Hi Jon, I must have missed the thread where you linked to the Collier video, but I think I found the one you are referring to. Also didn’t know who Griffiths was/is (textbook writer; I turned out not to have used any of his books), but now realize what those mentions refer to. (I had imagined he was some philosopher of science saying very obscure things; didn’t realize this was an undergrad E&M textbook, with a regular teaching point.)
I think the thing about intrinsic dipoles in Collier’s video is beside the point. It is something that happens to interest her, and her discussion of whether there can be a classical physics with intrinsic dipoles, so you don’t need to invoke QM, is a perfectly good subject. But the way she sets it up with the Lorentz force law is a separate matter, and the spin-question doesn’t follow from the classical Lorentz question. She could have done them as two separate videos, and would have been easier to parse. So, with apologies to the list, a short summary of what I think the answer is to what I think she introduced as the first question. If F = v x B, and work-rate = power P = F * v, then clearly power from the magnetic force on a charge is zero. Fine. But electromagnets pick up cars, so what gives? You could go off into QM, and the fact that cars are made of steel, and that it is the presence of spins that makes steel responsive to magnetic fields, and on and on. But we don’t need to. She makes the central point early on, and it is enough to get at the picking-up-cars thing. It is that in classical electromagnetism without intrinsic magnetic dipoles, magnetic dipoles are viewed as little current loops. That’s also fine, and that is where she could have stayed if her only point was to answer the question she opened with. There is no requirement that the “current loops” be taken to zero size or that they be intrinsic. We can just leave them finite-sized, and answer the original question. Instead of having the electromagnet lift a car made of steel, let the electromagnet lift another electromagnet, powered by a battery that drives a current through a solenoid. Where is the force coming from? We know this. The E field drives charge movement along the direction of the wire in our little solenoid. I don’t care whether the positive or the negative charges or both are mobile; everything below will have the same outcome whichever it is. What happens of there is then a B field from the external electromagnet? It won’t instill work on the electric charges in our solenoid; it will rather accelerate them in a direction perpendicular to their direction of travel in the wire. Which means they will be driven to the edge of the wire. But now it is the fact that the wire has an edge, and thus limited places for electrons to be, that keeps them from being driven off the edge, and that transduces work from the electric field that was originally driving the solenoidal current, into a distorted particle distribution where they are piled up against the boundary of the wire. And their direction-of-piling will be the direction in which the external B field is pulling our solenoid. So the solenoid will be forced, as a response to relax that potential energy from piling-up that was driven by the B field. Done. There is no need to bring in QM to work out this force problem. Interestingly, one can get to the same result in another way, which is close to my first email. The B field that the outside electromagnet builds up does have an energy density associated with it. So also does the B field of our little solenoid, when it is far away. If the solenoid can be moved in the direction toward the lifting electromagnet (and I think it will want to be oppositely oriented, but don’t ever trust me with a minus sign until I have calculated it on paper...), its little dipole field will partly cancel the field of the lifter. This is the remarkable thing about two somethings adding to make a nothing. Thus the energy density in the net field will be lower than it was when the two were distant. One can say, then, that the work is coming from a depletion of the energy density in the total B field from enabling a partial dislocation, and that it is put into the potential energy (or kinetic energy, depending on the set-up) of the forced solenoid. On this, whether one wants to grant potential energy, and credit it to the lifter or to the Earth — another point Collier pursues that is kind of fun but not essential to anything later — can be up to you. Need to reply briefly to the exchange with Glen; next, Eric > On Aug 21, 2024, at 1:04 PM, Jon Zingale <jonzing...