[FRIAM] This makes me think of this list...

Fri Aug 23 20:27:49 EDT 2024

For what it's worth, which is nothing, I rented Bob Griffiths' apartment a
few blocks from Carnegie Mellon when he moved out.  I met him when I went
to see the apartment before committing to it.  It was one room plus a tiny
kitchen and an equally small bathroom.  The rent was $65 per month.  At the
same time I had a '67 Corvette which cost me $350 per month.

---
Frank C. Wimberly
140 Calle Ojo Feliz,
Santa Fe, NM 87505

505 670-9918
Santa Fe, NM

On Fri, Aug 23, 2024, 5:54 PM Santafe <desmith at santafe.edu> wrote:

> 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 <jonzingale at 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
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