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| "Veritasium" (YT) - "The Big Misconception About Electricity" ? |
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| Naej:
Well now all EE's are under the protection of Carpenter's theorem; and can answer by saying "there is no energy in light". ^-^ |
| rfeecs:
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. --- End quote --- Yet another tool to use in the right situation. |
| HuronKing:
--- 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. --- End quote --- Yet another tool to use in the right situation. --- End quote --- Indeed and I was thinking about this when reading Carpenter's interesting papers posted by Naej. His mathematics is equivalent in terms of computing P_{out} versus P_{in} but I think there is some sly subtlety in the language he uses to describe, for example, in the section Magnetic Materials and Transformers he says: --- 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." --- End quote --- Hmm, what is this "remote action effect"? Does he just mean... field effect? But doesn't want to say fields? And then later, --- Quote ---Once f is known everywhere, then the energy transferred by the moving charges is given by in accordance with eqn. 13, and the antennas are no different in this respect from the plates in Fig. 1, or from the wires in Fig. 4a. Both examples illustrate the way in which removing the energy from empty space also removes the energy flow vector which is needed to account for it, and makes the question of the ‘mechanism’ of energy transfer wholly irrelevant. The mutual energy is divided between the charges, so that they necessarily become the vehicle by which the energy is transferred. It is, perhaps, remarkable that, although Maxwell’s attempts to use field theory to develop a mechanistic aether model have been universally abandoned, and with them the possibility of any useful meaning to the concept of the field as providing a force transfer mechanism, the properties of mass, energy and momentum in empty space are still interpreted literally, and go largely unchallenged --- End quote --- And earlier in energy exchange in a capacitor: --- Quote ---However, the mechanisms of the electromagnetic interactions, all of which are remote, are of no interest to the engineer if they are not needed to predict the behaviour of electromagnetic devices, and the general objective of Reference 2 was to show that this is so. As J4 satisfies the stored energy and force requirements, the point which remains is not whether it is a ‘correct’ description of energy flow (a question which is clearly meaningless in view of the variety of the alternatives), but whether or not it is sufficient for practical purposes. --- End quote --- 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. In QM/QED, we talk about photons (which I suppose from my understanding are themselves compositions of the energy in fields in QFT). PS If there is no energy in light - then what is radiation pressure? |
| Sredni:
--- Quote from: EEVblog on January 15, 2022, 09:45:45 pm --- --- Quote from: Naej on January 15, 2022, 06:43:35 pm ---Exactly! These concepts are tools. --- End quote --- Careful, I've been constantly put down on this thead for being a dumb arse engineer for dare suggesting that engineers have tools that solve problems, and not understanding that Poynting is the only true way that energy flows ::) --- End quote --- Let's see. Minute 8:50 of your video Derek's voice: "The charge on the surface of the conductor also creates an electric field outside the wires, and the current inside the wires creates a magnetic field outside the wires. So now there is a combination of electric and magnetic fields in the space around the circuit, so according to Poynting theory energy should be flowing. We can work out the direction of this energy flow using the right hand rule. Around the battery, for example, the electric field is down and the magnetic field is into the screen, so you find the energy flux is to the right, away from the battery..." Practical engineer using classical electrodynamics tool: "Now, the problem here is... this is something he doesn't address in the video. He's talking about the Poynting vector going out from the wire. Now, this is the case when you have AC. You have a... this is electromagnetic radiation, right? This is what happens. This is a big part of practical electrical engineering: designing products so that we can mantain the electromagnetic energy in the field surrounding the wire. This is why we have transmission lines, and coaxial cables. This is why we have transmission line theory on PCBs for example. But at DC, and DC steady state which we're going to take a look at, the Poynting vector is actually back INTO the wire. It's not going out, there's no electromagnetic radiation at DC. that only happens at AC. But at DC it's actually pointing in, it's not going out." (gestures in the opposite direction as that shown by Derek) It seems to me that the practical engineer has said - by using your language when you debunk dodgy tech - bullshit. The Poynting vector at DC is directed as Derek has shown because the battery is giving its energy to the rest of the circuit. Maybe that's the reason you are being put down as a dumb arse engineer. And make no mistakes, there is no interpretation of energy flow, integral over closed surfaces, momentum of the Maxwell fluid to add another intepretation here: it's just a cross product. With the fields shown the Poynting vector is pointing away from the battery. And even if you consider resistive wires, that will make the Poynting vector slightly impinge in the wires - well, that happens at DC as well as at AC. You made the same error Science Asylum did back in his original video on energy transfer. Both of you ended up quoting Feynman, IIRC. And my wild guess is that you saw that picture on Feynman that shows a piece of resistive wires at DC with the Poynting vector pointing radially in and assumed that is what happens with wires at DC. Am I wrong? |
| Sredni:
--- 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. --- End quote --- 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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