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Unformatted text preview: johnson (rj6247) – hw 6 – Opyrchal – (121014) 1 This printout should have 13 questions. Multiplechoice questions may continue on the next column or page – find all choices before answering. 001 10.0 points A conductor with crosssectional 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 : ℓ ′ = 8 . 1 ℓ . The volume V of the wire is given by V = ℓ A, where ℓ is the length of the wire and A is the crosssectional area. When the wire is stretched to a length ℓ ′ , the volume will re main the same. The new crosssectional area is then, ℓ A = ℓ ′ A ′ A ′ = ℓ A ℓ ′ = ℓ A 8 .....
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This note was uploaded on 03/11/2010 for the course PHYS 121 taught by Professor Opyrchal during the Spring '08 term at NJIT.
 Spring '08
 Opyrchal
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