In the equilateral-triangle setup with q1 = 1 nC, q2 = 2 nC, and q3 = 2 nC at the three corners, what is the direction of the net force on the bottom charge q1?

• A. Downward
• B. Right
• C. Upward
• D. Left

Answer: (C)

The net force is found by adding the two forces from the other charges, vectorially. Each force on the bottom charge lies along the line joining it to a top charge, and because the two top charges have the same magnitude and sit at equal distances from the bottom one, the two forces have equal magnitudes and are symmetrically placed.

The horizontal components from the top charges cancel each other due to symmetry, leaving only vertical components. If the top charges are opposite in sign to the bottom charge (so the interactions are attractive), both vertical components point upward. Since they have the same magnitude and direction, they add, giving a net upward force on the bottom charge.