| General > General Technical Chat |
| The Science Asylum -- How Special Relativity Fixed Electromagnetism. |
| (1/6) > >> |
| aetherist:
How Special Relativity Fixed Electromagnetism…….. The Science Asylum…… 636K subscribers… 19K… 369K views 3 years ago 2,058 Comments Electricity and Magnetism…….Electrodynamics (electricity and magnetism) is governed by Maxwell's equations and the Lorentz force law, but that left it a little broken. It would take Albert Einstein inventing special relativity to fix it. If magnets are based on motion and motion is relative, how does that work? Nick explains how Einstein's STR explains the magnetic field around a wire. Nick's squirrel (which has a positive charge) moving along parallel to the wire sees that the positive charges in the stationary wire are closer together & hence the wire has a positive charge & hence the squirrel is repelled from the wire. This repulsion is in effect identical to the repulsion described by the conventional right hand rule, based on the wire being surrounded by a cylindrical magnetic field. But i see at least 2 problems for the STR theory. Problem-1A. If i placed a (stationary) Cu sheet between the squirrel & the wire then the Cu sheet would block/shield the charge repulsion (ie block/shield the faux magnetic force). Whereas we all know that a Cu sheet has almost zero blocking/shielding effect re the magnetic field around a wire. Problem-1B. In other words, 2 parallel wires carrying currents experience a force of attraction or repulsion, & i reckon that if u placed a Cu sheet between the wires then that attraction or repulsion would not be much affected. Problem-1C. But if the attraction or repulsion is due to Nick's charge effect then a Cu sheet would reduce the attraction or repulsion to almost zero. Hence the STR explanation for magnetic force near a wire is wrong. Problem-2A. We know that a charge moving away from a current carrying wire experiences a magnetic force parallel to the wire (as per the right hand rule). Problem-2B. But, if Nick's squirrel is moving away from the wire (instead of parallel to the wire) then Nick karnt invoke some kind of STR length contraction of the wire & charges, hence Nick's squirrel would not experience a force in any direction, which would be contrary to what we know happens (as per 2A). My inclusion of Problem-1ABC might be wrong, praps the Cu sheet would not block charge. But my Problem-2AB i reckon on its own kills the STR theory. Has anyone here ever seen any discussion of these 2 problems or similar? |
| jasonRF:
My intro physics course (I took the ‘honors’ version) included special relativity, relativistic transformation of the EM field, etc. We used Electricity and Magnetism, 2nd edition, by Purcell. Not a great book for a first exposure to electromagnetism, in my opinion. It ‘derives’ the magnetic force using special relativity and coulomb’s law. There were only a few EEs in the class, as it was mostly full of physics and engineering physics majors. A book that does that same derivation but also includes a lot of the history, including details of the experimental basis for Coulomb’s law and special relativity, is Electromagnetics by Elliott. He was an EE prof that also wrote a very good book on antennas. The crazy thing is that he shows the table of data in Coulomb’s paper, and it just has a handful of data points I do hate the title of that youtube video, though, since Maxwell’s equations were already consistent with special relativity. It feels click-baity. |
| switchabl:
--- Quote from: jasonRF on May 29, 2023, 05:43:13 pm ---I do hate the title of that youtube video, though, since Maxwell’s equations were already consistent with special relativity. It feels click-baity. --- End quote --- It's true that Maxwell's equations turned out to be fine after all but that wasn't so clear at the time. Electromagnetism at the turn of the century was a theory in crisis. The interpretation of the equations was very different from how people think about them today. It was generally assumed that the electrical and magnetic fields were disturbances in some fixed "ether". So in a sense the problem described in the video wasn't really a problem at all. Because the moving reference frame would be moving relative to the ether and so it wasn't supposed to be equivalent. Unfortunately, experiments started to contradict the ether hypothesis (most famously the Michelson-Morley experiment). Physicists scrambled to make sense of it all. Heaviside FitzGerald suggested that objects moving relative to the ether might experience length contraction. That would have fixed some problems but not others (of course, a similar phenomenon occurs naturally in SR and features prominently in the video). Lorentz and Poincaré tinkered with the definitions of force and even time, trying to come up with something that would not depend on movement relative to the ether. A lot of ideas and mathematics (notably the Lorentz transform) was already in place at that point but Einstein was the first to present a coherent theory, based on a few simple assumptions, instead of a bunch of ad-hoc modifications. There seems to be this narrative that SR "fixes" Newtonian mechanics while electromagnetics was relativistic all along. But that is mostly a didactic fiction. Historically, special relativity is very much the culmination of an effort to find a consistent theory of electromagnetics. Deriving EM from electrostatics and relativity is elegant and fun but it is also egregious retcon. ;D EDIT: It was FitzGerald who first suggested deformation of objects depending on their movement relative to the ether, probably inspired by the work of Heaviside on moving charges. I mixed that up. |
| aetherist:
--- Quote from: jasonRF on May 29, 2023, 05:43:13 pm ---My intro physics course (I took the ‘honors’ version) included special relativity, relativistic transformation of the EM field, etc. We used Electricity and Magnetism, 2nd edition, by Purcell. Not a great book for a first exposure to electromagnetism, in my opinion. It ‘derives’ the magnetic force using special relativity and coulomb’s law. There were only a few EEs in the class, as it was mostly full of physics and engineering physics majors. A book that does that same derivation but also includes a lot of the history, including details of the experimental basis for Coulomb’s law and special relativity, is Electromagnetics by Elliott. He was an EE prof that also wrote a very good book on antennas. The crazy thing is that he shows the table of data in Coulomb’s paper, and it just has a handful of data points I do hate the title of that youtube video, though, since Maxwell’s equations were already consistent with special relativity. It feels click-baity. --- End quote --- How did Purcell & Elliott deal with my Problem-2AB? |
| IanB:
--- Quote from: aetherist on May 30, 2023, 12:35:26 am ---How did Purcell & Elliott deal with my Problem-2AB? --- End quote --- They only had to deal with their problems. Your problems are yours alone to deal with. |
| Navigation |
| Message Index |
| Next page |