@gmail.com> wrote: > > Eric, > > Thanks for the thoughtful response. I have always felt like a physics > tourist, even in my earlier days as a fly-on-the-wall in Dan Freed's TQFT > seminars in the RLM building. I couldn't agree more about Purcell, and I > still have my mom's copy from when I was a kid. Rasmus has loaned me Jackson > on a number of occasions and one of these days I hope to get into the > exercises like I do with other texts. If in what follows I am completely off > base, I am happy to learn something. > > That said, I don't feel competent to validate Angela Collier's exposition in > the video I referenced, but there is something I thought generalized well to > the discussion. The confusion, as she outlines it, does not appear to be the > existence of the Lorentz force law, but its consequence in a theory without > intrinsic magnetic dipoles. Griffith seems to express (in an interview Angela > managed to track down) that (from his perspective) there is no place in > classical electrodynamics for the notion of an intrinsic magnetic dipole > because spin doesn't belong to the classical theory. He seems to accept that > by slight generalization of the theory, by extending a term, one does get a > coherent theory. Of course this is the kind of pedantry only a father could > love. This is in-part what I read in your stating, > > "The issue of formal systems, and their role in bringing into being habits of > thought that, later, we wrongly suppose to be “natural attitudes” > > Still, I think pausing for a moment on Griffith's take speaks to the > formalization discussion well, and was in-part inspired by your earlier > discussion with Nick regarding the inheritance of properties to ensembles > from properties of individuals: > > "Curie and Weiss took a certain essentialist position w.r.t. the dipolarity > of magnetization. That there can be magnetization on the macroscale because > there was already-realized magnetization at the microscale." > > A continuing interest I have is how one theory derives as a theorem what > another theory states as an axiom. While theories of physics such as QFTs > seem to begin with assumptions of spacetime and fields as fundamental, others > such as Carroll (via Everett) seek to derive the geometry of spacetime from > the information theoretic arguments and the Shrodinger equation. When > modeled, both have their own bizarre and cursed consequences. Yet I hope that > at the end of the day, the various approaches will provide models of physics > indistinguishable to farmers in our time and perhaps any other. > > Jon > -. --- - / ...- .- .-.. .. -.. / -- --- .-. ... . / -.-. --- -.. . > FRIAM Applied Complexity Group listserv > Fridays 9a-12p Friday St. Johns Cafe / Thursdays 9a-12p Zoom > https://linkprotect.cudasvc.com/url?a=https%3a%2f%2fbit.ly%2fvirtualfriam&c=E,1,VN-wtAyYkqfy0TsACiKOoES_jKwb-7qpIhL4PhPwOrLglC4W2RTsW4PXuKl_Xlw7rbOxet6Mz7gosZFt4P-xGwf8puLEVck4dGnXrWlDxGQ,&typo=1 > to (un)subscribe > https://linkprotect.cudasvc.com/url?a=http%3a%2f%2fredfish.com%2fmailman%2flistinfo%2ffriam_redfish.com&c=E,1,Uhx_bzwJsnhkzRjMm4peZIdTHheLbw6Dr3ExfdHVjT51E97bB2LqUWaxO1uTvbqvuZDIS2scXv2NaYhfnnQT0jFAvAnIOCWos7FRUuCgTe6Gal-m&typo=1 > FRIAM-COMIC > https://linkprotect.cudasvc.com/url?a=http%3a%2f%2ffriam-comic.blogspot.com%2f&c=E,1,_XPVzdsN3Z8jm0SG3idpGr0HmCh6tobZItLqE354naeH55Yo1FR7HvhiwI8Bl6719_CpVMymAa_oKmz50GkB87agsmV1cfzeAbjToRRjtaVczLE,&typo=1 > archives: 5/2017 thru present > https://linkprotect.cudasvc.com/url?a=https%3a%2f%2fredfish.com%2fpipermail%2ffriam_redfish.com%2f&c=E,1,MsOivmv0QUdU0FOIDO6ci-LWDU5fcm_I-hO4oQXDQa71Yguw90dfABTEJTeCjZA7JhEwU92ZtIQgOG2CqahQ6Zw65_dNyWecPG2wTeOxiw,,&typo=1 > 1/2003 thru 6/2021 http://friam.383.s1.nabble.com/ -. --- - / ...- .- .-.. .. -.. / -- --- .-. ... . / -.-. --- -.. . 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