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Unformatted text preview: Flores, Michael Homework 9 Due: Mar 29 2004, 4:00 am Inst: Sonia Paban 1 This printout should have 21 questions. Multiplechoice questions may continue on the next column or page find all choices before making your selection. The due time is Central time. 001 (part 1 of 1) 10 points The recent discovery of high temperature superconductors with negligible resistance has raised many possibilities for technological applications (magnetically levitated trains, zerodissipation power transmission lines, in credibly strong refrigerator magnets, etc.) However, these materials lose their supercon ductivity if they are subjected to excessive magnetic fields. This limits the amount of current that a superconductor can carry, since the current results in a magnetic field at the surface of the wire. Calculate this critical current for a d = 3 . 81 mm diameter superconducting wire if the superconductivity is destroyed when the surface magnetic field exceeds B = 0 . 0182 T. Given = 1 . 25664 10 6 N / A 2 . Correct answer: 173 . 355 A. Explanation: Solution: This problem can be solved with Amperes law, I ~ B d~s = I Choose the integration path to be a loop around the surface. This gives us B ( d ) = I where d is the diameter. Or, I = B d = (0 . 0182 T)(0 . 00381 m) 1 . 25664 10 6 N / A 2 = 173 . 355 A 002 (part 1 of 1) 10 points An infinitely long straight wire carrying a current I 1 = 73 . 6 A is partially surrounded by a loop as in figure. The loop has a length L = 22 . 7 cm, a radius R = 21 . 3 cm, and carries a current I 2 = 21 . 2 A. The axis of the loop coincides with the wire. R L I 1 I 2 Calculate the force exerted on the loop. Correct answer: 665 . 15 N. Explanation: The central wire creates field ~ B = I 1 2 R counterclockwise . The curved portions of the loop feels zero force since ~ l ~ B = 0 there. The straight portions both feel I 2 ~ l ~ B forces to the right, amounting to ~ F = I 2 2 L I 1 2 R = I 1 I 2 L R to the right k ~ F k = (73 . 6 A)(21 . 2 A)(0 . 227 m) (21 . 3 cm) = 665 . 15 N . 003 (part 1 of 2) 10 points A superconducting solenoid is to generate a magneticfieldof14 . 9T. Thesolenoidwinding has 1290 turns per meter. What is the required current? Correct answer: 9191 . 51 A. Explanation: Magnetic field at the center of solenoid is B = N I l = nI , then I = B l N Flores, Michael Homework 9 Due: Mar 29 2004, 4:00 am Inst: Sonia Paban 2 = (14 . 9 T) (1290) = 9191 . 51 A . 004 (part 2 of 2) 0 points What force per unit length is exerted on the windings by this magnetic field? Correct answer: 136953 N / m. Explanation: Force per unit length is F l = I B = (9191 . 51 A)(14 . 9 T) = (136953 N / m) radially outward ....
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This note was uploaded on 02/18/2010 for the course PHYS 303l taught by Professor Panab during the Spring '04 term at North Texas.
 Spring '04
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