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Unformatted text preview: hapter 21 PHY213 1 Chapter 21 (cont) Electromagnetic Waves (Ch. 21.8-21.13) Note: There will be no quantitative questions on Doppler effect James Clerk Maxwell s 1831 – 1879 s Electricity and magnetism were originally thought to be unrelated s in 1865, James Clerk Maxwell provided a mathematical theory that showed a close relationship between all electric and magnetic phenomena More of Maxwell’s Contributions s Electromagnetic theory of light s Kinetic theory of gases s Nature of Saturn’s rings s Color vision s Electromagnetic field interpretation s Led to Maxwell’s Equations Maxwell’s Starting Points s Electric field lines originate on positive charges and terminate on negative charges s Magnetic field lines always form closed loops – they do not begin or end anywhere s A varying magnetic field induces an emf and hence an electric field (Faraday’s Law) s Magnetic fields are generated by moving charges or currents (Ampère’s Law) hapter 21 PHY213 2 Maxwell’s Predictions s Maxwell used these starting points and a corresponding mathematical framework to prove that electric and magnetic fields play symmetric roles in nature s He hypothesized that a changing electric field would produce a magnetic field s Maxwell calculated the speed of light to be 3x10 8 m/s s He concluded that visible light and all other electromagnetic waves consist of fluctuating electric and magnetic fields, with each varying field inducing the other Hertz’s Confirmation of Maxwell’s Predictions s 1857 – 1894 s First to generate and detect electromagnetic waves in a laboratory setting s Showed radio waves could be reflected, refracted and diffracted s The unit Hz is named for him Hertz’s Basic LC Circuit 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 electric field of the capacitor s At this time, the current is zero and no energy is stored in the inductor LC Circuit, cont s As the capacitor discharges, the energy stored in the electric field decreases s At the same time, the current increases and the energy stored in the magnetic field increases s When the capacitor is fully discharged, there is no energy stored in its electric field s The current is at a maximum and all the energy is...
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