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Unformatted text preview: gilvin (jg47854) – 15. RC Circuits – meyers – (21235) 1 This print-out should have 18 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. This homework is due Friday, April 1, at midnight Tucson time. 001 (part 1 of 2) 10.0 points Unlike most real bulbs, the resistance of the bulb in the questions below does not change as the current through it changes. A capacitor, a bulb, and a switch are in the circuit as shown below. C R S The switch is initially open as shown in the above diagram, and the capacitor is charged. C R S Which of the following correctly describes what happens to the bulb when the switch is closed? 1. The bulb is dim and remains dim. 2. At first the bulb is bright and it gets dimmer and dimmer until it goes off. correct 3. At first the bulb is dim and it gets brighter and brighter until the brightness levels off. 4. The bulb is bright and remains bright. 5. None of these is correct. Explanation: When the switch is closed, the bulb has the same potential difference as the capacitor. As the capacitor discharges, the potential dif- ference across the bulb decreases and it gets dimmer and dimmer. 002 (part 2 of 2) 10.0 points Which correctly describes what happens after the switch has remained closed for a long time? 1. None of these is correct. 2. The bulb is on and is bright. 3. The current in the circuit is steady. 4. The bulb is permanently off. correct 5. The potential difference across the capac- itor is steady. Explanation: When the switch remains closed for a long time, the capacitor has completely dis- charged. Its potential difference, which is also the potential difference across the bulb, comes to zero. The bulb goes off. 003 10.0 points The switch S has been in position b for a long period of time. C R 1 R 2 R 3 E S b a When the switch is moved to position “a”, find the characteristic time constant. 1. τ = R 2 C 2. τ = 1 R 1 C 3. τ = radicalbig R 1 R 2 C 4. τ = 2 ( R 1 + R 2 ) C 5. τ = 1 √ R 1 R 2 C gilvin (jg47854) – 15. RC Circuits – meyers – (21235) 2 6. τ = ( R 1 + R 2 ) C correct 7. τ = R 1 + R 2 2 C 8. τ = R 1 C 9. τ = 1 ( R 1 + R 2 ) C 10. τ = 1 R 2 C Explanation: In charging an R C circuit, the characteris- tic time constant is given by τ = R C , where in this problem R is the equivalent resistance, or R = R 1 + R 2 . 004 10.0 points C R 1 R 2 R 3 E S b a Let R 1 = R 2 = R 3 = R . Initially the switch is in position “b” and the capacitor is uncharged. At time t = 0 the switch is moved to position “a”....
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