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Unformatted text preview: johnson (rj6247) hw 6 Opyrchal (121014) 1 This print-out should have 13 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 10.0 points A conductor with cross-sectional area 9 cm 2 carries a current of 10 A. If the concentration of free electrons in the conductor is 9 10 28 electrons / m 3 , what is the drift velocity of the electrons? Correct answer: 0 . 000770556 mm / s. Explanation: Let : I = 10 A , n = 9 10 28 electrons / m 3 , q e = 1 . 60218 10 19 C , and A = 9 cm 2 = 0 . 0009 m 2 . The current in a conductor is given by I = n q v d A , where n is the number of charge carriers per unit volume, q is the charge per carrier, v d is the drift velocity of the carriers and A is the cross section of the conduction. Solving for v d , we have v d = I n q A = 10 A 9 10 28 electrons / m 3 1 1 . 60218 10 19 C 1 . 0009 m 2 1000 mm 1 m = . 000770556 mm / s . 002 10.0 points A wire with a resistance R is lengthened to 8 . 1 times its original length by pulling it through a small hole. Find the resistance of the wire after it is stretched. Correct answer: 65 . 61 R . Explanation: Let : = 8 . 1 . The volume V of the wire is given by V = A, where is the length of the wire and A is the cross-sectional area. When the wire is stretched to a length , the volume will re- main the same. The new cross-sectional area is then, A = A A = A = A 8 .....
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- Spring '08