SAT Subject Physics Practice Question 546: Answer and Explanation
2. A positively charged rod is brought near an uncharged pith ball that is being touched by a copper wire. Both the rod and the copper wire are simultaneously removed, and the pith ball is tested to find if any electrostatic charge is present. What is the result?
A. The pith ball has been positively charged by conduction.
B. The pith ball has been positively charged by induction.
C. The pith ball has been negatively charged by conduction.
D. The pith ball has been negatively charged by induction.
E. The pith ball has not been charged in any way.
Correct Answer: D
The correct answer is (D). Objects that are touched by the charging body are charged by conduction. Objects that are not touched by the charging body are charged by induction. The negative charge is placed on the pith ball when a positively charged rod is brought near the pith ball. The pith ball is touched by a body other than the charging rod, which provides a path for electrons to flow onto the pith ball and charges the pith ball oppositely from the charge on the rod.