The Lorentz force ($\vec{F} = q(\vec{v} \times \vec{B})$) acts on a moving charge in a magnetic field. The force is perpendicular to both the velocity ($\vec{v}$) and the magnetic field ($ \vec{B}$). For positive charges, the force direction is given by the right-hand rule applied to $ \vec{v} \times \vec{B}$. For negative charges (where $q$ is negative), the force is in the opposite direction. This difference in the direction of the force causes the positive and negative charges to be deflected in opposite directions, leading to separation.