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標題:

Physics - Wave Propagation 01

發問:

A light source has an average power of 100W. Calculate the intensity of the light source at a point 6m from the source. P = AI 200 = 4π(6)^2 x I 我想問 4π(6)^2 點黎 ? 更新: 修正：100 = 4π(6)^2 x I

最佳解答:

The term [4π(6)^2 m^2 ] is the surface area of an imaginary sphere with radius 6 m and centred at the source. [surface area of sphere = 4π x radius^2] Intensity is defined as the energy falling on unit area in unit time. A 100 w light source gives 100 J in 1 sec. Assume no energy is lost up to 6 m, this amount of energy will spread over the surface of a sphere of radius 6 m with its centre at the position of the souce. Thus, the energy on unit area on the sphere surface = 100 J/4π(6)^2 m^2 = 100/4π(6)^2 J/m^2 But this amount of energy is emitted in 1 sec, hence intensity = 100/4π(6)^2 J/s-m^2 = 100/4π(6)^2 w/m^2

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