lecture24 - Physics 2102 Gabriela Gonzlez Electromagnetic...

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Electromagnetic waves Physics 2102 Gabriela González
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A solution to Maxwell’s equations in free space: ) sin( t x k E E m ϖ - = ) sin( t x k B B m - = k = c , speed of propagation. mph s m B E c m m 163 , 187 954 , 462 , 299 1 0 0 = = = = ε μ Visible light, infrared, ultraviolet, radio waves, X rays, Gamma rays are all electromagnetic waves. Electromagnetic waves http://phys23p.sl.psu.edu/CWIS/
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Electromagnetic waves are able to transport energy from transmitter to receiver (example: from the Sun to our skin). The power transported by the wave and its direction is quantified by the Poynting vector . John Henry Poynting (1852-1914) 2 1 1 | | E c EB S 0 0 = = μ The Poynting vector E B S Units: Watt/m2 For a wave, since E is perpendicular to B: B E S × = 0 1 In a wave, the fields change with time. Therefore the Poynting vector changes too!!The direction is constant, but the magnitude changes from 0 to a maximum value.
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__ __________ 2 2 2 ___ ) ( sin 1 1 t kx E c E c S I m ϖ μ - = = = 0 0 The average of sin2 over one cycle is ½: 2 2 1 m E c I 0 = 2 1 rms E c I 0 = Both fields have the same energy density. B E u B cB E u = = = = 0 0 0 0 ε 2 0 2 2 2 1 ) ( 2 1 2 1 or, EM wave intensity, energy density A better measure of the amount of energy in an EM wave is obtained by averaging the Poynting vector over one wave cycle. The resulting quantity is called intensity . The total EM energy density is then 0 2 2 0 / B E u = =
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Solar Energy The light from the sun has an intensity of about 1kW/m2. What would be the total power incident on a roof of dimensions 8x20m? I=1kW/m2
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lecture24 - Physics 2102 Gabriela Gonzlez Electromagnetic...

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