Final answer:
Positively charged particle trajectories always follow magnetic field lines, as the force experienced by the particle is perpendicular to its direction of travel and causes it to move in a curved path.
Step-by-step explanation:
Positively charged particle trajectories always follow magnetic field lines. When a charged particle moves perpendicular to a uniform magnetic field, it experiences a force that is perpendicular to its direction of travel. This force causes the particle to follow a curved path, ultimately forming a complete circle. Charged particles often follow magnetic field lines rather than cross them because the force is zero when the particle's velocity is parallel to the magnetic field.