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HW 1 solution

# HW 1 solution - song(shs546 Homework 1 weathers(22202 This...

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song (shs546) – 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 10.0points 1) Two uncharged metal balls, X and Y , each stand on a glass rod and are touching. Y X 2) A third ball carrying a positive charge, is brought near the first two. Y X + 3) While the positions of these balls are fixed, ball X is connected to ground. Y X + 4) Then the ground wire is disconnected. Y X + 5) While X and Y remain in touch, the ball carring the positive charge is removed. Y X 6) Then ball X and Y are separated. Y X After these procedures, the signs of the charge q X on X and q Y on Y are 1. q X is positive and q Y is neutral. 2. q X is neutral and q Y is neutral. 3. q X is positive and q Y is positive. 4. q X is negative and q Y is neutral. 5. q X is neutral and q Y is negative. 6. q X is positive and q Y is negative. 7. q X is neutral and q Y is positive. 8. q X is negative and q Y is negative. cor- rect 9. q X is negative and q Y is positive. Explanation: When the ball with positive charge is brought nearby, the free charges inside X and Y rearrange themselves. The negative charges are attracted and go to the right ( i.e. move to Y ), whereas the positive charges are repelled and collect in the left hand side of the system XY , i.e., in X . When we ground X , the positive charges which have collected in X are allowed to es- cape (they strive to the left), whereas the negative charges in Y 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 Y is redistributed in the system XY , i.e. they share the negative charge (equally if identical). Finally we separate X and Y . The signs of the charge on X and that on Y are both negative. 002 10.0points Four point charges, each of magnitude 11 . 71 μ C, are placed at the corners of a square 26 cm on a side. If three of the charges are positive and one is negative, find the magnitude

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song (shs546) – Homework 1 – weathers – (22202) 2 of the force experienced by the negative charge. The value of Coulomb’s constant is 8 . 98755 × 10 9 N · m 2 / C 2 . Correct answer: 34 . 8979 N. Explanation: Let : k e = 8 . 98755 × 10 9 N · m 2 / C 2 , d = 26 cm = 0 . 26 m , and Q = 11 . 71 μ C = 1 . 171 × 10 5 C .
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