# Ch21s10 - Chapter 21 Electromagnetic Waves (Only 21.821.10...

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Chapter 21 Electromagnetic Waves (Only 21.8–21.10 and 21.12– 21.13)

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Four fundamental laws of electricity and magnetism prior to Maxwell s Electricity Gauss’s law: Φ E = Q enclosed 0 for any closed surface s Magnetism Φ B = 0 for any closed surface (magnetic charges do not exist) s Electricity Magnetism Ampère’s law: ∑B || = μ 0 I enclosed for any loop s Magnetism Electricity Faraday’s law: E =–N∆Φ B /∆t for any loops
Maxwell’s predictions s Maxwell postulated that Ampère’s law needs a second term, ∆Φ E /∆t , so that the four equations would become more “symmetric”. (In other words, the role of electricity in magnetism would become similar to that of magnetism in electricity: changing electric field produces magnetic field.) s Ampère’s law with Maxwell’s second term: ∑B || = μ 0 I enclosed + μ 0 ε 0 ∆Φ E /∆t μ 0 ε 0 is needed to make the units for the first and second terms the same. (The effect of the second term is usually so small that it took many years before its existence was directly confirmed by experiment.) s From the four equations, using calculus, Maxwell predicted the existence of electromagnetic waves (oscillating electric and magnetic fields, with each varying field inducing the other), which propagate at a peed . s This speed, 3x10 8 m/s, agreed with the known speed of light, leading Maxwell to conclude that light is nothing but just one type of electromagnetic wave s He also predicted: (1) whenever a charged particle accelerates, it radiates an EM wave; (2) EM wave carries energy and momentum 0 0 1 ε μ c =

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Hertz’s simple LC circuit (from book) s When the switch is closed, oscillations occur in the current and in the charge on the capacitor s When the capacitor is fully charged, the total energy of the circuit is stored in the capacitor s At this time, the current is zero and no energy is stored in the inductor
LC Circuit, cont (from book) s As the capacitor discharges, the energy stored in it decreases s At the same time, the current increases and the energy is stored in the inductor s When the capacitor is fully discharged, there is

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## This note was uploaded on 03/31/2010 for the course PHY 2054 taught by Professor Avery during the Spring '08 term at University of Florida.

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Ch21s10 - Chapter 21 Electromagnetic Waves (Only 21.821.10...

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