試題詳解

12. The figure shows six charged rods. Some have the same sign charge throughout and are uniformly charged. The rest have one sign on one half and the opposite sign on the other half, with each half uniformly charged. For each rod, a point P on a perpendicular bisector is indicated. In which situation is the net electric field at P in the positive direction of the y axis?

(A) 2 and 5,
(B) 2 and 6,
(C) 4 and 5,
(D) 1 and 4,
(E) 3 and 5.