# hw5sol - Flores, Michael Homework 5 Due: Feb 23 2004, 4:00...

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Unformatted text preview: Flores, Michael Homework 5 Due: Feb 23 2004, 4:00 am Inst: Sonia Paban 1 This print-out should have 38 questions. Multiple-choice 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 33 C to Earth (or vice versa ). How many electrons are transferred? Correct answer: 2 . 0597 10 20 . Explanation: Given : q = 33 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 =- 33 C- 1 . 60218 10- 19 C = 2 . 0597 10 20 . 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 17 . 9 g / mol. If each water molecule donates one electron, how much water is ionized in the lightning? Correct answer: 0 . 00612218 g. Explanation: Given : M = 17 . 9 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 = ( 2 . 0597 10 20 ) (17 . 9 g / mol) (6 . 02214 10 23 / mol) = . 00612218 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 1 . 02 cm 2 varies with time as q = q 1 t 3 + q 2 t + q 3 , where q 1 = 5 . 1 C / s 3 , q 2 = 3 . 3 C / s, q 3 = 5 . 8 C, and t is in seconds. What is the instantaneous current through the surface at t = 1 . 5 s? Correct answer: 37 . 725 A. Explanation: Given : q 1 = 5 . 1 C / s 3 , q 2 = 3 . 3 C / s , q 3 = 5 . 8 C , and t = 1 . 5 s . I d q dt = 3 q 1 t 2 + q 2 = 3 ( 5 . 1 C / s 3 ) (1 . 5 s) 2 + 3 . 3 C / s = 37 . 725 A . 004 (part 2 of 2) 10 points What is the value of the current density at Flores, Michael Homework 5 Due: Feb 23 2004, 4:00 am Inst: Sonia Paban 2 t = 1 . 5 s? Correct answer: 369853 A / m 2 . Explanation: Given : a = 1 . 02 cm 2 = 0 . 000102 m 2 and t = 1 . 5 s . J I A = 37 . 725 A . 000102 m 2 = 369853 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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## This note was uploaded on 02/18/2010 for the course PHYS 303l taught by Professor Panab during the Spring '04 term at North Texas.

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hw5sol - Flores, Michael Homework 5 Due: Feb 23 2004, 4:00...

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