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Unformatted text preview: Version PREVIEW HW11 feng (58230) 1 This printout should have 14 questions. Multiplechoice questions may continue on the next column or page find all choices before answering. Displacement Current 03 001 (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. Q RC e t/Q 2. RC Q e t/ ( RC ) 3. Q RC e t/ ( RC ) 4. RC Q e t/ ( RC ) 5. Q RC e t/ ( RC ) correct Explanation: Basic Concept RC circuits. Displacement Current. The displacement current is defined to be I d = d E dt . The electric field inside a capacitor is essen tially uniform and E = q 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 = d E dt = d dt parenleftbigg q 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 = 2 F, Q = 28 C, R = 391 k, and = 8 . 85419 10 12 C 2 / N m 2 , at what rate is the electric flux between the plates changing at time t = 0 . 17 s? Correct answer: 3 . 2538 10 6 Vm / s. Explanation: Let : = 8 . 85419 10 12 C 2 / N m 2 , t = 0 . 17 s , C = 2 F , Q = 28 C = 2 . 8 10 5 C , and R = 391 k = 3 . 91 10 5 . From the discussion in the previous part, we know that d E dt = I d = Q RC e t/ ( RC ) = 2 . 8 10 5 C (3 . 91 10 5 ) (2 10 6 F) e bracketleftbigg . 17 s (3 . 91 10 5 )(2 10 6 F) bracketrightbigg = 3 . 2538 10 6 Vm / s . Which Maxwell Equation 003 (part 1 of 6) 10.0 points Which of Maxwells equations can be used, along with a symmetry argument, to calculate the electric field of a point charge? 1. contintegraldisplay vector B d vectors = I + E t 2. contintegraldisplay vector E d vectors = B t 3. contintegraldisplay vector B d vectors = 0 4. contintegraldisplay vector E d vectors = Q Version PREVIEW HW11 feng (58230) 2 5. contintegraldisplay vector B d vector A = I 6. contintegraldisplay vector E d vector A = B t 7. contintegraldisplay vector B d vector A = Q 8. contintegraldisplay vector E d vector A = Q correct 9....
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This note was uploaded on 04/27/2010 for the course PHY 303L taught by Professor Turner during the Spring '08 term at University of Texas at Austin.
 Spring '08
 Turner
 Physics, Capacitance, Current

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