ECE-320 CH02.pdf - Unix.eng.ua.edu

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G. Rizzoni, Principles and Applications of Electrical Engineering Problem solutions, Chapter 2Solution:Problem 2.17Known quantities:Circuit shown in Figure P2.17 if the power delivered by the source is 40 mW; the voltage v = v1/4; andR = kΩ,R = 10kΩ,R = 12kΩ1823Find:The resistance R, the current i and the two voltages v and v1Analysis:P = v ⋅i= 40 mW (eq. 1)vv1 = R2⋅i= 10000⋅i= (eq. 2)4From eq.1 and eq.2, we obtain:i = 1.0 mA and v = 40 V.Applying KVL for the loop:− v + 8000 i + 10000i+ Ri + 12000i= 0 or, 0 .001R= 10Therefore,R = 10kΩ and v 1= 10V.________________________________________________________________________Solution:Problem 2.18Known quantities:Rated power; rated optical power; operating life; rated operating voltage; open-circuit resistance of thefilament.Find:a) The resistance of the filament in operationb) The efficiency of the bulb.Analysis:a)P = VIOL:PR∴ I =VR60 Va= = 521.7 ma115 VV VR115 VR = = = = 220. 4 ΩI I 521.7 mab)Efficiency is defined as the ratio of the useful power dissipated by or supplied by the load to the total powersupplied by the source. In this case, the useful power supplied by the load is the optical power. From anyhandbook containing equivalent units: 680 lumens=1 W2.14
G. Rizzoni, Principles and Applications of Electrical Engineering Problem solutions, Chapter 2WPo , out= Optical Power Out = 820 lum = 1. 206 W680 lumPo, out 1.206 Wη = efficiency = = = 0.02009 = 2.009 %P 60 WR________________________________________________________________________Solution:Problem 2.19Known quantities:Rated power; rated voltage of a light bulb.Find:The power dissipated by a series of three light bulbs connected to the nominal voltage.Assumptions:The resistance of each bulb doesn’t vary when connected in series.Analysis:When connected in series, the voltage of the source will divide equally across the three bulbs. The acrosseach bulb will be 1/3 what it was when the bulbs were connected individually across the source. Powerdissipated in a resistance is a function of the voltage squared, so the power dissipated in each bulb whenconnected in series will be 1/9 what it was when the bulbs were connected individually, or 11.11 W:Ohm’s Law:P = IVVB = VS= 110 VRBB( 110 V )= I2R2VB= = = 121 ΩP 100 Va2BV=R2BBConnected in series and assuming the resistance of each bulb remains the same as when connectedindividually:KVL: −VS+ VB1+ VB2+ VB3= 0OL: −V+ IR + IR + IR 0SB3 B2B1=VS 110 VI === 303maR R RVB1+B2+B3121+121+121a22 V 1PB = I RB1= 303 ma 121 = 11.11W= 100 a 9( ) .1W________________________________________________________________________Solution:Problem 2.20Known quantities:Rated power and rated voltage of the two light bulbs..2.15
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ECE-320 CH02.pdf - Unix.eng.ua.edu

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