General > General Technical Chat
"Veritasium" (YT) - "The Big Misconception About Electricity" ?
rfeecs:
--- Quote from: HuronKing on January 16, 2022, 01:10:50 am ---
--- Quote ---The core, represented largely by an equivalent surface current J, , has the effect of a flywheel, into which the primary injects and recovers momentum by the remote action effect of A. The electron stream in the secondary conveys energy into the load, which generates a p4/2 pressure in the winding, and hence controls the rate at which energy is extracted from the ‘fluid."
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Hmm, what is this "remote action effect"? Does he just mean... field effect? But doesn't want to say fields?
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I'm thinking he means retarded potential:
https://en.wikipedia.org/wiki/Retarded_potential
--- Quote ---In electrodynamics, the retarded potentials are the electromagnetic potentials for the electromagnetic field generated by time-varying electric current or charge distributions in the past. The fields propagate at the speed of light c, so the delay of the fields connecting cause and effect at earlier and later times is an important factor: the signal takes a finite time to propagate from a point in the charge or current distribution (the point of cause) to another point in space (where the effect is measured), see figure below.
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It is still fields propagating.
Naej:
--- Quote from: rfeecs on January 16, 2022, 12:25:36 am ---So, uh, what about photons?
From https://en.wikipedia.org/wiki/Photon:
--- Quote ---To explain the photoelectric effect, Einstein introduced the idea that light itself is made of discrete units of energy.
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Yet another tool to use in the right situation.
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Photon are quantum mechanical, there is no photon in Maxwell's equations.
An emitting antenna consumes energy (radiation resistance), a receiving antenna gives energy (radiation resistance).
--- Quote from: HuronKing on January 16, 2022, 01:10:50 am ---So, all I'm getting from his paper is that he doesn't give a damn what happens in the middle (and uses the ad hoc 'remote action effect' term) - all he cares about is the end result. How much power is delivered from source to load?
But, this interpretation seems to be insufficient to answer Derek's question about how long it takes for the bulb to receive energy in the transient period. The mechanism of EM interaction IS important to Derek's question (that EM energy can traverse empty space at speed c, and if so, how?) - otherwise you'd be led to the wrong answer.
Classically, it's all fields.
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A charge here creates scalar (Lorenz) potential V everywhere.
A moving charge here creates vector (Lorenz) potential A everywhere.
Potentials propagate at the speed of light, and this gives the answer: when you close the switch an EM disturbance is created, it propagates to the light and "switch it on".
(It's all fields until you remove them. See Liénard-Wiechert potential, for example in an Atoms & Sporks video or in Wiki)
--- Quote from: HuronKing on January 16, 2022, 01:10:50 am ---If there is no energy in light - then what is radiation pressure?
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Potential momentum qA being increased/decreased (remember that Lorenz' A propagates at the speed of light), and converted in a mechanical one.
So yes Carpenter's energy of light is 0, Carpenter's momentum of light is 0. It's vacuum after all! Why would you put energy in vacuum! 8)
--- Quote from: Sredni on January 16, 2022, 01:20:48 am ---
--- Quote from: Naej on January 15, 2022, 09:23:10 am ---Carpenter's theorem states that if Maxwell's equations are correct, then JV is a valid energy flow. And with JV, energy flows only in wires.
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Yes, it actually flows in the upper wire.
No, in the lower wire
No, wait, half and half.
No, no, wait again... it's 5/8 in the upper one and 3/8 in the lower one.
Or, the other way around?
The phi J representation is subject to as many representations as the potential. We have already been over that, some five or six pages ago.
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Yes all this are valid power flows. So it should start by yes or you are just making a controversy where there shouldn't be one.
I know it has been discussed, and also that if you accept JV then light has no energy …
But now you know that JV is correct ;) and your choice is essentially between "no energy in wires" and "no energy in light". Or "energy in wires" and "energy in light".
rfeecs:
--- Quote from: HuronKing on January 16, 2022, 01:10:50 am ---If there is no energy in light - then what is radiation pressure?
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Feynman has an explanation for radiation pressure just from the fields acting on the charges: https://www.feynmanlectures.caltech.edu/I_34.html
--- Quote ---34–9 The momentum of light
Now we turn to a different topic. We have never, in all our discussion of the past few chapters, said anything about the effects of the magnetic field that is associated with light. Ordinarily, the effects of the magnetic field are very small, but there is one interesting and important effect which is a consequence of the magnetic field. Suppose that light is coming from a source and is acting on a charge and driving that charge up and down. We will suppose that the electric field is in the x-direction, so the motion of the charge is also in the x-direction: it has a position x and a velocity v, as shown in Fig. 34–13. The magnetic field is at right angles to the electric field. Now as the electric field acts on the charge and moves it up and down, what does the magnetic field do? The magnetic field acts on the charge (say an electron) only when it is moving; but the electron is moving, it is driven by the electric field, so the two of them work together: While the thing is going up and down it has a velocity and there is a force on it, B times v times q; but in which direction is this force? It is in the direction of the propagation of light. Therefore, when light is shining on a charge and it is oscillating in response to that light, there is a driving force in the direction of the light beam. This is called radiation pressure or light pressure.
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adx:
--- Quote from: SiliconWizard on January 14, 2022, 10:15:38 pm ---A lot of the "debate" coming more from using different (or even vague) definitions and a philosophical approach of science rather than experimental, it can probably go on forever. =)
Just like with the KVL one.
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If physics is stuck, then what else is left other than vagueness and a philosophical approach?
HuronKing:
--- Quote from: Naej on January 16, 2022, 01:49:34 am ---A charge here creates scalar (Lorenz) potential V everywhere.
A moving charge here creates vector (Lorenz) potential A everywhere.
Potentials propagate at the speed of light, and this gives the answer: when you close the switch an EM disturbance is created, it propagates to the light and "switch it on".
(It's all fields until you remove them. See Liénard-Wiechert potential, for example in an Atoms & Sporks video or in Wiki)
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Maybe I'm an idiot but I really don't see how this interpretation removes any of the fields. And I looked at the Liénard-Wiechert potential - it's defined in terms of vector fields. We're still talking about the propagations of fields, through empty space.
--- Quote ---Potential momentum qA being increased/decreased (remember that Lorenz' A propagates at the speed of light), and converted in a mechanical one.
So yes Carpenter's energy of light is 0, Carpenter's momentum of light is 0. It's vacuum after all! Why would you put energy in vacuum! 8)
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Because since the 19th century we know that thermal energy can cross a vacuum? We've abandoned their aether interpretations of this, but, if the vacuum doesn't have energy, I'd be curious to know how you explain the Casimir Effect.
@rfeecs
Yes, I'm aware of Feynman's explanation of radiation pressure - that's why I question the statements Naej is making in their interpretation of Carpenter. After all, Feynman remarks a moment later,
--- Quote ---Therefore the force, the “pushing momentum,” that is delivered per second by the light, is equal to 1/c times the energy absorbed from the light per second! That is a general rule, since we did not say how strong the oscillator was, or whether some of the charges cancel out. In any circumstance where light is being absorbed, there is a pressure. The momentum that the light delivers is always equal to the energy that is absorbed, divided by c:
⟨F⟩=dW/dtc.(34.24)
That light carries energy we already know. We now understand that it also carries momentum, and further, that the momentum carried is always 1/c times the energy.
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