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# 21-3 - brown(twb493 Ch21-h3 chiu(56565 1 This print-out...

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brown (twb493) – Ch21-h3 – chiu – (56565) 1 This print-out should have 23 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0points A copper wire with square cross section car- ries a conventional current I to the left as in the figure below. There is a magnetic field B perpendicular to the wire. Take the + x axis to point toward the right, + y to point up- ward, and + z to point out of the plane of the page (toward you). Copper B I Which of the following unit vectors gives the direction of vector E , the transverse electric field inside the wire due to the Hall effect? 1. ( 0 , 1 , 0 ) 2. ( 1 , 0 , 0 ) 3. ( 0 , 0 , 1 ) 4. vextendsingle vextendsingle vextendsingle vector E vextendsingle vextendsingle vextendsingle = 0 so vector E has no direction. 5. ( 0 , 0 , 1 ) correct 6. (− 1 , 0 , 0 ) 7. ( 0 , 1 , 0 ) Explanation: vector E is in the z direction (into the page). Electrons are the mobile charge carriers, and they move in the + x direction. The magnetic force on the moving electrons is in the z di- rection. As a result, negative surface charges piles upon the back surface of the wire, and positive surface charge builds up on the front surface of the wire. The component of the electric field due to this surface charge points away from the + charge and toward the charge, so it points in the z direction. 002(part1of9)5.0points The Hall effect can be used to determine the sign of the mobile charges in a particu- lar conducting material. A bar of a new kind of conducting material is connected to a bat- tery as shown in the figure below. In this diagram, the x axis runs to the right, the y axis runs up, and the z axis runs out of the page toward you. A voltmeter is connected across the bar as shown, with the leads placed directly opposite each other along a vertical line. In order to answer the following ques- tions, you should draw a careful diagram of the situation, including all relevant charges, electric fields, magnetic fields, and velocities. + + Voltmeter Initially, there is no magnetic field in the region of the bar. Inside the bar, which unit vector gives the direction of the electric field vector E due to the charges on the batteries and surface of the wires and the bar? This is the electric field that drives the current in the bar. 1. ( 0 , 1 , 0 ) 2. ( 1 , 0 , 0 ) correct 3. (− 1 , 0 , 0 ) 4. ( 0 , 0 , 1 ) 5. ( 0 , 0 , 1 ) 6. ( 0 , 1 , 0 )

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brown (twb493) – Ch21-h3 – chiu – (56565) 2 Explanation: E tan is in the + x direction. 003(part2of9)5.0points If the mobile charges in the bar are positive, in what direction do they move when current runs? Choose the best unit vector. 1. ( 0 , 0 , 1 ) 2. ( 0 , 1 , 0 ) 3. (− 1 , 0 , 0 ) 4. ( 1 , 0 , 0 ) correct 5. ( 0 , 1 , 0 ) 6. ( 0 , 0 , 1 ) Explanation: Positive mobile charges move in the + x direction. 004(part3of9)5.0points If the mobile charges in the bar are negative, in what direction do they move when current runs? Choose the best unit vector.
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