Instead, they derived from Maxwell’s original attempts to model electromagnetic fields as altered states of a space-filling mechanical medium. Now here is the key point: Maxwell and his pre-Lorentz followers needed two vectors for electric fields and two vectors for magnetic fields, for ‘empty’ space as well as for materials. All I’m arguing is that the reasons for using D and H when materials are present gives no support at all for needing these extra vectors in a vacuum. Yes, many writers call the magnetic field without explanation or comment. This is called alpha and is the dimensionless charge value. "H" should be called "flux" and "B" should be called "field". This reflected a pre-Lorentzian, essentially ether-based, picture of electromagnetism. It is customary to call the magnetic induction and the magnetic field strength. Weber and Julius Stratton who call H "magnetic intensity." marcusl said: ↑ rbj, there's reason to call B "field," but please don't call H "flux" because this is a different quantity as I had noted earlier in the thread you quoted. marcusl said: ↑ robphy said: From a more abstract viewpoint, H and B aren't even the same type of geometrical object. We can then 'forget about' the medium, knowing that for isotropic media, D and H are related quite simply to E and B. The same equations applied both to empty space and materials; you just needed different values of permittivity and permeability constants. What has happened, unfortunately, is that the old ether-based conceptions are not completely dead, but exert a malign influence. E and h relationship. This was because he thought of empty space itself as full of a hard-to-detect medium, the ‘ether’. rbj said: ↑ you can define reality in terms of Planck Units and then every quantity measured really is unitless and dimensionless. This is, I suggest, why, even in the modern mks system, we are stuck with different units for D and E and for H and B. When materials are present it is useful to think of E as the sum of E due to charges which don’t form an integral part of the material itself, and E due to charges that do. without going to natural units, comparing the magnitude of actions from different fundamental forces is like comparing apples to oranges. So, for macroscopic purposes, we don’t need to calculate from first principles the contribution to E of the inherent charges in the material, as long as we know the value of the permittivity. We reject this custom inasmuch as is the truly fundamental field and is a subsidiary artifact. the gravitational attraction between two planets exceeds the electrostatic repulsion if they happened to be equally charged by a few elementary charges. I believe that this stress and strain, stimulus and response, conception is still held by some engineers, and perhaps by some physicists. [These electrons behave in accord with the basic laws of electromagnetism, so the explanation has a pleasing economy and 'closure'.] Where media are involved it is quite useful to have two electric vectors, D and E, and two magnetic vectors, H and B. it's what you use to look up the concept, even if it is now known that this concept sorta trancends the fine-structure splitting. Thank you for your interest. think of this in terms of the natural meaning of an inverse-square action. Now that we no longer believe in the ether, we surely need only one electric field vector and one magnetic field vector for electromagnetism in a vacuum. Thus I believe that some electrical engineers, and perhaps some physicists, regard B and H, and D and E in a medium as somehow analogous to stress and strain. What follows is a total rewrite of my last reply. D and H were, I suspect, deliberately defined to be different quantities from E and B, even in a vacuum. Dating latina. possibly to the point where they are not easily distinguishable. By using another vector, D, as well as E, we can write Maxwell’s equations in a way which doesn’t involve the charges integral to the material. I would only add: whatever system of units is used. I am glad we both agree that "[itex] \alpha [/itex] represents the strength of the electromagnetic action". They do not have the Lorentz viewpoint. marcusl said: ↑ Agreed, this is a nice feature of Gaussian units. The story of formulating electromagnetism without a metric starts in part IV. The ‘extra’ two vectors weren’t thought of as coming into their own just for materials. The frustrating thing is that it is probably no accident that D and H are defined to be different from E and B even in a vacuum.
Erowid. rbj said: ↑ actually, Meir, we need to be clear about a couple of things. For Maxwell, who didn’t know about electrons, there was no fundamental distinction between electromagnetism in a material medium and in empty space. In the case of electric fields they are related to stress and strain. that's true, but hard to conceptualize without thinking in terms of natural units. I am puzzled by your post, since I assume you read mine that you quote, and I think we are in almost complete agreement. I did mean to imply that alpha had somewhat different algebraic forms in different systems of units. [I'm considering only isotropic media.] Later Lorentz brilliantly explained the differences between what was observed in media and what was observed in a vacuum, in terms of the bound electrons in media. this is an arbitrary human decision. Now that we know that alpha is a standard ratio between many numbers in physics, it is probably time to not limit it to fine structure, and just call it alpha, but that is not an important point.
Dax Shepard Says Relationship With Kristen Bell Is Not 'Effortless' but They 'Work It Like a Job'. If these changes were made, then in a vacuum D and H would be the same vectors as E and B respectively. A key idea is that there are two independent field tensors that capture most of electromagnetism. In the presence of a metric or other structure, the two tensors are related.
E-plane and H-plane - Wikipedia. Weber and Julius Stratton who call H "magnetic intensity." okay, i meant to call H "flux density" like we call D. We shall call the magnetic field and leave the reader to deal with as he pleases. of atomic charges and currents." That is why E and B should be called fields, and why Mel Schwartz doesn't bother to even name H in his book. In a vacuum, there is clearly no need for the ‘supplementary’ vectors D and H, as there are no inherent charges or magnetic dipoles to take care of. the most common name for this quantity is the Fine-structure constant and it is proportional to the of the elementary charge, e. we have a point charge Q "emmiting" a total quantity of flux that is the same as the amount of charge Q.. actually, Meir, we need to be clear about a couple of things. The same sort of thing was done for magnetic fields, resulting in a vector H, additional to B, for use when magnetic materials are present. That is why I explicity mentioned the division by fourpiepsilonzero in SI. It might be better to join authors such as E. in my EE-based fields book "Hayt: Engineering Electromagnetics", D is "electrostatic flux density" and what comes out of the Gauss's Law integration is total "flux" which is proportional to the enclosed charge. It is, I think, partly responsible for why we are stuck with different units for E and D and for B and H. Meir Achuz said: ↑ Mel deserves his Nobel prize for that sentence alone. It is pre-Lorentzian, and essentially ether-based. rbj, there's reason to call B "field," but please don't call H "flux" because this is a different quantity as I had noted earlier in the thread you quoted