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Unformatted text preview: gonzales (pag757) Homework10 Hoffmann (56745) 1 This print-out should have 20 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 (part 1 of 2) 10.0 points A long, straight wire carries a current and lies in the plane of a rectangular loops of wire, as shown in the figure. 3 . 6cm 32 cm 9 . 5cm (50A)sin bracketleftBig (272rad / s) t + bracketrightBig 47 loops (turns) Determine the maximum emf , |E| , induced in the loop by the magnetic field created by the current in the straight wire. Correct answer: 27 . 8287 mV. Explanation: Let : N = 47 , = 272 rad / s , = 9 . 5 cm , a = 3 . 6 cm , b = 32 cm , and I = 50 A . a b r dr I = I sin( t + ) Magnetic field near a long wire is B = I 2 r . Faradays Law is E =- d B dt . The magnitude of the magnetic field is B = I 2 r . Thus the flux linkage is B = N B 1 = N I 2 integraldisplay a + b a dr r = N I 2 ln bracketleftbigg a + b a bracketrightbigg sin( t + ) . Finally, the induced emf E is E =- d B dt =- N I 2 ln bracketleftbigg a + b a bracketrightbigg cos( t + ) , which is a maximum when the cosine function yields 1. E max = N 2 I ln bracketleftbigg a + b a bracketrightbigg = (1 . 25664 10 6 N / A 2 )(47) 2(3 . 14159) (50 A)(9 . 5 cm)(272 rad / s) ln bracketleftbigg (3 . 6 cm) + (32 cm) (3 . 6 cm) bracketrightbigg parenleftBig 10 2 m cm parenrightBig parenleftbigg 10 3 mV V parenrightbigg = 27 . 8287 mV . gonzales (pag757) Homework10 Hoffmann (56745) 2 002 (part 2 of 2) 10.0 points If at t = 0, =0, and a positive I denotes an upward moving current in the figure, then which statement below is correct at t = 0. 1. There is zero current in the loop. 2. The current in the loop is counter- clockwise. correct 3. The sense of current is not determined with the information given. 4. The current in the loop is clockwise. Explanation: Note: d I dt ( t =0) = I . At t = 0, the magnetic field from the long straight wire is increasing into the loop from above which leads to a finite time rate of change of magnetic flux, although B ( t =0) = 0 . The induced current in the loop then at- tempts to produce magnetic field through the loop that penetrates the loop from below ( i.e. , from Lenzs law it is attempting to resist the change of flux). By the right hand rule, this requires counter-clockwise induced current in the loop. 003 10.0 points A toroid having a rectangular cross section ( a = 2 . 36 cm by b = 2 . 7 cm) and inner radius 2 . 77 cm consists of N = 460 turns of wire that carries a current I = I sin t , with I = 60 . 5 A and a frequency f = 34 . 3 Hz....
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- Spring '08