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Unformatted text preview: Chapter 18 Direct-Current Circuits Quick Quizzes 1. (a), (d). Bulb R 1 becomes brighter. Connecting a wire from b to c provides a nearly zero resistance path from b to c and decreases the total resistance of the circuit from R 1 + R 2 to just R 1 . Ignoring internal resistance, the potential difference maintained by the battery is unchanged while the resistance of the circuit has decreased. The current passing through bulb R 1 increases, causing this bulb to glow brighter. Bulb R 2 goes out because essentially all of the current now passes through the wire connecting b and c and bypasses the filament of Bulb R 2 . 2. (b). When the switch is opened, resistors R 1 and R 2 are in series, so that the total circuit resistance is larger than when the switch was closed. As a result, the current decreases. 3. (a). When the switch is closed, resistors R 1 and R 2 are in parallel, so that the total circuit resistance is smaller than when the switch was open. As a result, the total current increases. 4. (a) unchanged; (b) unchanged; (c) increase; (d) decrease. Neglecting internal resistance, the terminal potential difference of the battery remains constant (equal to the emf) as bulbs are added. Since each bulb is added in parallel with the battery, the current in each bulb, and hence the brightness of the bulb, is unchanged as new bulbs are added. The total current supplied by the battery increases as bulbs are added in parallel. Thus, the power delivered by the battery increases and its lifetime decreases. 5. ((a) decrease; (b) decrease; (c) decrease; (d) increase. Adding bulbs in series increases the total resistance of the circuit and results in a decrease in the current in the bulbs. Thus, the brightness of individual bulbs deceases as bulbs are added. Neglecting internal resistance, the terminal potential difference of the battery remains constant (equal to the emf) and the power supplied by the battery decreases as bulbs are added. This means that the battery will supply energy at a lower rate, and its lifetime will be increased. 6. (c). After the capacitor is fully charged, current flows only around the outer loop of the circuit. This path has a total resistance of 3 , so the 6-V battery will supply a current of 2 Amperes. 99 100 CHAPTER 18 Answers to Even Numbered Conceptual Questions 2. 4. (a) ii (b) i 6. (a) ii (b) i 8. A short circuit can develop when the last bit of insulation frays away between the two conductors in a lamp cord. Then the two conductors touch each other, opening a low resistance branch in parallel with the lamp. The lamp will immediately go out, carrying no current and presenting no danger. A very large current will be produced in the power source, the house wiring, and the wire in the lamp cord up to and through the short. The circuit breaker will interrupt the circuit quickly but not before considerable heating and sparking is produced in the short-circuit path....
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