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Unformatted text preview: Version 001 midterm 04 review chiu (56565) 1 This print-out should have 24 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. Hewitt CP9 28 R18 001 0.0 points Which travels most slowly in glass? 1. Green light 2. Brown light 3. Violet light correct 4. Yellow light 5. Blue light 6. Orange light 7. Red light Explanation: n = c v Since violet light is refracted more than the light of other frequencies its index of re- fraction is the highest, making its speed the smallest. In other words, violet light travels slowest in glass. Energy of Sunlight 002 0.0 points The intensity of sunlight under the clear sky is 1180 W / m 2 . How much electromagnetic energy is con- tained per cubic meter near the Earths sur- face? The speed of light is 2 . 99792 10 8 m / s. Correct answer: 3 . 93606 10- 6 J / m 3 . Explanation: Let : I = 1180 W / m 2 and c = 2 . 99792 10 8 m / s . If v is the velocity at which the energy is transferred (the velocity of the wave), then the relationship between the intensity I (en- ergy per unit time per unit area) and the energy density of a wave u is I = u c u = I c = 1180 W / m 2 2 . 99792 10 8 m / s = 3 . 93606 10- 6 J / m 3 . EM Wave Direction 02 003 0.0 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. 3. traveling right to left. correct Explanation: The E vector and 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. v z y x E B For instance, let us choose the point where the E vector is along the y axis, as shown in the above figures. At this same instant, the B vector is along the positive x axis (not the negative x axis as shown in the figure for a point 180 out of Version 001 midterm 04 review chiu (56565) 2 phase from the place on the velocity ( z ) axis where the E vector is drawn). Then E B is along the negative z axis. Therefore, the electromagnetic wave is traveling right to left. Electromagnetic Point Source 004 0.0 points At what distance from a 19 W electromag- netic wave point source (like a light bulb) is the amplitude of the electric field 46 V / m? o c = 376 . 991 . Correct answer: 0 . 733998 m. Explanation: Let : P = 19 W , E max = 46 V / m , and c = 376 . 991 . The wave intensity (power per unit area) is given by I = S av = E 2 max 2 ( o c ) = (46 V / m) 2 2 (376 . 991 ) = 2 . 80643 W / m 2 . The constant o c (impedance of free space) turns out to be 120 . You should verify that (V / m) 2 = W / m 2 ....
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- Spring '08