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Electromagnetic waves transport energy: The Poynting vector
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Electromagnetic energy density (Joules/m3)
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Poynting vector (Joules/ m2s)
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[Joules/ m2 s]
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S: power per unit area [Watt/ m2 ]
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Average power per unit area < S > : Light intensity
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Power per unit area [Watt/ m2 ]
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Intensity
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I = eo c Erms2
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Intensity
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Power per unit area [Watt/ m2 ]
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Intensity decreases with distance from the light source
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CASE: point source emitting energy all over the space
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Example: Consider an isotropic source of light with 250 W power. Calculate the electric field at 1.8 m from the center of the source
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Isotropic source
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Now we are left with plugging the corresponding values for eo and verify whether the expression is compatible with the corresponding units