well, my thoughts on this, not knowing entirely what is being asked and having little experience in physics, is inertia. an object in motion tends to stay in motion unless acted upon by an outside force. the two particles are moving in the same direction, magnetic field is generated (outside force) and they are attracted to each other --> <-- but once they break their paths, they are no longer traveling in the same plane, in the same direction, therefore the magnetic field is no more and the to -'s or +'s repel each other and for the split second when they are in the same plane in the same direction, they attract, but the attraction doesn't last long enough, nor is it strong enough to cancel out the inertia of the two particles. they are no longer moving in relation to each other, they're on their merry way, doing their own thing.
go for it... tear it apart.
\m/ \m/ \m/ \m/ \m/ \m/ \m/ \m/ \m/ \m/ \m/