A positive test charge does not follow electric field lines directly; instead, it experiences a force at its current position, which updates its trajectory. As the charge moves, it continually feels the force from the electric field at each new location. In the case of a parallel plate capacitor, the electric field lines are perpendicular to the plates, resulting in a constant field strength. If a charged particle starts with a velocity parallel to the plates, it will follow a parabolic path rather than a straight line. Thus, while the charge is influenced by the field, its path can be curved rather than strictly following the field lines.
