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Unformatted text preview: mcconnell (kam2342) oldhomework 13 Turner (60230) 1 This printout should have 15 questions. Multiplechoice questions may continue on the next column or page find all choices before answering. 001 10.0 points How long does it take electrons to get from the car battery to the starting motor? Assume the current is 141 A and the electrons travel through copper wire with cross sectional area 37 . 2 mm 2 and length 96 . 9 cm. The mass density of copper is 8960 kg / m 3 and the molar mass is 63 . 5 g / mol. Correct answer: 57 . 9815 min. Explanation: Let : I = 141 A , A = 37 . 2 mm 2 = 3 . 72 10 5 m 2 , L = 96 . 9 cm = 0 . 969 m , = 8960 kg / m 3 , and M molecule = 63 . 5 g / mol = 0 . 0635 kg / mol . Current is equal to the charge passing a cross section per unit time. In this problem, the current I comes from the motion of the electrons inside the copper wire. If the time that the electrons move from the battery to the starting motor is t , and the charge inside this region is Q , we have I = Q t . The volume of this copper wire segment is V = AL, so the mass is M = V = AL = (8960 kg / m 3 ) (3 . 72 10 5 m 2 ) (0 . 969 m) = 0 . 322979 kg . This mass corresponds to . 322979 kg . 0635 kg / mol = 5 . 08629 mol , so the number of electrons is N = (5 . 08629 mol) (6 . 02 10 23 / mol) = 3 . 06195 10 24 , and the total charge is Q = N e = (3 . 06195 10 24 ) (1 . 602 10 19 C) = 4 . 90524 10 5 C . Thus the time it takes to move this amount of charge from the battery to the starting motor is t = Q I = 4 . 90524 10 5 C 141 A parenleftbigg 1 min 60 s parenrightbigg = 57 . 9815 min . 002 10.0 points How much does it cost to watch a com plete 22 h long World Series on a 516 W television set? Assume that electricity costs $0 . 07 / kW h. Correct answer: 79 . 464 cents. Explanation: Let : P = 516 W = 0 . 516 kW , t = 22 h , and R = $0 . 07 / kW h . The cost is c = ( P t ) R = (0 . 516 kW) (22 h) ($0 . 07 / kW h) 100 cents dollar = 79 . 464 cents . 003 10.0 points Consider the circuit shown in the figure. In this circuit, r i is the internal resistance of the battery (it cannot be removed and must be considered as part of the battery); R o is an mcconnell (kam2342) oldhomework 13 Turner (60230)...
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 Spring '10
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