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Unformatted text preview: Physics 1B: Electricity & Magnetism Summer Session I, 2010 Quiz #1, 07/01/2010 1. A metallic object holds a total charge of- 6 . 6 10- 7 C. What total number of electrons does this amount of charge represent? a. 1 . 1 10 11 b. 1 . 6 10 13 c. 4 . 1 10 12 d. 1 . 6 10 12 e. 9 . 10 9 The unit of elementary charge, e, is 1 . 6 10- 19 C; each electron has a charge e. The number of electrons is (total charge)/( e) = (- 6 . 6 10- 7 C)/( 1 . 6 10- 19 C) = 4 . 1 10 12 . 2. Two identical balls have the same amount of charge, but the charge on ball A is positive and the charge on ball B is negative. The balls are placed on a smooth, level, frictionless table whose top is an insulator. Which of the following is true? a. Since the force on A is equal but opposite to the force on B, they will not move. b. They will move together with constant acceleration. c. Since the force on both balls is negative they will move in the negative direction. d. Since the forces are opposite in direction, the balls will move away from each other. e. None of the above is correct. The two balls will start moving towards each other, but their acceleration will not be constant. The acceleration a of each ball is related to the magnitude of the instantaneous electrostatic force F e via F e = ma , but F e at any given instant depends on the distance of separation r of the two balls, which is decreasing as the balls roll towards each other. So as r decreases, F e increases, and the instantaneous acceleration will also be increasing (not constant). So the answer is none of the above. 3. A ping-pong ball covered with a conducting graphite coating has a total mass of 4 grams and carries an excess charge of 0.2 C. What is the magnitude and direction of the electricC....
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