Lecture_25_(Chap.24)

# Lecture_25_(Chap.24) - Fi Final exam Thursday May 5 7-9 PM...

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i l Final exam: hursday May 7 - M Thursday, May 5, 7 9 PM LILY 3118 Covers all chapters: 14-25. Somewhat more emphasis on Chap. 22-25. All multiple choice problems. 1

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• Online course evaluations began Monday, pril 18. April 18. 2
Stability of Atoms Circular motion: Is there radiation emitted? v a Classical physics says “YES” rotating particle must lose energy! speed decreases p particle comes closer to center lassical model of atom: Classical model of atom: Electrons should fall on nucleus! To explain the facts - introduction of quantum mechanics: Electrons can move around certain orbits only and emit E/M radiation only when jumping from one orbit to another 3

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Sinusoidal Electromagnetic Radiation Acceleration: f  t y dt y d a sin 2 max 2 2 f T / 1 2 r c a q E radiative 2 0 4 1  ˆ n 1 2 ax qy   j sin 4 2 max 0 t r c E radiative Sinusoidal E/M field 4
Sinusoidal E/M Radiation: Wavelength Instead of period can f use wavelength: cT c f T / 1 2 f Freeze picture in time: Example of sinusoidal E/M di ti radiation: radio stations E/M noise from AC wires 5

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Energy and Momentum of E/M Radiation a q 1 r c E radiative 2 0 4  1. The direction of the field is opposite to qa 2. The electric field falls off at a rate 1/ r According to particle theory of light: h t h d t Photons have energy and momentum Classical E/M model of light: E/M radiation must carry energy and momentum 6
Energy of E/M Radiation A particle will experience electric rce during a short time /c: force during a short time d/c: qE F elec p p 0 F elec t qE d c hat will happen with the ball? What will happen with the ball? It will oscillate Energy was transferred from E/M field to the ball qEd p 1 2 2 Amount of energy in m c m K K 2 2 0 the pulse is ~ E 2 7

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Energy of E/M Radiation Ed 2 Ball gained energy: m c qEd K 2 1 ulse energy must decrease Pulse energy must decrease ) (J/m 3 2 2 0 1 1 1 B E olume Energy 0 2 2 Volume E/M radiation: E=cB 2 0 0 2 0 2 0 2 0 1 1 2 1 1 2 1 2 1 c E c E E Volume Energy 2 0 E 8
Energy Flux There is E/M energy stored in the pulse: ) (J/m 3 2 0 E Volume Energy ulse moves in space: Pulse moves in space: there is energy flux Units: J/(m 2 s) = W/m 2 During t : Energy   0 E 2 A c

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Lecture_25_(Chap.24) - Fi Final exam Thursday May 5 7-9 PM...

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