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| "Veritasium" (YT) - "The Big Misconception About Electricity" ? |
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| TimFox:
--- Quote from: aetherist on May 10, 2022, 07:37:16 am --- --- Quote from: TimFox on May 10, 2022, 05:08:54 am ---I know that the speed of a signal down a transmission line is the usual function of the inductance and capacitance per unit length. In my career, I have measured transmission lines to verify this. I have built resonant circuits using coax lines. I have built delay lines from discrete inductors and capacitors. I have used coax cables with and without the PVC outer insulation. I have never seen the mythical behavior you wave your hands about. I am getting tired of stepping through your “pearls”. --- End quote --- If the leading edge of a (DC) signal is slowed due to the presence of a parallel wire then that would be a big problem for my new electon electricity. --- End quote --- "the leading edge of a (DC) signal" is nonsensical: what you mean is the leading edge of a Heaviside step function, which is not DC. Such step-function pulses or waveforms never exist without a source impedance from the generator. The end of the transmission line where you connect the generator is a discontinuity, and may well have parallel capacitance. What happens to a step function applied through a resistance to a capacitance in parallel with another impedance (the characteristic impedance of the transmision line)? Spoiler: you get an increased rise time. Quantitative values depend on the actual parameters. Anytime you have two wires, you have a transmission line. Two wires lying on top of your bench makes a crappy transmission line, by which I mean that its parameters are not well-defined, nor are they constant down the length of the wire, as the would be in a well-made coaxial, twisted-pair, or twin-lead line. |
| aetherist:
--- Quote from: Berni on May 10, 2022, 12:06:26 pm --- --- Quote from: aetherist on May 10, 2022, 10:57:46 am ---The say 10 m wire could be stretched high above the dirt, from the say 3rd floor of a building, using plastic rope going to the say adjacent building. But i would not worry too much. Minor reflexions etc shouldn’t matter much, as long as there is a clear strong non-ambiguous signal. I would connect the wire to the scope (ie to receive a pulse), & not worry about completing some kind of circuit (or about using ground)(but i scored i think 51/100 for Electricity-1). I suppose that u would need the scope to measure voltage across a resistor stuck on the near end of the wire. Rather than using a (simple) pulse, i would prefer to use a (complicated) 12 V lead acid battery. Feed the wire from the negative terminal. Don’t have any connection to the positive terminal. Possibly use a special switch. No great need to looz sleep about all of the things needed to make the experiment perfect. The Nobel Committee will have an easy job. --- End quote --- A scope measures all signals in reference to ground, so you have to connect ground somewhere. The signal generator making the pulse (or a battery and switch) also has two terminals, so you need to connect the other end to something to see a signal at all. The source can only 'pump' electrons over. So if you want to push electrons into the 10m rod you have to pump them from somewhere. --- End quote --- I suspect that the scope is its own ground. In any case ground is only a worry if voltage is critical, which here it aint, what we need is good nanoseconds not good nanovolts. Re seeing a signal at all, that is old (electron) electricity. My new (elekton) elekticity don’t need no circuit. Hence the X will confirm my elektons whilst killing your electrons. The elektons are continuously circulating on the negative terminal of the lead acid battery. Fed from the electrolyte in the cell. They do not need any pumping or pushing. They merely need a contact, & off they go, at the speed of light (albeit slowed by the drag of the Cu surface)(ie the drag of the drifting electrons in the Cu)(plus a little bit of drag due to having to plough through free surface electrons). Sweden here i kum. |
| TimFox:
"what we need is good nanoseconds not good nanovolts" When dealing with young junior engineers doing digital hardware at work, I found that they had a similar cavalier attitude about the "nuisance" ground clip supplied with the oscilloscope probe, and generally lost it somewhere. The "bouncy-bouncy" on their displayed waveforms was the result. I responded by anathematizing "ground" as the "G word" that should not be used in polite company: specific terms such as "circuit common", "ground plane", "protective earth", "coax shield", etc. were required. There are well-defined methods, described in the oscilloscope literature, for obtaining high-fidelity displays of fast waveforms. My personal favorite are the coaxial test points that can be soldered directly to the circuit board, and fit the barrel and point of standard oscilloscope probes. Never, ever, depend on your scope dangling somewhere on the bench, possibly connected to a green wire, to be a good "ground" reference by itself. |
| aetherist:
