Practice Midterm 02-solutions

# Practice Midterm 02-solutions - sanne (as42476) Practice...

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Unformatted text preview: sanne (as42476) Practice Exam 02 Yao (59110) 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 A 1-megabit computer memory chip contains many 76 fF capacitors. Each capacitor has a plate area of 8 . 6 10 12 m 2 . The characteristic atomic diameter is 10 10 m = 1 A. The permittivity of a vac- uum is 8 . 85419 10 12 C 2 / N m 2 . Determine the plate separation of such a capacitor (assume a parallel-plate configura- tion). Correct answer: 10 . 0192 A. Explanation: Let : A = 8 . 6 10 12 m 2 , C = 76 fF = 7 . 6 10 14 F , and = 8 . 85419 10 12 C 2 / N m 2 . d = A C = ( 8 . 85419 10 12 C 2 / N m 2 ) 7 . 6 10 14 F ( 8 . 6 10 12 m 2 ) = 1 . 00192 10 9 m = 10 . 0192 A . 002 (part 1 of 4) 10.0 points Four capacitors are connected as shown in the figure. 2 2 . 1 F 59 . 3 F 38 . 7 F 8 4 F 98 . 8 V a b c d Find the capacitance between points a and b of the entire capacitor network. Correct answer: 129 . 517 F. Explanation: Let : C 1 = 22 . 1 F , C 2 = 38 . 7 F , C 3 = 59 . 3 F , C 4 = 84 F , and E = 98 . 8 V . C 1 C 3 C 2 C 4 E a b c d A good rule of thumb is to eliminate junc- tions connected by zero capacitance. C 2 C 3 C 1 C 4 a b The definition of capacitance is C Q V . The series connection of C 2 and C 3 gives the equivalent capacitance C 23 = 1 1 C 2 + 1 C 3 = C 2 C 3 C 2 + C 3 = (38 . 7 F) (59 . 3 F) 38 . 7 F + 59 . 3 F = 23 . 4174 F . The total capacitance C ab between a and b can be obtained by calculating the capacitance in sanne (as42476) Practice Exam 02 Yao (59110) 2 the parallel combination of the capacitors C 1 , C 4 , and C 23 ; i.e. , C ab = C 1 + C 4 + C 23 = 22 . 1 F + 84 F + 23 . 4174 F = 129 . 517 F . 003 (part 2 of 4) 10.0 points What is the charge on the 38 . 7 F centered- upper capacitor? Correct answer: 2313 . 64 C. Explanation: The voltages across C 2 and C 3 , respectively, (the voltage between a and b ) are V ab = V 23 = 98 . 8 V , and we have Q 23 = Q 3 = Q 2 = V ab C 23 = (98 . 8 V)(23 . 4174 F) = 2313 . 64 C . 004 (part 3 of 4) 10.0 points A dielectric with dielectric constant 2 . 9 is inserted into the 59 . 3 F capacitor (lower- centered capacitor) while the battery is con- nected. What is the charge on the 84 F lower-right capacitor? Correct answer: 8299 . 2 C. Explanation: Since the battery is still connected, the volt- age will remain the same. Thus the charge is simply Q 4 = V ab C 4 = (98 . 8 V) (84 F) = 8299 . 2 C . 005 (part 4 of 4) 10.0 points If the battery is removed before the dielectric in the above question is inserted, what will be the charge on the 84 F lower-right capacitor?...
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## This note was uploaded on 11/03/2009 for the course PHY 303L taught by Professor Turner during the Spring '08 term at University of Texas at Austin.

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Practice Midterm 02-solutions - sanne (as42476) Practice...

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