Ch34 - Physics 4B Lecture Notes Chapter 34 -...

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Physics 4B Lecture Notes 34-1 Chapter 34 - Electromagnetic Waves Problem Set #13 - due: Ch 34 - 2, 6, 8, 12, 16, 17, 20, 25, 28, 35, 45, 47 Since Maxwell's Equations summarize everything we know about electricity and magnetism, they should lead us to an understanding of the properties of electromagnetic waves. Lecture Outline 1. Producing and Detecting Electromagnetic Waves 2. Properties of Electromagnetic Waves 3. Maxwell's Equations and Waves in Free Space 4. The Electromagnetic Spectrum 5. The Poynting Vector 6. Radiation Pressure and Momentum Transfer 1. Producing and Detecting Electromagnetic Waves Electromagnetic waves can be produced by an oscillating circuit connected to an antenna as shown at the left. The oscillating charges in the antenna set up electric and magnetic fields. EM waves can be detected with an antenna as shown at the right. The charges in the antenna are forces to oscillate by the EM waves and the resulting voltages can be detected with a voltmeter. The waves are initially produced by the charges on the antenna. The charge itself produces an E-field and the motion of the charges (current) produces a B-field as shown at the left. Later when the current switches directions, the fields also switch directions near the antenna. However, the changing fields away from the antenna induce more fields according to Maxwell’s Laws of Electricity and Magnetism as shown at the right. The amazing thing is that these field are self sustaining. The changing magnetic field producing an electric field and the changing electric field produces a magnetic field. This is the nature of the EM waves shown at the left. L R C antenna EM Waves transformer antenna EM Waves V I I I I I
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Physics 4B Lecture Notes 34-2 2. Properties of Electromagnetic Waves • They continue to travel after the source is turned off. • They travel through empty space. • They always travel at the same constant speed. • The electric field is always perpendicular to the magnetic field. • The velocity is perpendicular to both the electric field and the magnetic field. • The ratio of the peak electric field to the peak magnetic field equals the speed of the waves. There are a couple of ways to represent these waves that illustrate these properties. The electric and magnetic fields oscillate in amplitude in space and they travel to the right as time goes on. This shows the varying strength of the fields, but it doesn't illustrate the fact that these waves are for all practical purposes infinite in the x and y direction. This illustration indicates the infinite extent in the x and y directions but it is hard to visualize that things are waving. Regardless of how you visualize the
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Ch34 - Physics 4B Lecture Notes Chapter 34 -...

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