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Unformatted text preview: Lee, In Seon Homework 1 Due: Sep 11 2006, 1:00 pm Inst: Hwang WY 1 This print-out should have 21 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. The due time is Central time. 001 (part 1 of 1) 10 points 1) Two uncharged metal balls, X and Y , stand on glass rods and are touching. Y X 2) A third ball, carrying a negative charge, is brought near the first two. Y X- 3) Then the first two balls are separated from each other, Y X- 4) and the third ball is finally removed. Y X When this is all four steps are done, it is found out that 1. Balls X and Y are both positive. 2. Ball X is positive and ball Y is negative. 3. Balls X and Y are both negative. 4. Balls X and Y are still uncharged. 5. Ball X is negative and ball Y is positive. correct Explanation: Basic Concept: Electric induction caused by nearby charges. Solution: When a negative ball is moved near a metallic object ( X and Y ), the negative charge will attract positive charges, causing X to have excess negative charge and Y to have excess positive charge ( X and Y are in contact, so the total net charge on X and Y should be zero). +-- Later, X and Y are separated, retaining their charges, so when the third ball is finally removed, X will have net negative charge and Y will have net positive charge. keywords: 002 (part 1 of 2) 10 points What can we conclude from the attractive na- ture of the force between a positively charged rod and an object? + + + F F ? 1. The object is an insulator. 2. The object is a conductor. 3. Cannot be determined. correct Explanation: The attractive nature of the force will be the same whether the object is a conductor or insulator. 003 (part 2 of 2) 10 points What can we conclude from the attractive na- ture of the force between a positively charged rod and an object? 1. Cannot be determined. Lee, In Seon Homework 1 Due: Sep 11 2006, 1:00 pm Inst: Hwang WY 2 2. The object has an appreciable positive charge. 3. The object does not have an appreciable negative charge. 4. The object has an appreciable negative charge. 5. The object has no net charge. 6. The object does not have an appreciable positive charge. correct Explanation: Because of the electric induction caused by the (positively or negatively) charged rod, there is always attractive force between the rod and the object unless the object has some net charge with the same sign as the charge of the rod, whether the object is an insulator or a conductor. keywords: 004 (part 1 of 1) 10 points Two identical small charged spheres hang in equilibrium with equal masses as shown in the figure. The length of the strings are equal and the angle (shown in the figure) with the vertical is identical....
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