解答
$\vec{F} = q(\vec{v} \times \vec{B})$, with magnitude $F = |q| v B \sin\theta$ where $\theta$ is the angle between $\vec{v}$ and $\vec{B}$.
解説
Because the force is always perpendicular to velocity ($\vec{F} \cdot \vec{v} = 0$), the magnetic field does no work on a moving charge and does not change its kinetic energy.