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Lecture24

# Lecture24 - Physics 344 Foundations of 21st Century Physics...

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Physics 344 Foundations of 21 st Century Physics: Relativistic and Quantum Systems Instructor: Dr. Mark Haugan Office: PHYS 282 [email protected] TA: Dan Hartzler Office: PHYS 7 [email protected] Grader: Fan Chen Office: PHYS 222 [email protected] Office Hours: If you have questions, just email us to make an appointment. We enjoy talking about physics! Help Session: Thursdays 2:00 – 4:00 in PHYS 154 Reading: M&I Volume II sections 23.2 through 23.5 2 nd edition or M&I Volume II sections 24.2 through 24.5 3 rd edition

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Light: Waves and Particles When we discussed Compton scattering recently we used the photon model and 4-momentum conservation to analyze the scattering of extremely high frequency electromagnetic waves, gamma rays, by electrons. on q by field F qE qv B = + × a a a a On the other hand, you learned last semester that such radiation consists of electromagnetic wave fields that satisfy Maxwell’s equations, are generated by accelerated charges and that exert forces on charges as described by the Lorentz force law incident wave scattered (re-radiated) wave incident wave plus re-radiated wave The wave model of such fields explains scattering as a process in which an incident wave accelerates the charge. This causes the charge to emit the scattered radiation we observe, and it explains the changes in the charge’s energy and momentum we measure.
x y z ˆ ˆ n z = Q1. At an instant of time t , the z component of the magnetic field is negative and decreasing in magnitude on the tiny surface shown. In what direction does the non-Coulomb electric field wind around ? A) B) ˆ n Answer: A ( 29 ( 29 ˆ C S S S d d E dl B ndA B d dt dt =∂ = - = - ⋅ Σ a a a a a i Q2. In what direction must we integrate around the boundary of the tiny surface to correctly evaluate the formal expression for the emf on the left-hand side of Faraday’s Law? S is a surface bounded by the closed curve C with the direction around the curve related to the direction of the surface’s unit normal by the right-hand rule. Answer: B ˆ n

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During the next week or so, we will blend what we the wave and photon models into a quantitative quantum mechanical model of single photon states of the electromagnetic field that is used in the field of quantum optics. We will blend the two theories together by focusing on the way each explains the transport of energy and momentum by electromagnetic radiation and on what photon-detection methods measure in different physical situations. An Electromagnetic Plane-Wave Pulse To review the wave model I asked you to read sections of the M&I text that use Maxwell’s equations to determine the properties of slab-pulse field. They require the pulse to travel at the speed of light, 0 0 1 c ε μ = and that the magnitudes of the pulse’s electric and magnetic fields be related by E B c =
0 0 1 1 ˆ in V S V S V E ndA E d dV Q ρ ε =∂ = ⋅ Σ = = a a a i i Gauss’ law ˆ 0 S V S B ndA B d =∂ = ⋅ Σ = a a a i i Gauss’ law for magnetism ˆ n = outward-directed normal of element of closed surface

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Lecture24 - Physics 344 Foundations of 21st Century Physics...

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