P18_025 - I = U/tA = uv . (b) The power output P of the...

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25. (a) We take the wave to be a plane wave and consider a region formed by the surface of a rectangular solid, with two plane faces of area A perpendicular to the direction of travel and separated by a distance d , along the direction of travel. The energy contained in this region is U = uAd .I ft h e wave speed is v then all the energy passes through one end of the region in time t = d/v .T h e energy passing through per unit time is U/t =
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Unformatted text preview: I = U/tA = uv . (b) The power output P of the source equals the rate at which energy crosses the surface of any sphere centered at the source. It is related to the intensity I a distance r away by P = AI = 4 πr 2 I , where A (= 4 πr 2 ) is the surface area of a sphere of radius r . Substitute I = uv to obtain P = 4 πr 2 uv , then solve for u : u = P 4 πr 2 v = 50 , 000 W 4 π (480 × 10 3 m) 2 (3 . 00 × 10 8 m / s) = 5 . 76 × 10 − 17 J / m 3 ....
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This note was uploaded on 11/12/2011 for the course PHYS 2001 taught by Professor Sprunger during the Fall '08 term at LSU.

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