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Unformatted text preview: yang (ey942) – ohw22 – turner – (56705) 1 This printout should have 15 questions. Multiplechoice questions may continue on the next column or page – find all choices before answering. 001 (part 1 of 4) 10.0 points The magnetic field is constant and it points into the paper. The instantaneous velocity vector of a positive charge particle is pointing to the right. + + + + + + + + + + + + + + + + q v The trajectory of the subsequent motion is 1. bending along a counterclockwise path. correct 2. bending along a clockwise path. 3. to continue to move along a straight line to the right. Explanation: vector F = q vector v × vector B vector F = I vector l × vector B E = d Φ dt The particle experiences Lorenz force vector F = q vector v × vector B , which is pointing upward at that instant. So the trajectory of subsequent motion is coun terclockwise. 002 (part 2 of 4) 10.0 points Two parallel wires carry opposite current as shown. i 1 i 2 Find the direction of the magnetic force on i 2 due to the magnetic field of i 1 . 1. into the paper 2. to the right correct 3. out of the paper 4. to the left Explanation: i 1 i 2 F 2 B 1 Wire 1, which carries a current i 1 , sets up a magnetic field vector B 1 , which points into the paper at the position of wire 2. The direction of vector B 1 is perpendicular to the wire, as illustrated below. The magnetic force on a length l of wire 2 is vector F 2 = i 2 vector l × vector B 1 . Since i 2 flows downward, vector l × vector B 1 is to the right. 003 (part 3 of 4) 10.0 points A metal bar pivoting at A is rotating in a uniform magnetic field (see the setup shown below). yang (ey942) – ohw22 – turner – (56705) 2 A B v The potential difference V A V B is 1. positive. correct 2. negative. 3. zero. Explanation: v q A B Since the electrons move with the bar, the direction of their velocity is the same as the direction of rotation. vector F = q vector v × vector B Noting that the charge of electrons is nega tive, we find out that the magnetic force is pointing radially outwards (see the figure be low). Since electrons move radially outwards, nega tive net charge will cumulate at the outward end of the metal bar, while there will be pos itive net charge at the inward end. Therefore the potential V A should be higher than V B . Alternatively, the magnetic force on positive charge is directed radially inward. Thus the magnetic force does work on positive charge in pushing it from B to A. In analogy to ski lift, the magnetic force acts as the external force in lifting the “chairlift” from lower poten tial point to higher potential point. Therefore V A > V B , or V A V B is positive. 004 (part 4 of 4) 10.0 points Imagine that Galileo had dropped a bar mag net down the copper tube from the Tower of Pisa....
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This note was uploaded on 11/21/2010 for the course PHY 56705 taught by Professor Turner during the Spring '10 term at University of Texas.
 Spring '10
 Turner

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