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121HW6

121HW6 - lew(dl9564 hw 6 Opyrchal(121006 This print-out...

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lew (dl9564) – hw 6 – Opyrchal – (121006) 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 4 cm 2 carries a current of 19 A. If the concentration of free electrons in the conductor is 5 × 10 28 electrons / m 3 , what is the drift velocity of the electrons? Correct answer: 0 . 00592943 mm / s. Explanation: Let : I = 19 A , n = 5 × 10 28 electrons / m 3 , q e = 1 . 60218 × 10 19 C , and A = 4 cm 2 = 0 . 0004 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 = 19 A 5 × 10 28 electrons / m 3 × 1 1 . 60218 × 10 19 C × 1 0 . 0004 m 2 × 1000 mm 1 m = 0 . 00592943 mm / s . 002 10.0 points A wire with a resistance R is lengthened to 6 . 62 times its original length by pulling it through a small hole. Find the resistance of the wire after it is stretched. Correct answer: 43 . 8244 R . Explanation: Let : = 6 . 62 ℓ . The volume V of the wire is given by V = ℓ 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, ℓ A = A A = ℓ A = ℓ A 6 . 62 = A 6 . 62 .

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