Lecture_19

Lecture_19 - 24.3 The Speed of Light Very fast!.but finite....

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24.3 The Speed of Light Very fast!!!!. ....but finite. m/s 10 00 . 3 8 × = c Moon to Earth 1.3 seconds Sun to Earth 8 minutes Distant stars and other astronomical objects are so far away that astronomers use a unit of distance called the light year (ly). 1 ly = The distance light travels in 1 year = 9.5 × 10 15 m
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24.4 Energy Carried by EM Waves EM waves carry energy just like any other wave. yg y j y An EM wave consists of both an electric and magnetic field, and energy is contained in both fields. A measure of the energy stored in an electric field is given by the energy density: 2 1 l Energy Electrical Density Energy Electrical E o κε = = 2 Volume κ is the dielectric constant, and it equals 1 for a vacuum. So in a vacuum, 2 2 1 Density Energy Electrical E o ε = We find a similar expression for the energy density in the magnetic field: otice that the 2 2 1 Density Energy Magnetic B o μ = Notice that the energy goes as the square of the fields.
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So the total energy density in an EM wave is the sum of these two: 2 2 2 1 2 1 B E u o o μ ε + = But in an EM wave, the electric field and magnetic field carry the same energy.
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Lecture_19 - 24.3 The Speed of Light Very fast!.but finite....

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