ohw17-solutions - howard (cah3459) ohw17 turner (56705)...

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howard (cah3459) – ohw17 – turner – (56705) 1 This print-out should have 15 questions. Multiple-choice questions may continue on the next column or page – fnd all choices beFore answering. 001 10.0 points A long straight wire 1 lies along the x -axis. A long straight wire 2 lies along the y -axis so as to pass very near, but not quite touch wire 1 at the origin. IF both wires are Free to move, what hap- pens when currents are sent simultaneously in the + x direction through wire 1 and in the + y direction through wire 2? Note that “clock- wise around origin” reFers to an observer look- ing down on an xy plane in which + x is to the right and + y upward. 1. 1 rotates counterclockwise, 2 clockwise around the origin. correct 2. 1 accelerates in the - y direction, 2 in the - x direction. 3. Neither wire moves. 4. 1 rotates clockwise, 2 counterclockwise around the origin. 5. 1 accelerates in the - y direction, 2 in the + x direction. 6. Both wires rotate clockwise around the origin. 7. Both wires accelerate along the direction oF current ±ow. 8. 1 accelerates in the + y direction, 2 in the - x direction. 9. 1 accelerates in the + y direction, 2 in the + x direction. 10. Both wires rotate counterclockwise around the origin. Explanation: AFter the currents are turned on, each wire creates a magnetic feld which applies a mag- netic Force on the other wire. Consider the Force on wire 1 frst. According to the right- hand rule, the magnetic feld From wire 2 goes into the xy plane on the right side while out oF the plane on the leFt side. Consequently, the right halF oF wire 1 will have a upward Force while the leFt halF will have a downward Force causing wire 1 to rotate counterclock- wise. Similarly, we fnd that wire 2 will rotate clockwise. 002 (part 1 of 4) 10.0 points Two wires each carry a current I in the xy plane and are subjected to an external uni- Form magnetic feld $ B , which is directed along the positive y axis as shown in the fgure. R I wire #2 I wire #1 B L L x y What is the magnitude oF the Force on wire #1 due to the external $ B feld? 1. ± $ B ± =2 I R L B 2. ± $ B ± = ± L + R 2 ² IB 3. ± $ B ± I L B 4. ± $ B ± I R B 5. ± $ B ± = (2 L + R ) 6. ± $ B ± = I ( L + R ) B 7. ± $ B ± I ( L + R ) B correct 8. ± $ B ± = (2 BL +2 R ) I 9. ± $ B ± = I 2 (2 L R ) B 10. ± $ B ± =4 I ( L + R ) B Explanation:
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howard (cah3459) – ohw17 – turner – (56705) 2 $ F = I ± d$s × $ B = I $ # × $ B. For wire 1, $ # $ B, so the magnitude of $ F is just F = I # B . For wire #1, # =2 L +2 R, sp F = 2 I ( R + L ) B .
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This note was uploaded on 11/28/2010 for the course PHY 56705 taught by Professor Turner during the Spring '10 term at University of Texas.

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ohw17-solutions - howard (cah3459) ohw17 turner (56705)...

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