18.20. Set Up: For a point charge, Solve: 18.21. Set Up: From Chapter 17 we know that for a spherical shell the electric field outside the shell is the same as for a point charge located at the center of the shell. The electric field determines the force on a test charge placed outside the shell, and the work done on the test charge as it moves between two points determines the potential difference between those points. Therefore, outside the shell the potential is the same as for a point charge. Solve: (a) (b) Now so Reflect: The potential is positive since the charge is positive and the potential increases at points closer to the sur-face of the sphere. 18.22. Set Up: For a single point charge The total potential is the sum of the potentials due to the two point charges. Solve: (a) (b) (c) 18.23. Set Up: Conservation of energy says Solve: (a) (b) (i) so (ii) so (iii) so Reflect: As the charge q moves away from Q the repulsive force does positive work on q and its kinetic energy increases. 18.24. Set Up:
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