These animations show the field and force on a point
charge as it moves from a region with no external magnetic
field to a region with a constant magnetic field (the
field points upwards, or out of the screen in the top
view). As indicated by the arrow, the particle feels
no force while in the empty region, but feels a varying
force in the region of constant external field. A moving
charge generates a magnetic field whose magnitude and
direction are proportional to the velocity of the charge.
Thus, the particle feels maximal force when it is moving
fastest through the external field, in the direction
given by the cross product of its velocity and the direction
of the external field. When it slows to a stop, its
field collapses and the force felt reduces to zero.
When it starts moving again, in the opposite direction,
the force returns. However, since the direction of the
velocity has reversed, so has the direction of the force.