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| Veritasium -- How Special Relativity Makes Magnets Work. |
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| TimFox:
Beam diagnostics for charged-particle accelerators is a mature field. https://cds.cern.ch/record/499098/files/p154.pdf https://cds.cern.ch/record/1982418/files/23-60%20Jones.pdf https://uspas.fnal.gov/materials/14UNM/Diagnostics.pdf |
| TimFox:
By coincidence, I stumbled across this cartoon while looking for something else: https://xkcd.com/675/ |
| aetherist:
--- Quote from: TimFox on April 20, 2022, 01:37:07 pm ---Beam diagnostics for charged-particle accelerators is a mature field. https://cds.cern.ch/record/499098/files/p154.pdf https://cds.cern.ch/record/1982418/files/23-60%20Jones.pdf https://uspas.fnal.gov/materials/14UNM/Diagnostics.pdf --- End quote --- The electron beam stuff & cyclotron stuff is re a pulsed beam. Their detectors, beam transformers (or beam current transformers), detect the leading edge of a pulse & the trailing edge. They detect the change in the magnetic field around the beam. Ok, this confirms that there is a magnetic field associated with an electron beam. I admit that. If i wanted to take a hard line i could suggest that the magnetic field itself arises from the change in current. But i wont, i accept that the magnetic field exists in the steady state current part of the pulse. Allbeit a current of free-ish electrons in vacuum, rather than the Veritasium current of free-ish electrons in/on a wire (the central topic of this thread). Today i want to take a closer look at that there pulses of current. But not so much re pulses of flying electron beams in vacuo, no, i want to look at pulses of drifting electrons in Veritasium's wire. Trailing Edge Catastrophe. The first problem with a pulse of drifting electrons (in a wire) is that a pulse of drifting electrons can't have a trailing edge. Or, they can have a trailing edge, if we invoke the momentum of the drifting electrons. It is easy to understand that the push force at the leading edge of a pulse can be very great, that’s ok, electrons push electrons, as hard as is needed. But, if we look at a short pulse, what is pushing the pulse? Nothing! We have to create a postulate that a pulse has its own momentum. But how powerful might that momentum be? Drifting electrons drift at say 0.3 mm/s. The pulse momentum would evaporate in the first few mm of the wire (due to resistance)(heat loss). I think this came up earlier in this thread, or was it on the other Veritasium thread. And i don’t want to revisit it today, except to say that the trailing edge catastrophe for old (electron) electricity is not a big problem for my new (electon) electricity – if one accepts that my electons propagate happily (at c)(along the surface of a wire) without needing anything to push them along, ie just like free photons (light) in vacuum (or in air or glass etc). Ok, where was i. Veritasium & Co explain (try to explain) that STR explains mmf around a wire. This STR explanation invokes length contraction (& dilation) of the line of drifting electrons or length contraction (but not dilation) of the line of protons or both. Neutral Wire Catastrophe (Strike-1). In my Reply#5 #20 #27 #29 #34 (all reproduced in part in my #210) i pointed out my main objection to Veritasium's & Purcell's explanation for the STR cause of mmf around a wire. Veritasium & Purcell invoke what is a new postulate, which they fail to call a new postulate, koz that would invite closer scrutiny. Today i will call this saga the Neutral Wire Catastrophe. Earlier i had called it Strike-1. Reply#5. The Veritasium youtube etc tells us that drifting electrons drifting at (Veritasium says) 0.3 mm/s in the lab frame have the same spacings (center to center) as the Cu nuclei (ie protons)(which are stationary in the lab frame). Veritasium & Co have to invoke that postulate, koz they know that a stationary charge is not attracted or repelled by a wire carrying an electric current. But, if, when there is zero electric current, the electron to electron spacing is the same as the proton to proton spacing, then, when the current is turned on, & electrons drift, surely STR demands that in the lab frame the electron to electron spacing will be contracted. In which case the wire will have a negative charge in the lab frame. Veritasium & Purcell say or infer that a wire with zero current has no charge (ie is neutral), &, when a current is flowing, the wire remains neutral. Here below