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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 
\[\vec F_B=q\vec v \times \vec B\]
Using definition of the cross product, we obtain its magnitude as
\[|\vec F_B|=|q|vB\, \sin \theta\]
Where $\theta$ is the smaller angle between $\vec v$ and $\vec B$.