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Unformatted text preview: brown (twb493) Ch16-h3 chiu (56565) 1 This print-out should have 19 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 (part 1 of 4) 10.0 points A solid metal ball of radius 1 . 5 cm bearing a charge of 16 nC is located near a solid plastic ball of radius 2 cm bearing a uniformly distributed charge of 4 nC on its outer surface. The distance between the centers of the balls is 11 cm. Given that the value of k is 9 10 9 N m 2 / C 2 . 16 nC 4 nC Metal Plastic Which one of the following best represents the charge distributions on the two balls? 1. 16 nC 4 nC Metal Plastic------------ + + + + + + + + 2. No static charge distribution will satisfy this setupthe charges will be moving for all times. 3. 16 nC 4 nC Metal Plastic------------ + + + + + + + + correct 4. 16 nC 4 nC Metal Plastic-------- + + + + + + + + 5. 16 nC 4 nC Metal Plastic-------- + + + + + + + + Explanation: Plastic is an insulator, so the initially uni- formly distributed positive charges will not be able to move, even when brought near the metal ball. Metal, however, is a conductor, and since the positive charges on the plastic ball will attract the negative charges on the metal ball, this will cause a net migration of negative charge to the right hand side of the metal ball. Therefore, the actual charge distribution wil most closely mirror this one: 16 nC 4 nC Metal Plastic------------ + + + + + + + + 002 (part 2 of 4) 10.0 points What is the magnitude of the electric field at brown (twb493) Ch16-h3 chiu (56565) 2 the center of the metal ball due only to the charges on the plastic ball? Correct answer: 2975 . 21 N / C. Explanation: Gausss law tells us that we can treat the field outside of a spherically symmetric charge distribution precisely as if it were a point charge with all of the charge concentrated at the center of the distribution. Since the center of the plastic ball is 11 cm away from the center of the metal ball, we find that the net contribution due to the plastic ball is E = k q plastic d 2 = (9 10 9 N m 2 / C 2 )(4 nC) (11 cm) 2 1 10 9 C / nC (0 . 01 m / cm) 2 = 2975 . 21 N / C 003 (part 3 of 4) 10.0 points What is the net electric field at the center of the metal ball? Correct answer: 0 N / C. Explanation: The field inside of a conductor in a static configuration must be zero, since if the field were not zero, then the net field would move charges in the conductor, creating a non-static charge distribution. 004 (part 4 of 4) 10.0 points What is the magnitude of the electric field at the center of the metal ball due only to charges on the surface of the metal ball?...
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- Fall '08