CourseNotes.56 - and the fields are always in phase with each other E = E m sin kx-ωt B = B m sin kx-ωt • EM waves in vacuum always travel at

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Figure 37: The Electromagnetic Spectrum. 14.1 Traveling Electromagnetic Waves As we have seen already, changing magnetic fields induce electric fields and changing electric fields induce magnetic fields. The basic idea of an electromagnetic wave is that an oscillating field in an antenna for example, induces a changing electric field which travels off, or radiates, at the speed of light together with a changing magnetic field. We will discuss the general properties here, but for a mathematical discussion of the specifics one can refer to the textbook. The general structure of an EM wave can be seen in figure 38. The general properties are The electric and magnetic waves ~ E and ~ B are always perpendicular to the direction of travel. The electric and magnetic waves are always perpendicular to each other. The fields vary sinusoidally in space and time, just like the waves on a string discussed earlier,
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Unformatted text preview: and the fields are always in phase with each other. E = E m sin( kx-ωt ) B = B m sin( kx-ωt ) • EM waves in vacuum always travel at the same speed, c , which is called the speed of light. This speed is given by the constants that we have been working with all semester. c = 1 √ μ ± (50) • The ratio of the amplitudes of the electric and magnetic waves is fixed in terms of this speed. E m B m = c (51) 14.2 Energy and Momentum in EM Waves Anyone who has been out in the sun for a while or stood next to a campfire at night knows that em waves carry energy. The warmth that we feel in these two cases is the energy from these waves begin deposited in our skin. The quantity which describes the energy carried by an em wave is the Poynting vector. Poynting is the last name of the physicist who first discussed its properties, but it turns out to be aptly 56...
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This note was uploaded on 12/05/2011 for the course PHY 2049 taught by Professor Any during the Spring '08 term at University of Florida.

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