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Unformatted text preview: brown (twb493) – Ch21-h2 – chiu – (56565) 1 This print-out should have 29 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 current-carrying wire is oriented along the y axis. It passes through a region 0 . 9 m long in which there is a magnetic field of 4 . 6 T in the + z direction. The wire experiences a force of 15 . 4 N in the − x direction. What is the magnitude of the conventional current in the wire? Correct answer: 3 . 71981 A. Explanation: We can find the current as follows: | F | = vextendsingle vextendsingle vextendsingle I vector L × vector B vextendsingle vextendsingle vextendsingle = I L B ⇒ I = F L B = 15 . 4 N (0 . 9 m)(4 . 6 T) = 3 . 71981 A . 002 (part 2 of 2) 10.0 points What is the direction of the conventional cur- rent in the wire? 1. ( , 1 , ) 2. ( , , 1 ) 3. ( , , − 1 ) 4. ( 1 , , ) 5. (− 1 , , ) 6. ( , − 1 , ) correct Explanation: Use the right-hand rule. The current flows in the − y direction. 003 (part 1 of 3) 10.0 points A metal rod of length 14 cm and mass 76 g has metal loops at both ends, which go around two metal poles as in the following figure. 14 cm 4 cm I = 3 A I = 3 A 70 gram rod I = 3 A The rod is in good electrical contact with the poles but can slide freely up and down. The metal poles are connected by wires to a battery, and a 3 A current flows through the region of the rod. An external magnetic field vector B is present in the region, large enough that you can neglect the magnetic fields due to the 3 A current. The magnetic field is oriented so as to have the maximum effect on the rod. The rod sits at rest 4 cm above the table. Take the + x axis to be toward the right, the + y axis to be upward, and the + z axis out of the plane of the page. What is the (external) magnetic field in the region of the rod? Begin by finding the x component, B x . If necessary, use g = 9 . 8 m / s 2 . Correct answer: 0 T. Explanation: The metal rod is in equilibrium, so vector F net = 0. The gravitational force by Earth on the rod is downward, so the magnetic force by the magnetic field on the rod must be upward. Since vector F mag = I vector L × vector B and I flows in the + x direction, vector B must be in the − z direction. Therefore we can immediately tell that the x and y components of the magnetic field are zero. 004 (part 2 of 3) 10.0 points Find B y . Correct answer: 0 T. Explanation: See the explanation for part 1. brown (twb493) – Ch21-h2 – chiu – (56565) 2 005 (part 3 of 3) 10.0 points Find B z . Correct answer: 1 . 77333 T. Explanation: Based on what we discussed in part 1, we can apply the Momentum Principle to find vector F net = 0 vector F mag + vector F grav = 0 ⇒ vextendsingle vextendsingle vextendsingle vector F mag vextendsingle vextendsingle vextendsingle = vextendsingle vextendsingle vextendsingle vector F grav vextendsingle vextendsingle vextendsingle I L B = m g ⇒ B = m g I L =...
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