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Unformatted text preview: roofner (bar784) Homework 1 Weathers (22202) 1 This print-out should have 8 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 10.0 points 1) Two uncharged metal balls, Y and X , each stand on a glass rod and are touching. X Y 2) A third ball carrying a positive charge, is brought near the first two. X Y + 3) While the positions of these balls are fixed, ball Y is connected to ground. X Y + 4) Then the ground wire is disconnected. X Y + 5) While Y and X remain in touch, the ball carring the positive charge is removed. X Y 6) Then ball Y and X are separated. X Y After these procedures, the signs of the charge q Y on Y and q X on X are 1. q Y is neutral and q X is neutral. 2. q Y is neutral and q X is negative. 3. q Y is negative and q X is negative. cor- rect 4. q Y is negative and q X is positive. 5. q Y is positive and q X is negative. 6. q Y is positive and q X is positive. 7. q Y is positive and q X is neutral. 8. q Y is negative and q X is neutral. 9. q Y is neutral and q X is positive. Explanation: When the ball with positive charge is brought nearby, the free charges inside Y and X rearrange themselves. The negative charges are attracted and go to the right ( i.e. move to X ), whereas the positive charges are repelled and collect in the left hand side of the system Y Y , i.e., in Y . When we ground Y , the positive charges which have collected in Y are allowed to es- cape (they strive to the left), whereas the negative charges in X are still held enthralled by the positive charge on the third ball. We break the ground. Now we remove the third ball with positive charge. The charge on X is redistributed in the system Y Y , i.e. they share the negative charge (equally if identical). Finally we separate Y and X . The signs of the charge on Y and that on X are both negative. 002 10.0 points Four point charges, each of magnitude 18 . 82 C, are placed at the corners of a square 27 . 3 cm on a side. The value of Coulombs constant is 8 . 98755 10 9 N m 2 / C 2 . roofner (bar784) Homework 1 Weathers (22202) 2 If three of the charges are positive and one is negative, find the magnitude of the force experienced by the negative charge. Correct answer: 81 . 7609 N. Explanation: Let : k e = 8 . 98755 10 9 N m 2 / C 2 , d = 27 . 3 cm = 0 . 273 m , and Q = 18 . 82 C = 1 . 882 10 5 C . F 24 F 34 F 14 + 3 + 1 + 2- 4 The forces are F 34 = F 24 = k e Q 2 d 2 = ( 8 . 98755 10 9 N m 2 / C 2 ) ( 1 . 882 10 5 C ) 2 (0 . 273 m) 2 = 42 . 7125 N F 14 = k e Q 2 ( 2 d ) 2 = 1 2 k e Q 2 d 2 = 1 2 F 34 The vector sum of F 24 and F 34 is in the same direction as F 14 , so F = F 14 + radicalBig F 34 2 + F 24 2 = 1 2 F...
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