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"Veritasium" (YT) - "The Big Misconception About Electricity" ?
SandyCox:
--- Quote from: rfeecs on January 01, 2022, 06:17:21 pm ---
--- Quote from: SandyCox on January 01, 2022, 05:50:47 pm ---
There’s nothing wrong with Maxwell’s equations. The problem is the misinterpretation of what the Poynting vector tells us. Here is what Haus and Melcher says in Section 11.3 of their book:
"we illustrate the danger of ascribing meaning to S evaluated at a point, rather than integrated over a closed surface."
--- End quote ---
They are using an alternative energy flux vector S, not the Poynting vector.
Starting with conservation of energy, looking at the energy in a volume of space, you get the change in that energy by integrating an energy flux over the surface of the volume. The form you choose for that flux may be arbitrary. If there are no sources or dissipation of energy in that volume, you may have zero flux on the surface or you may have equal flux going in as going out.
They chose an alternative definition of energy flux that only depends on current density. It still satisfies energy conservation. But it only works in certain cases.
Clearly it doesn't work for a propagating wave in free space.
The Poynting vector seems to work for all cases. So why use two different definitions of energy flux?
--- End quote ---
I attach the relevant pages from Haus and Melcher. They use the standard Poynting vector S=ExH in Example 11.3.1. They only introduce their alternative formulation after Example 11.3.1. Let's assume that the Poynting vector does indicate the path along which power is transferred. Then what is the mechanism that causes power to be transferred from the washer-shaped conductor to rod in this example?
The problem is that humans misinterpret the meaning of the Poynting vector. It has no meaning without taking the integral over the surface of an enclosed volume.
SandyCox:
--- Quote from: bdunham7 on January 02, 2022, 04:34:27 am ---
--- Quote from: SiliconWizard on January 02, 2022, 02:37:31 am ---It's only been observed for very small particles (notwithstanding the mentioned recent experiments, which are interesting, but for which I'm still prudent.) And as I said earlier, any theory that can't survive observation under certain conditions can't be claimed to be valid for those conditions. So, while many physicists believe that the same laws hold at any scale, the honest ones will tell you that they just don't know. That it appears plausible, but we have no proof. The others are believers.
--- End quote ---
OK, I see our point of departure is at the definition of 'failure' of a theory. So you are saying that if the theory is not feasibly falsifiable, we should remain skeptical. OK, so it is not currently possible to experimentally verify the de Broglie wavelength of a tuna swimming in the sea, so your position is that we should regard any theoretical statement of it as unproven. That's not an inherently unreasonable position, but keep in mind that it then also applies to common laws that we accept will apply universally, or at least over a broad set of conditions that we don't expect to be able to analyze experimentally. I don't see QM/QED as being any different than any other physical laws or models in that regard. It is verifiable until you simply reach experimental limits. It's not like string theory, which AFAIK has not made any experimentally falsifiable conclusions.
--- End quote ---
This is an interesting setup, but it isn’t DC. As soon as the magnetic starts moving, the magnetic flux changes with time, inducing a time-varying voltage across the coil.
adx:
--- Quote from: SandyCox on January 02, 2022, 09:45:36 am ---The problem is that humans misinterpret the meaning of the Poynting vector. It has no meaning without taking the integral over the surface of an enclosed volume.
--- End quote ---
This sounds like what I was getting at in the last section of this post:
https://www.eevblog.com/forum/chat/veritasium-(yt)-the-big-misconception-about-electricity/msg3855821/#msg3855821
(Incompletely integrating) the Poynting vector is kind of a self-fulfilling prophecy (always works), because magnetic field is defined everywhere current isn't, and electric field is defined everywhere electric potential isn't. And for both electric fields (point charges) and magnetic fields ('monopole' fields like 1A going through an infinitely long wire), effect does not reduce over space (in the sense that 1/r^2 and 1/r relations are kind of an illusion due to things appearing smaller when they get further away).
But I don't know about the washer and rod situation yet, the reference a bit too 'math dense' for me to get a grip on without more thought. Which is not something I can guarantee!
SilverSolder:
--- Quote from: bdunham7 on January 02, 2022, 04:38:42 am ---
--- Quote from: SilverSolder on January 02, 2022, 03:02:38 am ---Arguably, you are storing potential energy while moving the magnets closer? - and when they are standing still... that energy is still there!
--- End quote ---
If the magnets are aligned so as to attract, it takes negative energy to move them into position from any further position or as physics teachers would posit, from infinity.
--- End quote ---
Ah, but since energy cannot be created or destroyed, the universe must have put the "negative" energy into them in the first place - you are just taking it back out again! (and if you put the magnets back where they were, you would have to put that energy back!)
Fundamental law of the universe: "No matter which way you turn, your ass always points backwards!" :D
Sredni:
--- Quote from: SandyCox on January 02, 2022, 09:45:36 am ---There’s nothing wrong with Maxwell’s equations. The problem is the misinterpretation of what the Poynting vector tells us. Here is what Haus and Melcher says in Section 11.3 of their book:
"we illustrate the danger of ascribing meaning to S evaluated at a point, rather than integrated over a closed surface."
--- End quote ---
--- Quote ---I attach the relevant pages from Haus and Melcher.
--- End quote ---
The full text is also online:
http://web.mit.edu/6.013_book/www/chapter11/11.3.html
--- Quote ---They use the standard Poynting vector S=ExH in Example 11.3.1. [snip]
Let's assume that the Poynting vector does indicate the path along which power is transferred. Then what is the mechanism that causes power to be transferred from the washer-shaped conductor to rod in this example?
--- End quote ---
And what seems to be the problem?
The fact that you have lines going from a resistor to another resistor?
This is a kind of unusual geometry: we have a battery whose pole is directly connected to two resistors and then a perfect conductor shorting the other ends of these resistors.
Let's see if we can untangle the geometry and still see a problem. Consider this other example:
source: https://www2.oberlin.edu/physics/dstyer/CircuitSurveyor/help.html
--- Quote ---The problem is that humans misinterpret the meaning of the Poynting vector. It has no meaning without taking the integral over the surface of an enclosed volume.
--- End quote ---
Is the fact that the first resistor in the above figure is getting all the field lines coming from the battery what you find of concern?
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