After a great many contacts with the charged ball, when the charged ball is far away, how is the charge distributed on the rod?

• A. Negative charge spread evenly on both ends.
• B. Positive charges concentrate at end A.
• C. The rod becomes neutral.
• D. All charge resides at the center.

Answer: (A)

When a conductor is in electrostatic equilibrium, all charge resides on its surface, and the charges arrange themselves to minimize repulsion. For a long, slender rod, the surface ends are the places where charges can spread out most effectively, so the surface charge density is greatest at the ends. After many contacts with a charged object, the rod picks up excess charge and, once the object is removed, the excess negative charge redistributes to the surface, concentrating at both ends due to symmetry. This results in negative charge at both ends, with roughly equal amounts on each end, while the interior remains neutral.