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"Veritasium" (YT) - "The Big Misconception About Electricity" ?

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adx:
Wo - after a moment of clarity I was ready to call BS on the "energy doesn't travel in wires" claim a couple of pages back, but after getting year end tasks done I come back and find the thread has done it for me!

I wrote down what I could and never fleshed it out so may be a bit garbled now, but basically:

* power is one part force and one part movement
* it's a completely static magnetic field, nothing moves (it is an effect of moving charges)
* the (non) argument that "it's all fields" or "all energy anyway" only works to confirm the possibility of energy in the wires (the wires are energy)
* electric field is what causes the electrons in the wire to move, the magnetic field around the wire is a result of the electrons moving in the wire, nowhere else(The idea I had in mind was more convincing than that.)

But then I realised that it's not up to me to prove where power flows. I only need ask for proof that power doesn't flow in the wires, because of the bold certainty with which the claim is made. It becomes a kind of theological argument. Especially as SandyCox said "Poynting’s theorem and charge flow lead to exactly the same answer". Is the expectation of a difference an illusion?

But this thread has confused me again. I had sort of settled on the opinion (some pages back) that DC power is a product of moving charge carriers and pressure difference (not field, but work function in a conservative field which boils down to a potential difference not gradient), inside and outside of the wires respectively. But Hontas Farmer's diagram of the electrons in the wire made it seem even simpler than that, and I'm now confused over the need for the pressure difference outside of the wires (something I had thought of before). Imagine a system where space outside the wires (or hydraulic pipes) doesn't exist, or at least couldn't hold anything. The pressure difference still drives down the pipes, through the resistor where it does its work, and returns via a parallel path. It divides along the path. But the concept of "power flow" is totally dependent on axial distance, and trying to locate it in the transverse direction requires setting up a field with gradient - that need not exist, but must if space exists. (Which is basically why I called the latter into doubt.)

I'll have to do some more thinking.

bdunham7:

--- Quote from: HendriXML on January 01, 2022, 04:42:12 pm --- To me it seems that when using wires the movement of electric charge alone could do work and transfer energy.

--- End quote ---

For an electric charge to do work, it has to be in an electric field--IOW it does it's work by moving from one potential to another.  So the arrival of an electron at the load alone doesn't accomplish anything, there also needs to be an electric field across the load in order for it to do work.  In a simple DC circuit, both the charges themselves and the electric field that propels them through the load are provided by the movement of charges through the conductor and the resultant charge distribution.  My Millikan tongs example separates the two functions.  Or does it?

SandyCox:

--- Quote from: EEVblog on January 01, 2022, 01:04:53 pm ---I am not talking about the measurements, they will be as they always have been. I'm talking about the the title of this thread "The Big Misconception About Electricity". Does the energy flow in the field around the wire or does it flow in the wire at DC? Poynting/classical field theory says outside, QFT appears to say inside.
I want to know what you and others who have been so (not incorrectly) dogged about anyone that dares think of this in any other way than Maxwell/Poynting think about this apparent conundrum.

--- End quote ---
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."

Fig. 11.3.1 is a nice illustration of this misinterpretation.

I'm still writing a more detailed explanation of what they refer to. Veritasium is wrong about the steady-state transfer of energy.

Another example of this misinterpretation is the direction of the Poynting vector in the airgap of a transformer.

bdunham7:

--- Quote from: SandyCox on January 01, 2022, 05:50:47 pm ---Fig. 11.3.1 is a nice illustration of this misinterpretation.

--- End quote ---

Can you post the illustration for those of us that don't have the book?

rfeecs:

--- 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?

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