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Cecil Moore wrote:
It has been said that the energy stored in the standing waves of a transmission line just "sloshes" around. We can demonstrate standing waves using a laser beam normal to a perfect mirror. There are points of maximum irradiance and points of minimum irradiance in the standing waves. So does the EM energy in the standing waves of light in free space "slosh" around like the energy in the standing waves in a transmission line? Yes -- there's energy actively bouncing around in that there beam; if you could reduce it down to one laser burst that was shorter than the distance between the laser and the mirror you'd (in theory at least) be able to see it. If so, where does the inductance and capacitance in free space come from to generate that 377 ohms of characteristic impedance? They don't. The behavior of EM radiation in free space is described by Maxwell's laws. The 377 ohms of characteristic impedance comes from the permittivity and permiability of free space but inductance and capacitance are only meaningful concepts if you have conductors in your model. If not, then why do the EM waves in a transmission line behave differently than the EM waves in free space? Because they're bounded by conductors. What different laws of physics do photonic waves in transmission lines obey than do photonic waves in free space? None. They obey Maxwell's laws. Of the E-field and H-fields rules for EM waves in free space, which of those rules are violated by EM waves in a transmission line? None. Is there one set of Maxwell's equations for free space and a separate set for transmission lines? No, just different boundary conditions to start. All this is covered in a good college E&M course. I wish I had an E&M book that I could recommend for self-study, but I don't. Mine is "Elements of Engineering Electromagnetics", but I took a course. I don't think I would have been able to just pick up the book and learn it from there. Did Maxwell ever mention the scientific concept of "sloshing"? Who knows? And was he talking about light waves or a wee dram of whiskey at the end of the day? As hard as it may be to believe for anyone who's gone through an E&M course the original form of Maxwell's equations were more difficult to comprehend than the way there're usually presented now -- the vector notation that is currently used either wasn't around then or wasn't in widespread use. ------------------------------------------- Tim Wescott Wescott Design Services http://www.wescottdesign.com |
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