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Unformatted text preview: Wong, Lemuel – Homework 4 – Due: Feb 16 2007, 11:00 pm – Inst: Matt Mackie 1 This printout should have 31 questions. Multiplechoice questions may continue on the next column or page – find all choices before answering. The due time is Central time. Note: HW is due 11pm central time, which is midnight out time. 001 (part 1 of 3) 10 points We want to find how much charge is on the electrons in a nickel coin. Follow this method. A nickel coin has a mass of about 4 . 4 g. Each mole (6 . 02 × 10 23 atoms) has a mass of about 57 . 9 g. Find the number of atoms in a nickel coin. Correct answer: 4 . 57478 × 10 22 atoms. Explanation: Let : N a = 6 . 02 × 10 23 atoms , M = 57 . 9 g , and m = 4 . 4 g . Mass is proportional to the number of atoms in a substance, so for m grams in N atoms in the nickel coin and M grams in N a atoms in one mole, we have m M = N N a N = m M N a = (4 . 4 g) (57 . 9 g) (6 . 02 × 10 23 atoms) = 4 . 57478 × 10 22 atoms . 002 (part 2 of 3) 10 points Find the number of electrons in the coin. Each nickel atom has 28 electrons / atom. Correct answer: 1 . 28094 × 10 24 electrons. Explanation: Let : n Ni = 28 electrons / atom . If n Ni electrons are in each Nickel atom, then the total number of electrons n e in the coin is n e = N n Ni = ( 4 . 57478 × 10 22 atoms ) × (28 electrons / atom) = 1 . 28094 × 10 24 electrons . 003 (part 3 of 3) 10 points Find the magnitude of the charge of all these electrons. Correct answer: 205229 C. Explanation: Let : q e = 1 . 60218 × 10 19 C / electron . The total charge q for the n e electrons is q = n e q e = (1 . 28094 × 10 24 electrons) × ( 1 . 60218 × 10 19 C / electron ) = 205229 C , which has a magnitude of 205229 C . keywords: 004 (part 1 of 1) 10 points A particle with charge 5 μ C is located on the xaxis at the point 4 cm , and a second particle with charge 8 μ C is placed on the xaxis at 10 cm . The Coulomb constant is 8 . 9875 × 10 9 N · m 2 / C 2 . 2 4 6 8 10 2 4 6 8 10 5 μ C 8 μ C 5 μ C x → (cm) What is the magnitude of the total elec trostatic force on a third particle with charge 5 μ C placed on the xaxis at 2 cm? Correct answer: 536 . 753 N. Explanation: Let : q 1 = 5 μ C = 5 × 10 6 C , q 2 = 8 μ C = 8 × 10 6 C , q 3 = 5 μ C = 5 × 10 6 C , x 1 = 4 cm = . 04 m , x 2 = 10 cm = 0 . 1 m , and x 3 = 2 cm = . 02 m . Wong, Lemuel – Homework 4 – Due: Feb 16 2007, 11:00 pm – Inst: Matt Mackie 2 Coulomb’s law (in vector form) for the elec tric force exerted by a charge q 1 on a second charge q 3 , written ~ F 13 is ~ F 13 = k e q 1 q 3 r 2 ˆ r 13 , where ˆ r 13 is a unit vector directed from q 1 to q 3 ; i.e. , ~r 13 = ~r 3 ~r 1 ....
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 Spring '08
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 Physics, Work, Correct Answer, Electric charge, WONG, Matt Mackie

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