3.2 EM Waves

3.2 EM Waves - EM radiation is produced by accelerating...

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1 EM waves 3.2 • Producing EM waves • Electromagnetic Spectrum • Energy in EM waves EM radiation is produced by accelerating charges. example Radio waves Evolution of the E and B fields Time-> + + - - E field lines B field lines at t=0 Dipole Antenna - + Oscillating Dipole +q -q d(t) Electric field lines radiation to the dipole axis No radiation || to the dipole axis (Why?) Electric Field lines Dipole antenna λ /2 λ /4 Half-wave antenna Quarter wave antenna Question A cell phone uses a frequency of 2.0 GHz. Find the length of the quarter wave antenna. 1) 3.8mm 2) 0.38 cm 3) 3.8 cm 4) 38 cm Interactions of EM radiation with an Antenna. • An optimal antenna to capture energy from an EM wave has a size close λ . ( λ /2 for a dipole antenna) Oscillating Electric field drives the movement of charges in a conductor E + -
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2 Passage of EM radiation through holes in a conductor • EM waves pass easily through holes in a conductor that are larger than λ but are blocked by holes smaller than λ .
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This note was uploaded on 06/01/2009 for the course PHYS phys2c taught by Professor Okamura during the Spring '09 term at UCSD.

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3.2 EM Waves - EM radiation is produced by accelerating...

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