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Unformatted text preview: Husain, Zeena Homework 5 Due: Feb 23 2004, 4:00 am Inst: Sonia Paban 1 This printout should have 38 questions. Multiplechoice questions may continue on the next column or page find all choices before making your selection. The due time is Central time. 001 (part 1 of 2) 10 points A strong lightning bolt transfers an electric charge of about 11 C to Earth (or vice versa ). How many electrons are transferred? Correct answer: 6 . 86566 10 19 . Explanation: Given : q = 11 C and q e = 1 . 60218 10 19 C . The charge is proportional to the number of electrons, so q = n q e n = q q e = 11 C 1 . 60218 10 19 C = 6 . 86566 10 19 . The charge on the electron is negative. The direction of current flow in lightning can be either from Earth to the clouds or vice versa . 002 (part 2 of 2) 10 points Assume: One mole of water has a mass of 18 . 1 g / mol. If each water molecule donates one electron, how much water is ionized in the lightning? Correct answer: 0 . 00206353 g. Explanation: Given : M = 18 . 1 g / mol and N a = 6 . 02214 10 23 / mol Mass is proportional to the number of atoms in a substance, so for m grams in N atoms in the water and M grams in N a atoms in one mole, we have m M = N N a N = m M N a Hence the total mass M total of the water ion ized by the lightning bolt is give by M total = n H 2 O m H 2 O = n e M H 2 O N A = ( 6 . 86566 10 19 ) (18 . 1 g / mol) (6 . 02214 10 23 / mol) = . 00206353 g , Since every water molecule donates one elec tron to the discharge, the total number of transferred electrons n e equals the total num ber n H 2 O of ionized water molecules, so n e = n H 2 O . 003 (part 1 of 2) 10 points The quantity of charge passing through a sur face of area 2 . 89 cm 2 varies with time as q = q 1 t 3 + q 2 t + q 3 , where q 1 = 4 . 9 C / s 3 , q 2 = 6 . 1 C / s, q 3 = 7 . 4 C, and t is in seconds. What is the instantaneous current through the surface at t = 0 . 9 s? Correct answer: 18 . 007 A. Explanation: Given : q 1 = 4 . 9 C / s 3 , q 2 = 6 . 1 C / s , q 3 = 7 . 4 C , and t = 0 . 9 s . I d q dt = 3 q 1 t 2 + q 2 = 3 ( 4 . 9 C / s 3 ) (0 . 9 s) 2 + 6 . 1 C / s = 18 . 007 A . 004 (part 2 of 2) 10 points What is the value of the current density at Husain, Zeena Homework 5 Due: Feb 23 2004, 4:00 am Inst: Sonia Paban 2 t = 0 . 9 s? Correct answer: 62308 A / m 2 . Explanation: Given : a = 2 . 89 cm 2 = 0 . 000289 m 2 and t = 0 . 9 s . J I A = 18 . 007 A . 000289 m 2 = 62308 A / m 2 . 005 (part 1 of 5) 10 points The mass density of Cu is 8 . 92 g / cm 3 , its molar mass is 63 . 5 g / mol, and Avogadros number is 6 . 02214 10 23 atoms / mole. The electron mass is 9 . 10939 10 31 kg, and the resistivity of copper is 1 . 7 10 8 m....
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