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Unformatted text preview: brown (twb493) – Ch18-h3 – chiu – (56565) 1 This print-out should have 14 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 straight wire of length 0 . 59 m carries a conventional current of 0 . 9 A. What is the magnitude of the magnetic field made by the current at a location that is a perpendicular distance 2 . 8 cm from the center of the wire? Taking μ 4 π = 1 × 10 − 7 T · m / A , first use the approximate formula to find the field. Correct answer: 6 . 42857 × 10 − 6 T. Explanation: The approximate formula for the magnetic field due to a straight wire is B = μ 4 π 2 I r , so we have B = μ 4 π 2(0 . 9 A) 2 . 8 cm = 6 . 42857 × 10 − 6 T . 002 (part 2 of 2) 10.0 points Now use the exact formula. Correct answer: 6 . 39981 × 10 − 6 T. Explanation: The exact formula for the magnetic field due to a straight wire is B = μ 4 π L I r radicalbig r 2 + ( L/ 2) 2 , so we have B = μ 4 π (0 . 59 m)(0 . 9 A) 2 . 8 cm radicalbig (2 . 8 cm) 2 + (0 . 59 m / 2) 2 = 6 . 39981 × 10 − 6 T . 003 (part 1 of 2) 10.0 points When you bring a current-carrying wire down onto the top of a compass, aligned with the original direction of the needle and 6 mm above the needle, the needle deflects by 12 degrees, as in the figure below. 12 ◦ B Earth Wire Assuming the compass needle was origi- nally pointing toward the north, what direc- tion is the conventional current traveling in the wire, and what is the direction of the force on the compass needle due to the mag- netic field caused by the current in the wire? 1. North, West 2. South, West 3. South, East correct 4. North, East Explanation: We just need to think about how the com- pass needle responds to the presence of the current-carrying wire to answer this question. Since the compass needle deflects to the east, that is the direction of the magnetic force on the needle due to the current in the wire. To decide which way current is traveling in the wire, we use the right hand rule. Your fingers should point toward the east, so in order for your fingers to curl around the wire and point toward the east, your thumb must point to- ward the south. So the current is traveling toward the south. 004 (part 2 of 2) 10.0 points Calculate the amount of current flowing in the wire. The measurement was made at a brown (twb493) – Ch18-h3 – chiu – (56565) 2 location where the horizontal component of the Earth’s magnetic field is B Earth = 2 × 10 − 5 T . Use μ 4 π = 1 × 10 − 7 A . Correct answer: 0 . 127534. Explanation: Ultimately, we want to use the expression B wire = μ 4 π 2 I r to find the current in the wire. We know r , but not B wire yet. To find B wire , we can use what we know about the Earth’s magnetic field and the deflection of the compass needle....
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- Fall '08
- Physics, Magnetic Field