification and does not represent the actual situation."
--- End quote ---
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This (italics mine). Is an example of an academic sermonising scientific hypothesis as fact. It may seem harmless, but results in generations(s) of disciples believing stuff.
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I know this was already a page ago, but can we pause for a moment to appreciate that John D. Kraus, the inventor of the helical antenna and corner reflector array, is being called a sermonizing academic?
https://en.wikipedia.org/wiki/John_D._Kraus
I mean, you can call him wrong, and you'd be wrong to call him wrong, but Kraus was hardly an ivory tower academic who didn't build anything practical... he used his exquisite knowledge of Maxwell's equations and Poynting theory to create entirely new types of antennas and waveguides - and all without numerical EM-simulators that we take for granted.
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I Wikipediad him before making that comment, just to be sure I wasn't saying anything silly. I had a lecturer who may have idolised someone maybe him, he also had a friend (a different one I guess) who used to go up with a hammer and "beat the side lobes out of the antenna" such was his deep and instinctive understanding of radio and stuff (my words at the end there). It is extremely hard to refuse Poynting's vector in RF, unlike DC. But Kraus commented specifically about DC and it is profoundly unscientific in the context of Poynting theory as presented. I don't mean academic in a pejorative way, just the way that getting focused on one area of work tends to focus a person into one area of work. Add some boffinness, perhaps a bit of age and a lot of experience, and it tends to happen.
It's not lost on me that Veritasium basically committed the same faux pas. But he is intentionally asking to get called out on it, not blithely stating a fact in a textbook.
Right, better leave it there for the night and get to bed, or make a strudel with an apple I promised to use before it totally rots.
bdunham7:
--- Quote from: bsfeechannel on January 03, 2022, 08:02:52 am ---What she said is that an electron has a much higher probability of interacting with another electron a billionth of a meter apart than with one 1 m away. Just that.
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I don't think any of her comments were directed at Derek's specific question regarding time. It took me a few minutes to figure that out, as I didn't see the relevance of her points at first.
--- Quote ---Dave says it doesn't apply to DC. He's wrong. But the issue here is not technical, as Maxwell himself pondered on.
--- End quote ---
I haven't seen Dave claim that Poynting's Theorem is incorrect in any circumstance. If I've missed it, could you point me there?
--- Quote --- It's difficult to accept that an analogy that you held dear turns out in the end to be superficial and misleading.
--- End quote ---
Indeed. :)
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