## College Physics (7th Edition)

The magnitude of magnetic field on a charged particle with charge q and speed v is $F=qvBsin\theta$, where $\theta$ is the angle between velocity of the particle and magnetic field B. In this case, $v\ne0$, $B\ne0$, thus for F=0, we must have $\theta=0^{\circ}$ or $\theta=180^{\circ}$. Therefore the magnetic field could be in the same or opposite direction .