# So far we have assumed that connecting wires are

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Unformatted text preview: apply the concepts developed in this chapter: electrical lighting systems and design of dc meters. So far, we have assumed that connecting wires are perfect conductors (i.e., conductors of zero resistance). In real physical systems, however, the resistance of the connecting wire may be appreciably large, and the modeling of the system must include that resistance. 2.8.1 Lighting Systems Lighting systems, such as in a house or on a Christmas tree, often consist of N lamps connected either in parallel or in series, as shown in Fig. 2.55. Each lamp is modeled as a resistor. Assuming that all the lamps are identical and Vo is the power-line voltage, the voltage across each lamp is Vo for the parallel connection and Vo /N for the series connection. The series connection is easy to manufacture but is seldom used in practice, for at least two reasons. First, it is less reliable; when a lamp fails, all the lamps go out. Second, it is harder to maintain; when a lamp is bad, one must test all the lamps one by one to detect the faulty one. 1 2 + Vo − 1 + Vo − Power plug 2 3 3 N Lamp (a) Figure 2.55 N (b) (a) Parallel connection of lightbulbs, (b) series connection of lightbulbs. EXAMPLE 2.16 Three lightbulbs are connected to a 9-V battery as shown in Fig. 2.56(a). Calculate: (a) the total current supplied by the battery, (b) the current through each bulb, (c) the resistance of each bulb. I 9V 15 W 20 W 9V 10 W (a) | v v Figure 2.56 | I1 I2 + V2 − R2 + V3 − + V1 − R1 R3 (b) (a) Lighting system with three bulbs, (b) resistive circuit equivalent model. e-Text Main Menu | Textbook Table of Contents | Problem Solving Workbook Contents 56 PART 1 DC Circuits Solution: (a) The total power supplied by the battery is equal to the total power absorbed by the bulbs, that is, p = 15 + 10 + 20 = 45 W Since p = V I , then the total current supplied by the battery is p 45 I= = =5A V 9 (b) The bulbs can be modeled as resistors as shown in Fig. 2.56(b). Since R1 (20-W bulb) is in parallel with the battery as well as the series combinatio...
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## This note was uploaded on 07/16/2012 for the course KA KA 2000 taught by Professor Bkav during the Spring '12 term at Cambridge.

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