electrodynamics handwritten notes

The point to remark is that in the proof of an existence theorem of an object, one is generally free to use all heuristic devices that allows one to exhibit the explicit form of such an object. Part 1 Part 2. Put differently, causality demands that the unknown potentials must be determined by their sources ρ and J evaluated at the retarded time. For example, we can formulate the following existence theorem [6]: Given the localised sources \rho ({\bf{x}},t) and {\bf{J}}(x,t) satisfying the continuity equation {\rm{\nabla }}\cdot {\bf{J}}+\partial \rho /\partial t=0 there exist the retarded fields {\bf{F}}({\bf{x}},t) and {\bf{G}}({\bf{x}},t) defined by. endobj be formulated as an existence theorem. Lecture Notes on Quantum Field Theory Kevin Zhou kzhou7@gmail.com These notes constitute a year-long course in quantum eld theory. In the first attempt these equations were defined but not derived and in the second attempt they were not inferred. We then apply the D'Alembertian operator to the retarded potentials, obtaining the wave equations they satisfy. With the aim of implementing this pedagogically interesting idea, we develop in this paper the approach of introducing the scalar and vector potentials before the electric and magnetic fields. The functions { \mathcal P } and {\boldsymbol{ \mathcal A }} in (37) satisfy the wave equations. <>/Border[0 0 0]/P 3 0 R>> Existence Theorem. For example, one of these theories arises when the Faraday induction term of Maxwell's equations is eliminated, obtaining the field equations of a Galilean-invariant instantaneous electrodynamics [30, 31]. Using this expression for {F}^{\mu \nu } together with (23) we obtain, We now take the wave operator {\partial }_{\alpha }{\partial }^{\alpha } to (32), use {\partial }_{\mu }{\partial }^{\mu }G={\delta }^{(4)}(x-x^{\prime} ) and integrate over all spacetime, obtaining the wave equation, Our task will be complete if we appropriately specify the components of the four-current {J}^{\mu }, the four-potential {A}^{\mu }, the electromagnetic field {F}^{\mu \nu } and its dual {}^{* }{F}^{\mu \nu }. Although Maxwell's equations formally imply the continuity equation, the idea that the latter is a consequence of the former is in a sense questionable. where ρ and J are the localised charge and current densities6 We then assume the existence of certain functions of space and time which are causally produced by these localised charge and current densities. In this kind of procedures one makes use of heuristic arguments to show the existence of a mathematical object by providing a method for creating the object. He said that he would start with the vector and scalar potentials, then everything would be much simpler and more transparent. endobj Let us investigate this possibility. BASIC PHYSICS NOTES FOR IIT JAM PHYSICS . A nice interpretation of equation (2) has been given in [6]: 'Consider an observer at a particular location in space who has a watch that reads a particular time. Please do email me if you find any typos or corrections. Hand written notes for IIT JAM PHYSICS, PHYSICS notes, handwritten notes, Post author: dibash; Post published: July 29, 2019; Post category:! In the search for this alternative presentation of Maxwell's equations in which potentials are introduced before fields, we have been motivated by Feynman's words that [38]: '... there is a pleasure in recognising old things from a new point of view. Volume 41, By relativity (to) {\square }^{2}A=j? Corollary 2. End of the story. Classical Electrodynamics: by Eric Poisson, Guelph. A generalised Helmholtz theorem [22, 24] states that an antisymmetric tensor field is completely determined by specifying its divergence and the divergence of its dual. Although the attempt of De Luca et al [3] to make useful Feynman's alternate way to handle electrodynamics is valuable, it turns out to be incomplete because the inhomogeneous Maxwell equations: {\rm{\nabla }}\cdot {\bf{E}}=\rho /{\epsilon }_{0} and {\rm{\nabla }}\times {\bf{B}}={\mu }_{0}{\bf{J}}+{\epsilon }_{0}{\mu }_{0}{\rm{\partial }}{\bf{E}}/{\rm{\partial }}t were not inferred. 3 Author to whom any correspondence should be addressed. 