2010 PHYSICS 002C Lecture 10

2010 PHYSICS 002C Lecture 10 - PHYSICS 002C Lecture 10...

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PHYSICS 002C Lecture 10 April 19, 2010 Serway and Jewett Chapter 24.4-7 – Electromagnetic Waves DEMO 1: Hertzian waves from a 220 MHz oscillator. Standing wave measurements reveal the wavelength. Wire with a gap Clicker Q1: Heinrich Rudolf Hertz discovered a) the wave equation; b) electromagnetic waves; c) light; d) Maxwell’s equations; e) vacuum polarization. Clicker Q2: The intensity of a light wave is obtained from the time average of the a) Pionting vector; b) Paynting vector; c) Puntyng vector d) Poynting vector; e) Pontying vector. Chap 24.4 Hertz’s (1857-1894) discoveries LC resonator used by Hertz was A capacitor C with charge Q and stored energy 2 1 2 1 Q C U Gauss’ Law: . 0 2 /  Q E r A d E Voltage across capacitor: Ed V Thus CV V d r Q 2 0  or Q CV An inductor L carrying current I and storing energy 2 2 1 LI K Faraday’s Law: . I Bl z d B 0 Ampère’s Law: dt dB a dt d V s d E 2 . Thus dt dI L dt dI l a V 2 0  or dt dI L V Put them together and what have you got? 2 2 2 2 dt V d LC dt Q d L dt dI L V The Harmonic Oscillator 0 2 2 LC V dt V d ! The total energy 2 2 1 2 1 2 1 LI Q C U K E slops back and forth between the L and the C just as in the mechanical harmonic oscillator, at an angular frequency which is the square root of the ratio of the springy item ( C -1 ) to the to the inertial item ( L ), LC L C 1 1 1
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In 1888 Hertz excited his LC circuit with an induction coil (transformer) by periodically interrupting and reconnecting the primary current. Hertz detected electromagnetic radiation at a distance from the transmitter by using an identical passive resonator loop with a tiny spark gap to make it sensitive to weak oscillating electromagnetic fields. Radio transmission is relatively easy compared to signaling with light
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2010 PHYSICS 002C Lecture 10 - PHYSICS 002C Lecture 10...

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