Three charges form a triangle: top is positive; bottom left is positive; bottom right is negative with equal magnitudes for bottom charges. What is the horizontal direction of the net force on the top positive charge?

Three charges form a triangle: top is positive; bottom left is positive; bottom right is negative with equal magnitudes for bottom charges. What is the horizontal direction of the net force on the top positive charge?

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

Answer: (C)

Think in terms of vector forces and symmetry. The top positive charge experiences a repulsive force from the left-bottom positive charge and an attractive force toward the right-bottom negative charge. The force from the left-bottom charge points up and to the right, while the force toward the right-bottom charge points down and to the right. If the bottom charges have equal magnitude and the top is centered above the base, the two forces have equal magnitudes and their vertical components cancel (one goes up, the other goes down). The horizontal components, both to the right, add up, giving a net force directed horizontally to the right.

Think in terms of vector forces and symmetry. The top positive charge experiences a repulsive force from the left-bottom positive charge and an attractive force toward the right-bottom negative charge. The force from the left-bottom charge points up and to the right, while the force toward the right-bottom charge points down and to the right. If the bottom charges have equal magnitude and the top is centered above the base, the two forces have equal magnitudes and their vertical components cancel (one goes up, the other goes down). The horizontal components, both to the right, add up, giving a net force directed horizontally to the right.