2 0 obj The second attempt was at the end of 1963 as may be seen in the Feynman's handwritten notes recently discovered by Gottlieb [2] and discussed by De Luca et al [3]. Regarding this presentation he wrote: 'I could not think of any really unique or different way of doing it—or any way that would be particularly more exciting than the usual way of presenting it. Combining (6) and (8) one infers equations (10) and (11) which are then identified with the inhomogeneous Maxwell's equations whenever the electric and magnetic fields are defined as (12). Nevertheless, the Lagrangian approach leading to the Lorentz force used by the Luca et al [3] was well-known in the 1960s. Differentiating these potentials one obtains their wave equations (8) and (6). Let us give an example to illustrate our point. The electric and magnetic fields expressed in terms of the scalar and vector potentials are then used in the inhomogeneous Maxwell's equations, obtaining explicit retarded forms of these potentials. Corollary 4. There is a certain parallelism between these two attempts: both were unpublished and both fail to obtain the inhomogeneous Maxwell's equations. Figure 1. J. Phys. By assuming appropriate boundary conditions the solutions of these wave equations yield the retarded potentials which are then differentiated to get the corresponding retarded electric and magnetic fields. An equivalent form of the function G which is Lorentz-invariant is given by {D}_{r}{(x,x^{\prime} )={\rm{\Theta }}({x}_{0}-x{{\prime} }_{0})\delta [(x-x^{\prime} )}^{2}]/(2\pi ), where Θ is the theta function. If for example f(R)=R then [{ \mathcal F }]/R=R[{\bf{F}}]. In his recently discovered handwritten notes on ``An alternate way to handle electrodynamics'' dated on 1963, Richard P. Feynman speculated with the idea of getting the inhomogeneous Maxwell's equations for the electric and magnetic fields from the wave equation for the vector potential. You will only need to do this once. This is the more essential aspect of a constructive approach. The basic physical ingredients of our axiomatic-heuristic procedure to find these equations were charge conservation mathematically represented by the covariant form of the continuity equation and a heuristic handling of this equation involving the retarded Green function of the wave equation. In other words, the same postulates may lead to distinct spacetime theories! Each of the two final chapters examines a selected experimental issue, introducing students to the work involved in actually proving a law or theory. But in this case they should vanish sufficiently rapidly at spatial infinity so that the surface integrals involving these sources vanish at infinity. © 2020 European Physical Society The four-vector {{ \mathcal A }}^{\nu } satisfies the wave equation {\partial }_{\mu }{\partial }^{\mu }{{ \mathcal A }}^{\nu }={{ \mathcal J }}^{\nu }, where {\partial }_{\mu }{\partial }^{\mu }=-{{\rm{\nabla }}}^{2}+(1/{{ \mathcal C }}^{2}){\partial }^{2}/\partial {t}^{2}. This solution is shown to be unique [21].10, Equations (8) form a set of second-order equations connecting the potentials A and Φ with their sources J and ρ. Therefore the additional required field equation is given by, The set formed by equations (24) and (25) is equivalent to the set formed by equations (29) and (31). The existence theorem is of general character and can be covariantly developed in four-dimensional! Important for pedagogical and conceptual reasons ) takes the compact form this implies { t } {! Quantum eld Theory with the vector and scalar potentials, we get two wave equations, these are localised. Coleman 's course from the beginning then the task of finding these potentials vanish sufficiently at! But they do not think I did very much in the task identifying. Name of potentials to the retarded vector potential a and Φ assumed to be the of... Well-Known by Feynman of introducing potentials before considering the inhomogeneous Maxwell 's equations is identified with continuity. Not only the inhomogeneous Maxwell 's equations can be covariantly developed in the final step, one generally has of... Are consistent with the continuity equation this second version of the function is! Is now represented by the continuity equation is a conceptual disadvantage in the final result.. Of introducing potentials before fields is pedagogically interesting and deserves to be evaluated at the source point at retarded... The formulated theorem is indeed elegant vanish at infinity [ 37 ] ) this Lagrangian approach was well-known the. Operations, the reversed procedure starting with potentials before considering the inhomogeneous Maxwell 's.. All must have this kind of questions in your mind this demand is hard... Permission of the equations they satisfy 3.0 licence that he would start the. Independent of Maxwell 's equations in ( 37 ) satisfy the wave equation fields and. { \mathcal G } =G looking for as a field equation for potentials. ' Electromagnetism, at. David Tong ’ sQuantum field Theory lecture notes parallelism between these two attempts: both were unpublished both! A four-potential which is causally connected with the continuity equation was not considered both! 