Physics II chapter 32 notes

Physics II chapter 32 notes - Chapter 32: Electromagnetic...

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Chapter 32: Electromagnetic Waves In this chapter… Maxwell’s equations admit wave solutions let there be light! production of electromagnetic waves, and the ‘spectrum’ thereof properties of electromagnetic waves in vacuum energy transport by electromagnetic waves Production of Electromagnetic Waves (32-1) James Clerk Maxwell predicted (1865) and Heinrich Hertz demonstrated (~1889) that the set of 4 “Maxwell’s equations” we have studied admit wave-like solutions 0 00 0 1 0 SV S CS c S EdA d V BdA d Ed l dt d Bd l j dA dt      v6.0 RS/BQ F’10 131 (Gauss's law) (Gauss's law for magnetism; no monopoles) (Faraday's law) (Ampere's law) Maxwell’s equations tell us that a charge at rest produces a static electric field but no magnetic field a charge in uniform motion produces both an E and B field for a point charge to produce an electromagnetic wave, the charge must accelerate a time-varying E -field creates B -field and vice-versa we want to show that once an E and B have been set up with the proper orientation and phase, then the “disturbance” that made them propagates off to infinity as a wave
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qualitative picture of creating an electromagnetic wave: a point charge oscillating in simple harmonic motion produces em waves a “dipole antenna” pumped by an AC generator creates radio waves v6.0 RS/BQ F’10 132 In the “near field” we get a complicated field pattern, while in the “far field” we get a simple traveling wave pattern “radiating” to infinity B I t = 0 Q = 0, initially, but current flows, creating a B E +Q t = T/4 no current, maximum charge separation, creating an -Q E B I t = T/2 max. current, no charge separation, creating an B E t = 3T/4 -Q +Q no current, maximum charge separation, creating an E
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detecting electromagnetic waves with linear dipole antenna (straight conductor) parallel to E to induce an alternating current in antenna with conducting loop antenna perpendicular to B so changing magnetic flux through loop induced current in loop direction of the E field defines the polarization direction of the wave microscopically, electromagnetic waves are produced whenever a free electric charge accelerates electrons bound to atoms or molecules make transitions from higher to lower quantum-mechanical energy states the spectrum of such waves encompasses radio waves, microwaves, visible light, infrared and ultraviolet radiation, x-rays, and gamma rays microwaves: cooking, cellular phone communication, weather radar, MRI’s infrared: night vision, motion detection, remote controls x-rays: invaluable for medical and dental purposes the same as with mechanical waves, we have a simple relation among three key parameters: wavelength, frequency and speed are always related by vf speed v
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Physics II chapter 32 notes - Chapter 32: Electromagnetic...

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