Читать книгу Introduction To Modern Planar Transmission Lines - Anand K. Verma - Страница 23

At this stage of developments in the EM‐theory, the electric field was described in terms of the scalar electric potential, and the magnetic field was described by the magnetic vector potential. Several laws were in existence, such as Faraday's law, Ampere's law, Gauss's law, and Ohm's law. Now Maxwell, Newton of the EM‐theory, arrived at the scene to combine all the laws in one harmonious concept, i.e. in the Dynamic Electromagnetic Theory. He introduced the brilliant concept of the displacement current, created not by any new kind of charge but simply by the time‐dependent electric field. Unlike the usual electric current supported by a conductor, this new current was predominantly supported by the dielectric medium. However, both currents were in a position to generate the magnetic fields. Thus, Maxwell modified Ampere's circuital law by incorporating the displacement current in it. The outcome was dramatic; the electromagnetic wave equation. Despite such success, the concept and physical existence of displacement current created a controversy that continues even in our time, and its measurement is a controversial issue [J.6–J.8].

In the year 1856, Maxwell formulated the Faraday's law of induction mathematically, and modified Ampere's circuital law in 1861 by adding the displacement current to it. Finally in 1865 after a time lag of nearly 10 years, Maxwell could consolidate all available knowledge of the electric and magnetic phenomena in a set of 20 equations with 20 unknowns. However, he could solve the equations to get the wave equations for the EM‐wave with velocity same as the velocity of light. Now, the light became simply an EM‐wave. In the year 1884, Heaviside reformulated the Maxwell equations in a modern set of four vector differential equation. The new formulation of Maxwell equations was in terms of the electric and magnetic field quantities and completely removed the concept of potentials, considering them unnecessary and unphysical. Hertz has independently rewritten the Maxwell equation in the scalar form using 12 equations without potential function. Hertz worked out these equations only after Heaviside. In 1884, Poynting computed the power transported by the EM‐waves. Recognizing the contributions of both Heaviside and Hertz in reformulating Maxwell's set of equations, Lorentz called the EM‐fields equations Maxwell–Heaviside–Hertz equations. However, in due course of time, the other two names were dropped and the four‐vector differential equations are now popularly known as “Maxwell’s Equations” [J.1, J.6, J.9, J.10, B.5–B.7].