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Halliday 2 Cap 18 - P18 093

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    December 2018
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93. The source being isotropic means Asphere = 4πr2 is used in the intensity definition I = P/A. Since intensity is proportional to the square of the amplitude (see Eq. 18-27), this further implies I2 = I1  sm2 sm1 2 = P/4πr22 = P/4πr12  r1 r2 2 or sm2 /sm1 = r1 /r2 . (a) With I = P/4πr2 = (10 W)/4π(3.0 m)2 = 0.088 W/m2 . (b) Using the notation A instead of sm for the amplitude, we find 3.0 m A4 = A3 4.0 m =⇒ A4 = 3 A3 . 4