Calculate the horizontal component of the net force, in newtons, on the charge which lies at the lower left corner of the rectangle.

Question

Four point charges of equal magnitude Q = 55 nC are placed on the corners of a rectangle of sides D₁ = 21 cm and D₂ = 11 cm. The charges on the left side of the rectangle are positive while the charges on the right side of the rectangle are negative. Use a coordinate system where the positive y direction is up and the positive x direction is to the right.

I get 1.10 * 10^-3 as the answer but its wrong.

Answer 1

I reproduce your answer (actually 1.05 * 10^-3) if I assume the long side of the rectangle is horizontal. Is there any additional clue to the orientation? If it is .21m tall and .11m wide, then:

Angle θ to the upper right corner has tan θ = .21/.11, and θ = 62.4º

F_x = k (55 * 10^-9)² [ 1/(.11²) + (cos θ)/(.11² + .21²)] = 2.47 * 10^-3