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Unformatted text preview: Solutions to Homework #7 Chapter 25 38. Picture the Problem: To obtain optimal reception of a 66.0MHz TV signal, the length of the antenna should be equal to half a wavelength. Strategy: Set the length of the antenna equal to half a wavelength, where the wavelength is given in terms of the frequency using equation 254. Solution: Calculate the length of the antenna: ( 29 8 7 1 3.00 10 m/s 2.27 m 2 2 2 6.60 10 Hz c L f λ = = = = Insight: To listen to an FM radio station at 107 MHz the antenna should be 1.40 m long. 44. Picture the Problem: The radiation pressure exerted by beam of light 1 is half the radiation pressure of beam of light 2. Strategy: Use the expressions 2 I c E ε = (equation 2510) and av av pressure I c = (equation 2512) to determine the rms electric field in beam 2. Solution: We note from equation 25–12 that the radiation pressure depends on the intensity of a wave. The intensity, in turn, depends on the square of the fields, as equation 25–10 shows. Therefore, the electric field in beam 2 has a magnitude of 2 . E 2. (b) The intensity of an electromagnetic wave, on the other hand, depends on the square of the electric and magnetic fields. As a result, doubling the fields quadruples the intensity of the wave. We conclude the rms average intensity of the wave changes by a multiplicative factor of 4.00. Insight: The answer can also be determined by a ratio: av, 2 av, 2 av, 2 av, 2 2 2 2 1 1 av, 1 av, 1 av, 2 2 av, 1 pressure pressure 2 2 pressure pressure I c I c E E E E E E I c I c ε ε = = = = = = = 62. Picture the Problem: A 75.0W light bulb emits electromagnetic waves uniformly in all directions. Strategy: Use equation 147 to calculate the intensity of the light 2.5 m from the source. Insert the intensity into equation 2510 to calculate the rms electric field, and then solve equation 259 for the magnetic field....
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This note was uploaded on 02/10/2012 for the course PHY 102 taught by Professor Korotkova during the Spring '09 term at University of Miami.
 Spring '09
 KOROTKOVA
 Physics, Work

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