Lect25_[Compatibility_Mode]

Lect25_[Compatibility_Mode] - Physics 344 Foundations of...

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Physics 344 Foundations of 21 st Century Physics: Relativity, Quantum Mechanics and heir Applications Their Applications Instructor: Dr. Mark Haugan Office: PHYS 282 haugan@purdue.edu TA: Dan Hartzler Office: PHYS 7 dhartzle@purdue.edu Grader: Shuo Liu Office: PHYS 283 liu305@purdue.edu Office Hours: If you have questions, just email us to make an ppointment. e enjoy talking about physics! appointment. We enjoy talking about physics! Reading: Sections 24.1 and 24.2 in Matter & Interactions, Vol. II, 2 nd Edition
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inusoidal (Monochromatic) Electromagnetic Waves Sinusoidal (Monochromatic) Electromagnetic Waves We will use waves with specific frequencies and wavelengths to build generic wave fields by superposition. Before analyzing the structure and propagation of such waves in detail, we review what you know about the emission of ,y electromagnetic radiation to put them in context. Consider a positive charge that is given a sudden kick (acceleration) downward. a G This causes a rapid change in the electric field near the charge’s location, hich causes a rapid change in the magnetic field, which causes … which causes a rapid change in the magnetic field, which causes … The result is transverse electromagnetic wave that travels away from the location of the acceleration event at the speed of light.
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The magnitude of the transverse G ˆ electric field in a given direction is determined by the component of the charge’s acceleration that perpendicular to the direction a a G r 2 1 radiation qa E r = G G is perpendicular to the direction from the charge to the field point 0 4 cr πε Notice that as the wave travels farther from the source event its local structure becomes more and more like that of the plane “slab” wave we studied last time. Sinusoidal acceleration of a charge leads to the emission of a inusoidal electromagnetic wave radiation E G sinusoidal electromagnetic wave. This picture shows the wave structure traveling away from the harge in one direction radiation B G charge in one direction.
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his figure superimposes circular This figure superimposes circular “wavefronts” as surfaces on which the electric field component has a maximum positive value. Focusing on parts of these wavefronts at greater and reater distance from the so rce helps s see ho greater distance from the source helps us see how the wave becomes a locally plane wave. Far from the source, the electromagnetic field in relatively large regions (large compared to the yg g ( g p field’s wavelength) is essentially the same as that of the plane wave fields that we now study in detail. We use Maxwell’s equations to establish their structure and the way in which they propagate. Knowing this, we will be able to superpose them to predict the structure and behavior of more general wave fields.
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Question 1: The wavelength and frequency of an electromagnetic plane wave are related by A) B) C) c ν λ = c νλ= c = Answer: The field pattern moves forward one wavelength during each cycle of the electric or magnetic fields through their complete range of variation. The
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Lect25_[Compatibility_Mode] - Physics 344 Foundations of...

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