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# ch21-sol - Chapter 21 Solution 21.43 From v = f the...

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Chapter 21 Solution 21.43 From v f λ = , the wavelength is 8 6 3.00 10 m s 4.00 10 m 4000 km 75 Hz v f λ × = = = × = The required length of the antenna is then, 4 1000 km L λ = = , or about 621 miles. Not very practical at all. 21.45 (a) The frequency of an electromagnetic wave is f c λ = , where c is the speed of light, and λ is the wavelength of the wave. The frequencies of the two light sources are then Red: 8 14 -9 3.00 10 m s 4.55 10 Hz 660 10 m re f d red c λ × = = = × × and Infrared: 8 14 -9 3.00 10 m s 3.19 10 Hz 940 10 m IR f IR c λ × = = = × × (b) Ignore 21.47 The distance between adjacent antinodes in a standing wave is 2 λ Thus, ( ) 2 6.00 cm 12.0 cm 0.120 m λ = = = ( ) , and ( ) 9 8 0.120 m 2.45 10 Hz c f λ = = × 2.94 10 m s = × 21.48 At Earth’s location, the wave fronts of the solar radiation are spheres whose radius is the Sun-Earth distance. Thus, from av av 2 4 Intensity A r π = = Ã Ã , the total power is ( ) ( ) ( ) 2 2 11 av 2 W 4 1340 4 1.49 10 m 3.74 10 W m Intensity r π π = = × =

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ch21-sol - Chapter 21 Solution 21.43 From v = f the...

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