State the superposition principle. Three charges of +2 μC each sit at the corners of an equilateral triangle of side 10 cm. Find the net force on any one charge.

1. 1 · State the principle. The net force on a charge is the vector sum of the forces due to all the other charges, each taken alone.
2. 2 · Draw the figure. [Equilateral triangle ABC, side 10 cm, +2 μC at each corner; at A, the two repulsive forces 60° apart and their resultant along the angle bisector]
3. 3 · Find one force. F = 9 × 10⁹ × (2 × 10⁻⁶)² ÷ (0.10)² = 3.6 N.
4. 4 · Add as vectors. Resultant = √(F² + F² + 2F² cos 60°) = √3 F.
5. 5 · Work out the answer. √3 × 3.6 N ≈ 6.2 N, along the bisector, away from the opposite side.
6. 6 · Close in one line. Each charge feels about 6.2 N, pushed outward from the centre of the triangle.

Scoring tip: Draw the force arrows before calculating. The diagram shows the examiner that you know the directions, and it earns a mark.