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Unformatted text preview: Husain, Zeena Homework 11 Due: Apr 12 2004, 6:00 pm Inst: Sonia Paban 1 This printout should have 24 questions. Multiplechoice questions may continue on the next column or page find all choices before making your selection. The due time is Central time. 001 (part 1 of 1) 10 points Given: c = 2 . 99792 10 8 m / s . The distance from the Earth to Polaris is approximately 6 . 31 10 18 m. If the North Star, Polaris, were to burn out today, in how many years would it disappear from our vision? Correct answer: 667 . 425 years. Explanation: Since the light travels at a finite velocity c , the last ray of light going out from Polaris would travel t = d c = 6 . 31 10 18 m 2 . 99792 10 8 m / s = 2 . 10479 10 10 s seconds before arriving at the Earth. In years, this is t y = t 1 60 min / s 1 60 hr / min 1 24 day / hr 1 365 yr / day = 667 . 425 years . 002 (part 1 of 5) 10 points Given: c = 2 . 99792 10 8 m / s . Consider a monochromatic electromagnetic plane wave propagating in the x direction. At a particular point in space, the magnitude of the electric field has an instantaneous value of 625 V / m in the positive ydirection. The wave is traveling in the positive xdirection. x y z E wave propagation Compute the instantaneous magnitude of the magnetic field at the same point and time. Correct answer: 2 . 08478 10 6 T. Explanation: For plane, electromagnetic waves, we know that the ratio of the instantaneous magni tudes of the electric and magnetic fields is E B = c . This means that the instantaneous magnitude of the magnetic field is B = E c = 625 V / m 2 . 99792 10 8 m / s = 2 . 08478 10 6 T . 003 (part 2 of 5) 10 points What is the instantaneous magnitude of the Poynting vector at the same point and time? Correct answer: 1036 . 88 W / m 2 . Explanation: The Poynting vector, ~ S , is given by ~ S = 1 ~ E ~ B . For a plane, electromagnetic wave, ~ E and ~ B are always perpendicular to each other and to the direction of propagation of the wave. In this case, the Poynting vector is in the direction of propagation and has magnitude S = E B = (625 V / m)(2 . 08478 10 6 T) 1 . 25664 10 6 N / A 2 = 1036 . 88 W / m 2 . 004 (part 3 of 5) 10 points What are the directions of the instantaneous magnetic field and the instantaneous Poynt ing vector respectively? Husain, Zeena Homework 11 Due: Apr 12 2004, 6:00 pm Inst: Sonia Paban 2 1. b B = + k and b S = 2. b B = + and b S = k 3. b B = k and b S = correct 4. b B = k + and b S = 5. b B = and b S = 6. b B = k and b S = + 7. b B = k and b S = 8. b B = and b S = 9....
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