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Unformatted text preview: Platt, David Homework 19 Due: Oct 24 2005, 4:00 am Inst: Ken Shih 1 This print-out should have 10 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. The due time is Central time. 001 (part 1 of 1) 10 points A conductor suspended by two flexible wires as in the figure has a mass per unit length of . 86 kg / m. The acceleration of gravity is 9 . 8 m / s 2 . B in What current must exist in the conductor in order for the tension in the supporting wires to be zero when the magnetic field is 3 . 7 T into the page? Correct answer: 2 . 27784 A. Explanation: Let : B = 3 . 7 T , g = 9 . 8 m / s 2 , and = m = 0 . 86 kg / m . The magnetic force acting on a current car- rying wire is ~ F = I ~ l ~ B . To balance the wire, the magnetic force must be equal to the gravity fl fl fl ~ F fl fl fl = mg , so I = mg l B = . 86 kg / m 9 . 8 m / s 2 3 . 7 T = 2 . 27784 A where is the mass per unit length. 002 (part 1 of 1) 10 points An infinitely long straight wire is bent as shown in the figure. The circular portion has a radius of 190 cm with its center a distance r from the straight part. I I 190 cm r Find r so that the magnetic field at the center of the circular portion is zero. Correct answer: 60 . 4789 cm. Explanation: Let : R = 190 cm . For a net magnetic field at the center to be zero, B line + B loop = 0 I 2 r + - I 2 R = 0 I 2 1 r- 1 R = 0 1 r- 1 R = 0 r = R = 190 cm = 60 . 4789 cm ....
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