A negatively charged ball is brought near end A of a rod that has a net positive charge at end A (with some negative charge in the center). What happens to end A when the ball approaches closely for the first time?

• A. It is repelled.
• B. It experiences no force.
• C. It is attracted.
• D. It causes the rod to rotate.

Answer: (C)

When charges are involved, opposite charges attract. The end A of the rod carries a net positive charge, while the approaching ball is negatively charged. The negative ball feels an attractive force toward the positive end because Coulomb’s law tells us that the force between opposite charges is directed along the line joining them and toward each other.

Even though there is some negative charge in the center of the rod, that region is farther from the ball when it first comes near end A. Since the electrostatic force weakens with distance, the stronger, closer attraction from the positive end dominates. So the end A is pulled toward the ball (and equivalently, the ball is pulled toward end A).

If the rod were free to move, that localized force could also produce torque, but the immediate effect at the first approach is an attractive interaction between the ball and the positively charged end.