Final answer:
The net electric field at the origin in a system of symmetrically placed charged insulators points entirely in the z-direction, with the horizontal components canceling each other out.
Step-by-step explanation:
The direction of the net electric field at the origin in a system where rectangular insulators with uniformly charged distributions are symmetrically placed is determined by the properties of symmetry within the electric fields created by each charge distribution.
Due to the equal and opposite charges, the horizontal (x)-components of the field cancel out, leaving a net field that is in the vertical direction. The field lines from each charge distribution push against each other in the horizontal direction, creating a situation where the net field is entirely in the vertical, or z-direction.
By using a symmetry argument or calculations, we can determine that the net electric field would point either directly upwards or downwards along the z-axis, depending on whether the charges on the insulators are positive or negative.
Moreover, due to the symmetry and the fact that we are considering the point at the origin, we can argue that any x- or y-component of the field from one charge distribution would be exactly negated by the corresponding component from the charge distribution on the opposite side.