Here's why:
* The magnetic force is determined by the cross product of the velocity of the charge and the magnetic field. This means the force is perpendicular to both the velocity and the magnetic field.
* When a charge moves parallel to the magnetic field, the angle between its velocity and the field is 0 degrees. The sine of 0 degrees is 0, so the cross product (and therefore the magnetic force) becomes zero.
In simpler terms, the magnetic force only acts on a charge when it's moving across the magnetic field lines. If it's moving along the lines, there's no force.
