Name___________________________ Ch 18 Sample Multiple Choice Questions – Electric Charge and Force Note: Unless otherwise noted, use g = 10 m/s2 and neglect air resistance. 1. If two objects are electrically attracted to each other, (A) both objects must be negatively charged. (B) both objects must be positively charged. (C) one object must be negatively charged and the other object must be positively charged. (D) the objects could be electrically neutral. (E) None of the above statements are absolutely true. 2. A plastic rod is charged up by rubbing a wool cloth and brought to an initially neutral metallic sphere that is insulated from ground. It is allowed to touch the sphere for a few seconds, and then is separated from the sphere by a small distance. After the rod is separated, the rod (A) is repelled by the sphere. (B) is attracted to the sphere. (C) feels no force due to the sphere. 3. When a positively charged rod is brought near, but does not touch, the initially uncharged electroscope shown above, the leaves spring apart (I). When the electroscope is then touched with a finger, the leaves collapse (II). When the finger and the rod are removed, the leaves spring apart a second time (III). The charge on the leaves is (A) positive in both I and III (B) negative in both I and III (C) positive in I, negative in III (D) negative in I, positive in III (E) impossible to determine in either I or III 4. X and Y are two initially uncharged metal spheres on insulating stands, and they are in contact with each other. A positively charged rod R is brought close to X as shown in part (a) of the figure. Sphere Y is now moved away from X, as shown in part (b). What are the final charge states of X and Y? (A) Both X and Y are neutral. (B) X is positive and Y is neutral. (C) X is neutral and Y is positive. (D) X is negative and Y is positive. (E) Both X and Y are negative. 5. Two conducting spheres of equal size have a charge of – 3 C and +1 C, respectively. A conducting wire is connected from the first sphere to the second. What is the new charge on each sphere? (A) – 4 C (B) + 4 C (C) – 1 C (D) + 1 C (D) zero 6. Two charges q1 and q2 are separated by a distance r and apply a force F to each other. If both charges are doubled, and the distance between them is halved, the new force between them is (A) ¼ F (B) ½ F (C) 4F (D) 8F (E) 16F 7. Two identical conducting spheres are charged to +2Q and Q, respectively, and are separated by a distance d (much greater than the radii of the spheres) as shown above. The magnitude of the force of attraction on the left sphere is F1. After the two spheres are made to touch and then are reseparated by distance d the magnitude of the force on the left sphere is F2. Which of the following relationships is correct? (A) 2F1 = F2 (B) F1 = F2 (C) F1 = 2F2 (D) F1 = 4F2 (E) F1 = 8F2 Use the following to answer the next two questions: Two small plastic balls are 25 cm apart with no other charges or fields present. Ball A has a mass of 10 g and carries a charge of 10 µC while ball B has a mass of 1 g and carries a charge of 1 µC. 8. Which one of the following statements is true about the magnitudes of the electric forces on these balls? (A) The force on A is 10 times the force on B. (B) The force on B is 10 times the force on A. (C) The force on A is exactly equal to the force on B. (D) The force on A is 100 times the force on B. (E) The force on B is 100 times the force on A. 9. Which of the following statements are true about the two balls as they move away from each other? Select two answers. (A) The magnitude of the acceleration of the 10-µC ball is 10 times that of the 1-µC ball. (B) The magnitude of the acceleration of the 1-µC ball is 10 times that of the 10-µC ball. (C) The balls always have accelerations of equal magnitude. (D) The speed of the balls keeps increasing. (E) The magnitude of the acceleration of the balls keeps increasing. 10. In outer space, a positive charge q is released near a positive fixed charge Q, as shown in the figure. As q moves away from Q, what is true about the motion of q? Select two answers. (A) It will move with decreasing speed. (C) It will move with increasing acceleration. (E) It will move with increasing speed. (B) It will move with constant acceleration. (D) It will move with decreasing acceleration. Use the following to answer the next two questions: As shown below, two particles, each of charge +Q, are fixed at opposite corners of a square that lies in the plane of the page. A positive test charge +q is placed at a third corner. 11. What is the direction of the force on the test charge due to the two other charges? (A) (B) (C) (D) (E) 12. If F is the magnitude of the force on the test charge due to only one of the other charges, what is the magnitude of the net force acting on the test charge due to both of these charges? (A) Zero (B) (C) F (D) √2 (E) 2F √ Use the following to answer the next two questions: As shown in the figure below, three charges are at the vertices of an equilateral triangle. 13. Which of the following arrows best indicates the direction of the net electrical force on the 1.0 nC charge due to the other two charges? (A) (B) (C) (D) 14. What is the magnitude of the net electrical force on the 1.0 nC charge due to the other two charges? (A) 3.1 × 10 N (B) 4.5 × 10 N (C) 5.4 × 10 N (D) 6.3 × 10 N 15. A piece of plastic has a net charge of +2.00 μC. How many more protons than electrons does this piece of plastic have? 13 13 19 23 (B) 1.25 × 1019 (A) 1.25 × 10 (C) 2.50 × 10 (D) 2.50 × 10 (E) 3.01 × 10 16. The force of attraction that a 40.0 μC point charge exerts on a +108 μC point charge has magnitude 4.00 N. How far apart are these two charges? (A) 2.10 m (B) 3.67 m (C) 3.12 m (D) 2.49 m (E) 1.13 m 17. Three point charges are located on the x-axis at the following positions: Q1 = +2.00 μC is at x = 1.00 m, Q2 = +3.00 μC is at x = 0.00, and Q3 = 5.00 μC is at x = 1.00 m. What is the magnitude of the electric force on Q2? (A) 0.0540 N (B) 0.135 N (C) 0.158 N (D) 0.0810 N (E) 0.189 N 18. Two tiny particles having charges of +5.00 μC and +7.00 μC are placed along the x-axis. The +5.00-µC particle is at x = 0.00 cm, and the other particle is at x = 100.0 cm. Where on the x-axis must a third charged particle be placed so that it does not experience any net electrostatic force due to the other two particles? (A) 4.58 cm (B) 45.8 cm (C) 50 cm (D) 9.12 cm (E) 91.2 cm 19. A charged plastic ball with mass m is vertically above another charged ball in a frictionless test tube as shown in the figure. The balls are in equilibrium a distance d apart. If both balls have the same positive charge, what is the charge on each ball? (A) = (B) = (C) = (D) = (E) = 20. The figure shows two tiny 5.0-g spheres suspended from very light 1.0-m-long threads. The spheres repel each other after each one is given the same positive charge and hang at rest when θ = 4.1°. What is the charge on each sphere? (A) 22 nC Answers 1. E 2. A 11. C 12. D (B) 90 nC 3. C 13. C (C) 360 nC 4. D 14. A 5. C 15. A (D) 180 nC 6. E 16. C 7. E 17. E (E) 45 nC 8. C 18. B 9. B,D 10. D,E 19. D 20. B

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