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214-31_Lec-22

# 214-31_Lec-22 - Physics 241 Lecture 22 Y E Kim Chapter 31...

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Physics 241 Lecture 22 Y. E. Kim November 11, 2010 Chapter 31, Sections 7-10 November 12, 2010 Physics for Scientists & Engineers, Chapter 31 1

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Traveling Electromagnetic Waves Poynting Vector and Energy Transport Radiation Pressure Polarization Electromagnetic Waves
November 12, 2010 University Physics, Chapter 31 3 Traveling Electromagnetic Waves (1) Electromagnetic waves can be produced by an oscillator connected to an antenna The connection between the circuit on the left and the circuit on the right is accomplished using a transformer A dipole antenna is used to approximate an electric dipole The voltage and current in the antenna vary sinusoidally with time and cause charge in the antenna to oscillate with frequency of the circuit The electromagnetic waves created by moving charges travel from the antenna with speed c and frequency f = /(2 )

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November 12, 2010 University Physics, Chapter 31 4 Traveling Electromagnetic Waves (2) We can think of these traveling electromagnetic waves as wave fronts spreading out spherically from the antenna However, at a large distance from the antenna, the wave fronts will appear to be almost flat, or planar So we can use the electromagnetic wave in terms of our assumed form of the plane wave If we now place a second RLC circuit (as a receiver) at a distance and tune it to the same frequency 0 as the emitting circuit, then voltage and current will be induced in this second circuit This principle of transmission of electromagnetic waves is used to transmit wireless signals     max max ( , ) sin ( , ) sin E r t E kx t B r t B kx t 

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November 12, 2010 University Physics, Chapter 31 6 Energy Transport (1) Previously we learned that the energy density of an electric field is given by and the the energy density of a magnetic filed is given by If we substitute in We get So the total energy density E cB 00 1 c    2 2 2 0 1 1 1 2 2 2 EB B u cB B u     2 0 1 2 E uE 2 0 / (2 ) B uB 2 0 /2 E uu 0 0 2 2 0 B EB u u E c E 2 2 2 0 0 2 0 0 0 0 rms rms rms rms E B E B u E c B c E B 
Energy Transport (2) 00 1 rms rms EB E P I u c B A      • Intensity I =Average energy flux density (W/m 2 ) 0 0 rms rms c u c U U P U uP Volume Ac t Ac t  P u A c  P A Energy flux density P/A= Energy transmitted through unit time per unit area

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November 12, 2010 University Physics, Chapter 31 8 Energy Transport (3) The rate of energy transported by an electromagnetic wave is usually defined as This quantity is called the Poynting vector after British physicist John Poynting who first discussed its properties For an electromagnetic wave, in which B is perpendicular to E , we can write S is the instantaneous power per unit area The units of the Poynting vector are W/m 2 0 1 S E B  0 1 S EB 0 1 P I u c A EB 
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214-31_Lec-22 - Physics 241 Lecture 22 Y E Kim Chapter 31...

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