If a particle of charge q and velocity $\vec v$ enters a region of space occupied by magnetic field $\vec B$, which is establishes by some source, it experiences a magnetic force $\vec F_B$ given by
$\vec F_B=q\vec v \times \vec B$
$|\vec F_B|=|q|vB\, \sin \theta$
Where $\theta$ is the smaller angle between $\vec v$ and $\vec B$.