Question

Three identical metal spheres are hung from a ceiling on rigid non-conducting rods. Sphere 1, sphere 2, and sphere 3 have charges 69 µC, −44 µC, and 48 µC, respectively. Each sphere is separated by 11 cm. (a) What is the net electric force on the middle sphere due to spheres 1 and 3? (Enter the magnitude only.)

Answer 1

net F = Σ kQq / d²

net F = 8.99e9N·m²/C² * 44e-6C * (48e-6C - 69e-6C) / (0.11m)²
Note that I've done the subtraction because the 48 µC charge is going to pull sphere 2 to the right (positive) and the 69 µC sphere is going to pull sphere 2 to the left (negative).

net F = -686 N ◄
That is, 686 N to the left, toward sphere 1.