General > General Technical Chat
"Veritasium" (YT) - "The Big Misconception About Electricity" ?
SandyCox:
--- Quote from: rfeecs on January 03, 2022, 05:44:29 pm ---
--- Quote from: SandyCox on January 03, 2022, 03:40:10 pm ---
So all the power entering the washer from the voltage source is dissipated in the washer and all the power entering the rod from the voltage source is disspiated in the rod.
There is no power being transferred in the region between the washer and the rod.
--- End quote ---
You cannot conclude that from your calculation. It could be that power is exiting the washer and flowing into space (into the fields) and the same amount of power is flowing into the rod from the space (the fields) around it.
There is no double counting of power. Both views are equally valid in terms of conservation of energy.
But the "alternative" approach is only valid for DC. Using the Poynting vector is valid for all cases.
--- End quote ---
Let's look at the problem without making use of the Poynting vector. We can simply calculate the integral of sigma E.E over the volume of each conductor. This gives us the total power dissipated (in the form of thermal energy) in each conductor. The power delivered by the voltage source to the rod is equal to the power dissipated in the rod. The same is true for the washer. So there is no net flow of power from the washer to the rod, as the Poynting vector seems to suggest.
There is nothing wrong with the Poynting vector or with Poynting’s theorem. The problem is that people misinterpret the meaning of the Poynting vector. It has no meaning unless it is integrated over the surface of a closed volume in space. This simple example makes this misinterpretation painfully clear.
adx:
I can't get my head around the washer and rod exercise without leading to a circular argument, at which point my brain shuts down and spits out the most recent result with an error flag set. The results are the same either way. I cannot see any point in considering the washer and rod simultaneously if I know they are independent. The power that's stored in any magnetic field may come out of it and go back in somehow, but it has no known effect. (Plus we have the benefit these days of knowing what makes the magnetic field, and being able to test the 'moral' reasonableness of any weird situations.)
So roll with that idea, and don't just consider the washer and rod separately, but split them into infinitesimally small threads, or in the case of that example, sectors. This alters the electric field, but has no effect on the potential difference that drives energy transfer (work function in a conservative field). Compute the Poynting vector of that and superpose. Which should be possible if the energies add up. The Poynting vector should then show something quite different.
Now apply that to Veritasium's example (at DC), which is something I did think of earlier but thought it mightn't work (as in, be a reasonable partitioning of the current flow).
snarkysparky:
In the linked section from Haus and Melcher about the S vector.
Eq 23
S = phi( J + part_D / part_t)
In the free space surrounding the wires for DC current both J and part_D / part_t are zero.
S is zero
snarkysparky:
SandyCox:
--- Quote from: snarkysparky on January 04, 2022, 11:43:55 am ---In the linked section from Haus and Melcher about the S vector.
Eq 23
S = phi( J + part_D / part_t)
In the free space surrounding the wires for DC current both J and part_D / part_t are zero.
S is zero
--- End quote ---
Eq. (23) is an alternative formulation. By comparing Figs. 11.3.1. and 11.3.2 it is clear that the new electroquasitatic flux in not the same as the original Poynting vector, even in the static (DC) case. The point is that their alternative formulation is not prone to the same misinterpretation as the Poynting vector.
The physics’ professors are also still arguing about the interpretation of the Poynting vector:
https://www.researchgate.net/publication/321440917_Energy_in_Electromagnetism_The_Poynting_Vector_Historical_Corner
My personal viewpoint is not to attach any meaning to the Poynting vector without integrating it over the surface of a volume in space.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version