is what Purcell wrote……. ……It happens that the linear density of negative charge along the line of electrons is exactly equal in magnitude. That is, any given length of "wire" contains at a given instant the same number of electrons and protons [9]. The net charge on the wire is zero……. [9] It doesn't have to, but that equality can always be established, if we choose, by adjusting the number of electrons per unit length. In our idealized setup, we assume this has been done. Purcell says that the wire is neutral (in the lab frame)(ie in the wire's frame) before & after the current starts to flow. He infers that the wire is neutral at all points & at all times, including at or near the leading edge of the current (which is propagating at nearly c). He duznt explain how it is that an electron or electrons can exit the wire so as to satisfy his demand for neutrality. Well, i suppose that exiting is easy, but what i mean is that electrons have to make their way from the leading edge of the current to exit at one end or the other. This must take time. Ok, the electrons don’t have to drift that far, all they have to do is to create a wavefront, & this wavefront propagates at nearly c (lets say), & it results in an electron being spat out somewhere, to give the demanded neutrality. But, even if that wavefront propagates at nearly c, the neutrality needs time. So, during that time, the wire has a charge, either briefly, or, continuously (depending on how u visualise the whole process). So, we have 2 wavefronts, each propagating at nearly c. One is the new electricity flowing along the wire, one is the electricity trying to give neutrality. Two electricitys. Purcell can try to ignore that & hide that, ie he duznt worry about the DC wavefront of the electricity, he jumps straight to the case where the DC has reached steady state. Pulse Catastrophe. But, today, i am looking at a pulse of DC propagating along a wire. This pulse has a leading edge, & a trailing edge. And this demands two electricity wavefronts at each, ie 4 wavefronts in all. And, it demands a charge in/on the wire at the leading edge & at the trailing edge, briefly, or continuously. But, we dont see any evidence of such a charge(s). No, the STR cause of mmf around a wire is a disaster. |
| TimFox:
I already pointed out to you that one of the Bergoz beam monitor units measures down to DC, presumably using a flux gate instead of the simple coil (current transformer) in their other units. CERN, Fermilab, and the other major accelerator labs know how to do this stuff. The smaller electron linear accelerators made by my former employer were pulsed, but the pulse duty factor was quite low so the difference between an AC-coupled pulse transformer and a true DC measurement was quite small. Cockcroft-Walton and Van der Graaf accelerators achieve > 1 MeV with DC (unpulsed) beams. Spell-check tried to replace “Bergoz” with “Berlioz”, just like it tried to replace “Poulsen” with “Poulenc”—must be biased towards composers. |
| aetherist:
--- Quote from: TimFox on April 21, 2022, 02:29:17 am ---I already pointed out to you that one of the Bergoz beam monitor units measures down to DC, presumably using a flux gate instead of the simple coil (current transformer) in their other units. CERN, Fermilab, and the other major accelerator labs know how to do this stuff. The smaller electron linear accelerators made by my former employer were pulsed, but the pulse duty factor was quite low so the difference between an AC-coupled pulse transformer and a true DC measurement was quite small. Cockcroft-Walton and Van der Graaf accelerators achieve > 1 MeV with DC (unpulsed) beams. Spell-check tried to replace “Bergoz” with “Berlioz”, just like it tried to replace “Poulsen” with “Poulenc”—must be biased towards composers. --- End quote --- Yes i remember that that unit measured down to 0 Hz. I accept that an electron beam has a magnetic field. In thems youtubes Sutterlin says that he measured the magnetic field around an electron beam in a straight glass tube & the field agreed with the standard equation. However, he also added that he was confident that that magnetic field would be zero if measured after a 90 deg bend (according to his own theory)(but he hasn’t dunn the X yet). I must have a close look at what Orman said…… Mathew Orman 2 months ago. The same experiment with compass near moving Van De Graaff generator charged belt show no magnetic filed... Draw your own conclusion.. |
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