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S E C T I O N 2 3 . 7 • Motion of Charged Particles in a Uniform Electric Field 725 Figure 23.23 shows the electric field lines in the vicinity of two equal positive point charges. Again, the lines are nearly radial at points close to either charge, and the Finally, in Figure 23.24 we sketch the electric field lines associated with a positive charge # 2q and a negative charge " q. In this case, the number of lines leaving # 2q is (a) + + C A B (b) Figure 23.23 (a) The electric field lines for two positive point charges. (The locations A, B, and C are discussed in Quick Quiz 23.7.) (b) Pieces of thread suspended in oil, which align with the electric field created by two equal-magnitude positive charges. Courtesy of Harold M. W aage, Princeton University +2q – –q + Active Figure 23.24 The electric field lines for a point charge # 2q and a second point charge " q. Note that two lines leave # 2q for every one that terminates on " q. At the Active Figures link at http://www.pse6.com, you can choose the values and signs for the two charges and observe the electric field lines for the configuration that you have chosen. Quick Quiz 23.7 Rank the magnitudes of the electric field at points A, B, and C shown in Figure 23.23a (greatest magnitude first). Quick Quiz 23.8 Which of the following statements about electric field lines associated with electric charges is false? (a) Electric field lines can be either straight or 23.7 Motion of Charged Particles in a Uniform Electric Field When a particle of charge q and mass m is placed in an electric field E, the electric force exerted on the charge is q E according to Equation 23.8. If this is the only force exerted on the particle, it must be the net force and causes the particle to accelerate The acceleration of the particle is therefore (23.12) If E is uniform (that is, constant in magnitude and direction), then the acceleration is constant. If the particle has a positive charge, its acceleration is in the direction of the a ! q E m F e ! q
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