# hw 16 - nguyen (jmn727) – homework 16 – Turner –...

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Unformatted text preview: nguyen (jmn727) – homework 16 – Turner – (59070) 1 This print-out should have 10 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points A 0 . 14 μ F capacitor is given a charge Q . After 14 s, the capacitor’s charge is 0 . 5 Q . What is the effective resistance across this capacitor? Correct answer: 144 . 27 MΩ. Explanation: Let : C = 0 . 14 μ F = 1 . 4 × 10- 7 F and t = 14 s . The charge on the capacitor is Q ( t ) = Q e- t/τ . e t/τ = Q Q t τ = ln parenleftbigg Q Q parenrightbigg τ = t ln parenleftbigg Q Q parenrightbigg . The effective resistance is R = τ C = t C ln parenleftbigg Q Q parenrightbigg = 14 s (1 . 4 × 10- 7 F) ln parenleftbigg Q . 5 Q parenrightbigg · 1 MΩ 10 6 Ω = 144 . 27 MΩ . 002 10.0 points The switch S has been in the position “a” for a long time. Then at t = 0, it is moved from “a” to “b”. C R 1 R 2 E S b a Find the time when the charge in the ca- pacitor is reduced to 1 e of its value at t = 0. 1. τ = radicalbig R 1 R 2 C 2. τ = R 1 C 3. τ = 1 ( R 1 + R 2 ) C 4. τ = ( R 1 + R 2 ) C correct 5. τ = 1 √ R 1 R 2 C 6. τ = 1 R 2 C 7. τ = R 2 C 8. τ = R 1 + R 2 2 C 9. τ = 2 ( R 1 + R 2 ) C 10. τ = 1 R 1 C Explanation: In charging an R C circuit, the characteris- tic time constant is given by τ = R C , where in this problem R...
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## This note was uploaded on 01/21/2010 for the course PHY 303L taught by Professor Turner during the Spring '08 term at University of Texas at Austin.

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hw 16 - nguyen (jmn727) – homework 16 – Turner –...

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