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HW08-solutions - Munoz(gm7794 HW08 TSOI(92515 This...

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Munoz (gm7794) – HW08 – TSOI – (92515) 1 This print-out should have 19 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points The emf E can drive the circuit below at any given frequency. E C L R A B D At what frequency does the light bulb glow most brightly? 1. very high frequencies 2. the frequency ω = LC 3. very low frequencies 4. the frequency ω = 1 LC 5. both very low frequencies or very high frequencies correct Explanation: Since the brightness of the bulb is propor- tional to the power dissipated in it, P = I 2 rms R, . In order to obtain maximum brightness we should adjust the frequency so that the rms current I rms through the bulb is the largest. The capacitive reactance X C = 1 ω C is in- versely proportional to ω , and the inductive reactance X L = ω L is proportional to ω , so Z = R + 1 1 X C - 1 X L 2 = R + 1 ω C - 1 ω L 2 = R + ω L ω 2 C L - 1 I R = E R + ω L ω 2 C L - 1 ω I light bulb ω 0 ω 0 = 1 L C Since the current is a minimum at reso- nance, the maximum brightness will occur at both very low frequencies and very high fre- quencies. 002 10.0 points The emf E can drive the circuit below at any given frequency. E C L R A B D When does the light bulb glow most brightly? 1. the frequency ω = 1 LC correct 2. the frequency ω = LC 3. both very low frequencies or very high frequencies 4. very low frequencies 5. very high frequencies Explanation: Since the brightness of the bulb is propor- tional to the power dissipated in it P = I 2 rms R, in order to get maximum brightness we should adjust frequency so that the rms current I rms
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Munoz (gm7794) – HW08 – TSOI – (92515) 2 through the bulb is the largest. The capac- itive reactance X C = 1 ω C is inversely pro- portional to ω , and the inductive reactance X L = ω L is proportional to it, so Z = R 2 + ( X L - X C ) 2 = R 2 + ω L - 1 ω C 2 I R = E R 2 + ω L - 1 ω C 2 ω I light bulb ω 0 = 1 L C ω 0 Since the current is a maximum at reso- nance, the maximum brightness will occur at resonance. 003 10.0 points Comparing radio waves and ultraviolet light, which of the following is true? 1. Radiowaves have lower frequency, same speed, and longer wavelength than ultraviolet light. correct 2. Radiowaves have lower frequency, faster speed, and shorter wavelength than ultravio- let light. 3. Radiowaves have higher frequency, same speed, and longer wavelength than ultraviolet light. 4. Radiowaves have lower frequency, slower speed, and longer wavelength than ultraviolet light. Explanation: Radiowaves have lower frequency, same speed, and longer wavelength than ultravi- olet light. 004 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.
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