General > General Technical Chat
"Veritasium" (YT) - "The Big Misconception About Electricity" ?
adx:
--- Quote from: adx on January 04, 2022, 10:40:01 am ---The Poynting vector should then show something quite different.
--- End quote ---
Not for that 2D example - might be best to quit while I'm behind :horse:.
snarkysparky:
More beating of the dead horse.
https://phys.libretexts.org/Bookshelves/Electricity_and_Magnetism/Book%3A_Electromagnetics_II_(Ellingson)/03%3A_Wave_Propagation_in_General_Media/3.01%3A_Poynting’s_Theorem
""The quantity E×H is the Poynting vector, which quantifies the spatial power density (SI base units of W/m 2 ) of an electromagnetic wave and the direction in which it propagates. The reader has likely already encountered this concept. Regardless, we’ll confirm this interpretation of the quantity E×H in Section 3.2. For now, observe that integration of the Poynting vector over S as indicated in Equation 3.1.20 yields the total power flowing out of V through S . The negative sign in Equation 3.1.20 indicates that the combined quantity represents power flow in to V through S . Finally, note the use of a single quantity Pnet,in does not imply that power is entirely inward-directed or outward-directed. Rather, Pnet,in represents the net flux; i.e., the sum of the inward- and outward-flowing power.""
bold text is my $.02
adx:
--- Quote from: snarkysparky on January 04, 2022, 05:41:09 pm ---... of an electromagnetic wave and the direction in which it propagates. ...
bold text is my $.02
--- End quote ---
italic text is mine
--- Quote from: adx on January 04, 2022, 10:40:01 am ---... at which point my brain shuts down and spits out the most recent result with an error flag set.
--- End quote ---
That result is something like a standing wave of order 0, even though though that does not make sense (since the theory holds that the resistor(s) is dissipating the power), but neither does the first iteration of the endless loop; how can a travelling wave "propagate" in DC? It implies continuous conduction, moving charges, static fields.
SandyCox:
--- Quote from: adx on January 05, 2022, 12:18:25 am ---
--- Quote from: snarkysparky on January 04, 2022, 05:41:09 pm ---... of an electromagnetic wave and the direction in which it propagates. ...
bold text is my $.02
--- End quote ---
italic text is mine
--- Quote from: adx on January 04, 2022, 10:40:01 am ---... at which point my brain shuts down and spits out the most recent result with an error flag set.
--- End quote ---
That result is something like a standing wave of order 0, even though though that does not make sense (since the theory holds that the resistor(s) is dissipating the power), but neither does the first iteration of the endless loop; how can a travelling wave "propagate" in DC? It implies continuous conduction, moving charges, static fields.
--- End quote ---
I am only aware of two mechanisms by which electromagnetic energy is transferred:
1. Flow of charge
2. Time-varying electric and/or magnetic fields
At DC, energy is only transferred by means of the flow of charge through an electric field. In this case the Poynting vector is the result of the flow of charge.
For AC, in the far field, only time-varying electric and magnetic fields.
But this is only the picture I have in my mind. Maybe there are more mechanisms.
I’m not sure if Maxwell’s equations can answer this question. Maybe we need to peel another layer of Feynman’s onion.
adx:
--- Quote from: SandyCox on January 05, 2022, 11:26:00 am ---
--- Quote from: adx on January 05, 2022, 12:18:25 am ---
--- Quote from: snarkysparky on January 04, 2022, 05:41:09 pm ---... of an electromagnetic wave and the direction in which it propagates. ...
bold text is my $.02
--- End quote ---
italic text is mine
--- Quote from: adx on January 04, 2022, 10:40:01 am ---... at which point my brain shuts down and spits out the most recent result with an error flag set.
--- End quote ---
That result is something like a standing wave of order 0, even though though that does not make sense (since the theory holds that the resistor(s) is dissipating the power), but neither does the first iteration of the endless loop; how can a travelling wave "propagate" in DC? It implies continuous conduction, moving charges, static fields.
--- End quote ---
I am only aware of two mechanisms by which electromagnetic energy is transferred:
1. Flow of charge
2. Time-varying electric and/or magnetic fields
At DC, energy is only transferred by means of the flow of charge through an electric field. In this case the Poynting vector is the result of the flow of charge.
For AC, in the far field, only time-varying electric and magnetic fields.
But this is only the picture I have in my mind. Maybe there are more mechanisms.
I’m not sure if Maxwell’s equations can answer this question. Maybe we need to peel another layer of Feynman’s onion.
--- End quote ---
I suppose time-varying electric fields will be produced by flowing charge. If you consider that a circuit must have a return path, then an electron going forward will push on the electron coming back, taking work from it and putting it into the field, which is returned after it passes. I guess topped up by the relativistic effect which produces magnetism, but that will be unequal and store energy in the electric field while current flows. Maybe something like that is responsible for the Poynting vector behaving as it does here.
There must be an explanation for the mathematical behaviour even if it doesn't make physical sense, the physical is magic (in the sense that nobody knows everything about how it works) but the maths shouldn't be. If it is simply a way of quantifying one's thinking.
I don't see anything (or any way) for QED or QFT to alter the Poynting vector beyond the different insights it brings to those people skilled in that field, but I don't know (and barely have a handle on Maxwell's equations themselves). I was looking at Feynman lecture #1 earlier tonight and wondering if I should finally bite the bullet and try to properly learn what I never did at school or uni! And field solvers again, but only as a way to avoid the maths.
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