--- Quote from: TimFox on May 10, 2022, 02:23:34 pm --- --- Quote from: aetherist on May 10, 2022, 07:37:16 am --- --- Quote from: TimFox on May 10, 2022, 05:08:54 am ---I know that the speed of a signal down a transmission line is the usual function of the inductance and capacitance per unit length. In my career, I have measured transmission lines to verify this. I have built resonant circuits using coax lines. I have built delay lines from discrete inductors and capacitors. I have used coax cables with and without the PVC outer insulation. I have never seen the mythical behavior you wave your hands about. I am getting tired of stepping through your “pearls”. --- End quote --- If the leading edge of a (DC) signal is slowed due to the presence of a parallel wire then that would be a big problem for my new electon electricity. --- End quote --- "the leading edge of a (DC) signal" is nonsensical: what you mean is the leading edge of a Heaviside step function, which is not DC. Such step-function pulses or waveforms never exist without a source impedance from the generator. The end of the transmission line where you connect the generator is a discontinuity, and may well have parallel capacitance. What happens to a step function applied through a resistance to a capacitance in parallel with another impedance (the characteristic impedance of the transmision line)? Spoiler: you get an increased rise time. Quantitative values depend on the actual parameters. Anytime you have two wires, you have a transmission line. Two wires lying on top of your bench makes a crappy transmission line, by which I mean that its parameters are not well-defined, nor are they constant down the length of the wire, as the would be in a well-made coaxial, twisted-pair, or twin-lead line. --- End quote --- I appreciate your input as usual. However that is old electricity. A single wire is a transmission line. All i need is the leading edge of a rise, ie of a step, the rise time itself is not critical. I suppose that Heaviside didn’t ever consider a single wire to be a transmission line. Whether the initial rise can be called DC is probly a side issue. The source might be problematical. I like to invoke a lead acid battery, koz i know that elektons live (mostly) on the positive terminal. I would be happy to try using a balloon that has been rubbed. Just touch the balloon on the Cu (ie on the transistor on the end of the Cu). But i am not sure whether my elektons live on balloons. I would be wary about letting electrodacus touch the resistor with one of his capacitors. Capacitors have more elektons on one plate than the other. And i am unsure about pulses provided by scopes. Are there any elektons in that there pulse? Dunno. Howardlong had no trouble getting a good pulse along & around & back on his 4 ft of crappy transmission ladderline on his kitchen table using a 20 GHz scope. And he got a delay of 79 ps which confirmed Veritasium. But, Howardlong didn’t show us the arrival of the main pulse, he only showed us the initial current due to inductance across the 25 mm from one Cu to the other Cu of his ladderline. Pity, it would have proven my elektons, alltho as i have said i would prefer tests on a single line (with reflexion) rather than using a circuit. |
| TimFox:
With transmission lines, especially those of substantial length, the voltage and current are local, and don't include uncontrolled long wires outside the measurement. Ideally, one defines a plane, perpendicular to the long direction of the coaxial cable (in this example) and the ideal voltage probe/oscilloscope measures the voltage from center to outer conductors in that plane, and the ideal current probe measures the current through the inner conductor passing through that plane. Practical oscilloscope and current probes try to emulate that ideal situation. When the diameter of the coax (again, in this example) becomes too large at the time scale of the measurement, we are crossing into waveguide territory, where we concentrate on the fields in the interior of the guide. Note that waveguides can be built with dielectric walls instead of conductive walls, with well-defined (in conventional electrodynamic theory) results similar to conductive guides, with the possibility of having a large DC (or lower-frequency than the waves) voltage from one end to the other. Similarly, if the interior of the waveguide is evacuated, one can run current in the form of electron or ion beams down the length of the guide, which will interact with the high-frequency fields. As mentioned in someone else's textbook citation, at the frequencies where waveguides are common, the skin depth in the metal walls is very small, and the energy is transported mainly in the traveling wave down the guide. With a bad termination, or a resonant cavity, energy can be stored in the standing wave. There is a large literature and industrial history of both waveguides in general and particle accelerators in particular. Again, they both work. When Heaviside made his huge breakthrough (realizing that adding lumped inductors in series with the telegraph lines improved their bandwidth), he may have used the telegraph system grounding as the return (bandwidth is not so high with manual Morse code). When AT&T successfully adopted his method to trunk telephone lines, they used twisted-pair transmission lines (balanced with respect to ground). By the way, with respect to the outer insulation (jacket) on coaxial cables. I have used precision coaxial cables, some of which had armored jackets to prevent damage, and coaxial cables with various dielectrics (usually PE or PTFE), and various jackets (usually PVC), but also "semi-rigid" coaxial cables, where the outer conductor is essentially a copper tube, with no outer insulation. Careful use of these semi-rigid cables requires proper tooling for making bends, so as not to destroy the inner geometry. See https://www.pasternack.com/pages/Featured_Products/hand-formable-semi-rigid-cable-assemblies-up-to-18-ghz-new-from-pasternack.html?utm_campaign=usa_cable_assemblies&keyword=semi-rigid%20coaxial%20cable%20assemblies&gclid=eaiaiqobchmi2dv1-dzv9wivshrnch1uia-oeaayasaaegkoypd_bwe for such assemblies available with or without outer insulating jacket. Again, they work the same either way. |
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