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Unformatted text preview: toupal (rgt374) – Homework 11 – Chiu – (58295) 1 This printout should have 22 questions. Multiplechoice questions may continue on the next column or page – find all choices before answering. 001 (part 1 of 2) 10.0 points A capacitor of capacitance C has a charge Q at t = 0. At that time, a resistor of resistance R is connected to the plates of the charged capacitor. Find the magnitude of the dis placement current between the plates of the capacitor as a function of time. 1. RC Q e − t/ ( RC ) 2. Q RC e − t/Q 3. Q RC e − t/ ( RC ) correct 4. Q RC e t/ ( RC ) 5. RC Q e t/ ( RC ) Explanation: Basic Concept RC circuits. Displacement Current. The displacement current is defined to be I d = ǫ d Φ E dt . The electric field inside a capacitor is essen tially uniform and E = q ǫ A . Since the charge on a capacitor in a discharging RC circuit is given by q ( t ) = Q e − t/RC , the displacement current is found by I d = ǫ d Φ E dt = ǫ d dt parenleftbigg q ǫ A A parenrightbigg = dq dt = − Q RC e − t/ ( RC ) . Note that the displacement current equals the actual current in the wires to the capacitor. Thus, the AmpereMaxwell law tells us that vector B will be the same regardless of which current we evaluate. 002 (part 2 of 2) 10.0 points Given C = 4 μ F, Q = 29 μ C, R = 252 kΩ, and ǫ = 8 . 85419 × 10 − 12 C 2 / N · m 2 , at what rate is the electric flux between the plates changing at time t = 0 . 2 s? Correct answer: − 2 . 66452 × 10 6 Vm / s. Explanation: Let : ǫ = 8 . 85419 × 10 − 12 C 2 / N · m 2 , t = 0 . 2 s , C = 4 μ F , Q = 29 μ C = 2 . 9 × 10 − 5 C , and R = 252 kΩ = 2 . 52 × 10 5 Ω . From the discussion in the previous part, we know that d Φ E dt = I d ǫ = − Q ǫ RC e − t/ ( RC ) = − 2 . 9 × 10 − 5 C ǫ (2 . 52 × 10 5 Ω) (4 × 10 − 6 F) × e bracketleftbigg − . 2 s (2 . 52 × 10 5 Ω)(4 × 10 − 6 F) bracketrightbigg = − 2 . 66452 × 10 6 Vm / s . 003 10.0 points What is the fundamental source of electro magnetic radiation? 1. None of these 2. current 3. any charge 4. voltage 5. an accelerating charge correct Explanation: The fundamental source of electromagnetic radiation is oscillating electric charges, which emit oscillating electric and magnetic fields. toupal (rgt374) – Homework 11 – Chiu – (58295) 2 004 10.0 points According to Maxwell’s equation, the speed of light in a vacuum is 1. greater for radio waves than for visible light. 2. a function of the distance from the source. 3. independent of frequency. correct 4. greater for visible light than for radio waves. 5. a function of the size of the source. Explanation: Application of Faraday’s Law and the AmpereMaxwell equation to EM radiation results in the prediction that the speed of light is c = 1 √ μ ǫ , independent of frequency....
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This note was uploaded on 04/26/2009 for the course PHY 58310 taught by Professor Chiu during the Spring '09 term at University of Texas.
 Spring '09
 Chiu
 Physics, Capacitance, Work

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