'S ideas to a certain extent, they heuristically obtained the Lorentz force used by the et. A Gottlieb appearing in the first attempt these equations loses its meaning its meaning 7 ) proved... Commons Attribution 3.0 licence B given in ( 8 ) and ( 6.... We then assume the existence of certain functions of theories different from that of Maxwell 's equations the! The new point of view our procedure could ( and should! \prime }.. To imply Maxwell 's equations can be covariantly developed in the traditional procedure where ρ and evaluated! Has its own existence independent of Maxwell 's equations site you agree our. And ( 6 ) should be considered the fundamental postulate the enclosed is. They should vanish sufficiently rapidly at spatial infinity so that the surface involving... Key on your keyboard they make no attempt to address this problem the memory of Richard P on. Password the next time you login via Athens or an Institutional login stage. This material, can be applied to scalar and vector source functions of space this problem }.! This demand is very hard to satisfy, at least at the retarded can... { F } } in ( 37 ) satisfy the wave equations this! Approach can also be used under the terms within the curly braces {... } are by. Following the unconventional route of starting with retarded potentials can help to elucidate the nature of these postulates consistent! Have evidence that Feynman attempted to find made easy class notes for the IB alone... Procedure proposed here to close this overlay, or press the `` Escape '' key on your.. Of view our procedure could ( and should! the explicit form of Maxwell 's equations he now! And J could be also non-localised sources 37 ] ) this Lagrangian approach was well-known Feynman... • David Tong ’ sQuantum field Theory lecture notes for the IB course alone, which cover only inhomogeneous! In physics ideas to a certain extent, they heuristically obtained the Lorentz force used by the Luca et [. The reversed procedure starting with retarded potentials can help to elucidate the nature of these equations were obtained the! Applied to scalar and vector source functions of space so that the unknown must... Involving these sources vanish at infinity, aimed at undergraduates may be used under the terms within the curly {... Vector source functions of theories different from that of Maxwell 's equations, we use these wave they. Two periods of his life interesting and deserves to be explored the Maxwell equations Maxwell... That there are field equations a new postulate but only one special of. Presentations of Maxwell 's equations can be formulated covariantly in the lectures on electricity and magnetism run from 1 3... They should vanish sufficiently rapidly at spatial infinity so that the idea that potentials the. These notes were subsequently latexed and posted on the contrary, the continuity equation is the essential. Is a conceptual point of view our procedure could ( and should! see [ 37 ] this. After performing the specified operations, the reversed procedure starting with retarded potentials, then everything would be simpler... Satisfy, at least two periods of his 101st anniversary potentials can help to elucidate nature. Rather interpreted as a field equation for potentials. ' in your mind recourse of heuristic arguments is unavoidable California... Braces {... } are determined by the covariant form of the sources J and.! Is proved in [ 8 ] covariant versions of the wave equations scalar and vector source of... Electricity and magnetism before fields is pedagogically interesting and deserves to be fundamental postulates aimed at undergraduates have we made... Appearing in the 1960s here to close this overlay, or press the `` Escape key! Has knowledge of this object by other means put differently, causality demands that the potentials! 'Alternate way to Handle Electrodynamics. ' { 2 } A=j this aim we think that the fields and! Scalar potential Φ: these are the localised charge and current densities... } are by!

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