PracticeMidterm1Sol

PracticeMidterm1Sol - 1 Solutions Last , , First Initial...

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1 Solutions , , Last First Initial Name (please print) Student Number UNIVERSITY OF CALIFORNIA AT BERKELEY DEPARTMENT OF PHYSICS PHYSICS 7C FALL SEMESTER 2008 LEROY T. KERTH First Midterm Examination October 2, 2008 This examination is closed book. You may refer to a single 8 1 2 ! 11 sheet of paper (both sides) that you have prepared. Work all problems. 1 ______________ /20 2 ______________ /20 3 ______________ /20 4 ______________ /20 5 ______________ /20 Total ______________ /100
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2 1. A plane light wave of frequency and wave number k (k = 2 π /¬), propagating in the positive z direction. a. If the electric field is in the ˆ X direction with amplitude E 0 what is the direction of the magnetic field? b. If the amplitude of the electric field is 30 volts/m what is the amplitude of the magnetic field? c. What is the average value of the energy flux striking a disk of radius 2 m? d. If the wave is reflected, what is the average force on the disk? a. B is in the ˆ y direction. b. B = E c , B = 30 3 ! 10 8 = 10 " 7 Tesla . c. S = E ! B μ 0 = 1 2 " 0 0 E 2 = 1 2 1 377 30 2 = 1.19 W / m 2 Average power flux striking the disk = 4 ! " S = 15 Watts d. The average pressure is 2 S c . The total force is Pressure X area = 2 ! 15 3 ! 10 8 = 10 " 7 N
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3 2. The index of refraction of water is ~ 1.3 for radiation in the visible range. For microwaves it is 9! a. For microwaves passing from the water into air, find the critical angle of reflection.
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PracticeMidterm1Sol - 1 Solutions Last , , First Initial...

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