222 a v1 9 by kcl the current through the series

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Unformatted text preview: n of R2 and R3 , V1 = V2 + V3 = 9 V The current through R1 is p1 20 = = 2.222 A V1 9 By KCL, the current through the series combination of R2 and R3 I1 = is I2 = I − I1 = 5 − 2.222 = 2.778 A (c) Since p = I 2 R , p1 20 = = 4.05 2 2.2222 I1 p2 15 R2 = 2 = = 1.945 2.7772 I2 p3 10 R3 = 2 = = 1.297 2.7772 I3 R1 = PRACTICE PROBLEM 2.16 Refer to Fig. 2.55 and assume there are 10 lightbulbs, each with a power rating of 40 W. If the voltage at the plug is 110 V for the parallel and series connections, calculate the current through each bulb for both cases. Answer: 0.364 A (parallel), 3.64 A (series). 2.8.2 Design of DC Meters a Max Vin b + − + Vout Min − c Figure 2.57 | v v The potentiometer controlling potential levels. | e-Text Main Menu By their nature, resistors are used to control the flow of current. We take advantage of this property in several applications, such as in a potentiometer (Fig. 2.57). The word potentiometer, derived from the words potential and meter, implies that potential can be metered out. The potentiometer (or pot for short) is a three-terminal device that operates on the principle of voltage division. It is essentially an adjustable voltage divider. As a voltage regulator, it is used as a volume or level control on radios, TVs, and other devices. In Fig. 2.57, Rbc Vout = Vbc = Vin (2.58) Rac where Rac = Rab + Rbc . Thus, Vout decreases or increases as the sliding contact of the pot moves toward c or a , respectively. | Textbook Table of Contents | Problem Solving Workbook Contents CHAPTER 2 Basic Laws 57 Another application where resistors are used to control current flow is in the analog dc meters—the ammeter, voltmeter, and ohmmeter, which measure current, voltage, and resistance, respectively. Each of these meters employs the d’Arsonval meter movement, shown in Fig. 2.58. The movement consists essentially of a movable iron-core coil mounted on a pivot between the poles of a permanent magnet. When current flows through the coil, it creates a torque which causes the pointer to deflect. The amount of current through the coil determines the de...
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