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Unformatted text preview: Version 003/AAAAD midterm 04 Turner (59130) 1 This printout should have 19 questions. Multiplechoice questions may continue on the next column or page find all choices before answering. 001 10.0 points Consider an electromagnetic wave pattern as shown in the figure below. E B The wave is 1. a standing wave and is stationary. 2. traveling left to right. correct 3. traveling right to left. Explanation: The vector E vector and vector B vector are not at the same point on the velocity axis. Pick an instant in time, where the E and B fields are at the same point on the velocity axis. z v x y E B For instance, let us choose the point where the vector E vector is along the x axis, as shown in the above figures. At this same instant, the vector B vector is along the negative y axis (at a point with a phase difference of 360 from the place on the veloc ity ( z ) axis where the vector E vector is drawn). Then vector E vector B is along the negative z axis. Therefore, the electromagnetic wave is traveling left to right. 002 10.0 points A thin tungsten filament of length 1 . 26 m radiates 95 . 2 W of power in the form of elec tromagnetic waves. A perfectly absorbing surface in the form of a hollow cylinder of radius 5 . 09 cm and length 1 . 26 m is placed concentrically with the filament. Assume: The radiation is emitted in the radial direction, and neglect end effects. The speed of light is 2 . 99792 10 8 m / s. Calculate the radiation pressure acting on the cylinder. 1. 2.27698e06 2. 6.22351e07 3. 7.8804e07 4. 7.42096e07 5. 8.64836e07 6. 3.20558e07 7. 9.01525e07 8. 1.77578e07 9. 9.28679e07 10. 1.45489e06 Correct answer: 7 . 8804 10 7 N / m 2 . Explanation: Let : r = 5 . 09 cm = 0 . 0509 m , = 1 . 26 m , c = 2 . 99792 10 8 m / s , and P = 7 . 8804 10 7 N / m 2 . The intensity of the radiation reaching the walls of the cylinder is I = ( S ) = P 2 r , so the radiation pressure on the walls is p = I c = P 2 r c = 7 . 8804 10 7 N / m 2 2 (0 . 0509 m) (1 . 26 m) (2 . 99792 10 8 m / s) = 7 . 8804 10 7 N / m 2 . 003 10.0 points A 101 mW laser beam is reflected back upon itself by a mirror. Version 003/AAAAD midterm 04 Turner (59130) 2 Calculate the force on the mirror. The speed of light is 2 . 99792 10 8 m / s. 1. 5.00346e07 2. 3.00208e07 3. 1.26754e07 4. 4.73661e07 5. 6.00415e08 6. 6.47114e07 7. 4.20291e07 8. 2.00138e08 9. 6.73799e07 10. 3.80263e07 Correct answer: 6 . 73799 10 7 N. Explanation: Let : P = 101 mW = 0 . 101 W and c = 2 . 99792 10 8 m / s . The intensity of the laser is P A with P the power and A the area of the beam. The average momentum per unit area per unit time (force per unit area) transferred to the wall is p tA = F A = 2 I c = 2 P Ac , so F = 2 P c = 2 (0 . 101 W) 2 . 99792 10 8 m / s = 6 . 73799 10 7 N ....
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 Spring '08
 Turner

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