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Unformatted text preview: yang (ey942) – ohw17 – turner – (56705) 1 This print-out should have 15 questions. Multiple-choice questions may continue on the next column or page – find 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 accelerates in the + y direction, 2 in the + x direction. 2. Both wires rotate clockwise around the origin. 3. 1 rotates counterclockwise, 2 clockwise around the origin. correct 4. Neither wire moves. 5. 1 rotates clockwise, 2 counterclockwise around the origin. 6. 1 accelerates in the − y direction, 2 in the- x direction. 7. Both wires rotate counterclockwise around the origin. 8. Both wires accelerate along the direction of current flow. 9. 1 accelerates in the + y direction, 2 in the- x direction. 10. 1 accelerates in the − y direction, 2 in the + x direction. Explanation: After the currents are turned on, each wire creates a magnetic field which applies a mag- netic force on the other wire. Consider the force on wire 1 first. According to the right- hand rule, the magnetic field 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 find 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 field vector B , which is directed along the positive y axis as shown in the figure. 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 vector B field? 1. bardbl vector B bardbl = I 2 (2 L + 2 R ) B 2. bardbl vector B bardbl = 2 I R B 3. bardbl vector B bardbl = (2 L + R ) I B 4. bardbl vector B bardbl = 2 I L B 5. bardbl vector B bardbl = parenleftbigg L + R 2 parenrightbigg I B 6. bardbl vector B bardbl = 2 I R L B 7. bardbl vector B bardbl = I ( L + R ) B 8. bardbl vector B bardbl = 4 I ( L + R ) B 9. bardbl vector B bardbl = (2 B L + 2 R ) I 10. bardbl vector B bardbl = 2 I ( L + R ) B correct Explanation: yang (ey942) – ohw17 – turner – (56705) 2 vector F = I integraldisplay dvectors × vector B = I vector ℓ × vector B . For wire 1, vector ℓ ⊥ vector B , so the magnitude of vector F is just F = I ℓ B . For wire #1, ℓ = 2 L + 2 R , sp F = 2 I ( R + L ) B ....
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- Spring '10
- Rotation, Magnetic Field, Right-hand rule