P18_018 - r = 1 . 0 m), or it may be done by setting up a...

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18. (a) Since intensity is power divided by area, and for an isotropic source the area may be written A =4 πr 2 (the area of a sphere), then we have I = P A = 1 . 0W 4 π (1 . 0m) 2 =0 . 080 W / m 2 . (b) This calculation may be done exactly as shown in part (a) (but with r =2 . 5 m instead of
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Unformatted text preview: r = 1 . 0 m), or it may be done by setting up a ratio. We illustrate the latter approach. Thus, I I = P/ 4 ( r ) 2 P/ 4 r 2 = r r 2 leads to I = (0 . 080 W / m 2 )(1 / 2 . 5) 2 = 0 . 013 W / m 2 ....
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