CBSE Class 12 · 2009 · Magnetic Effects of CurrentShort Answer

(i) With $\vec{v}$ along X and $\vec{B}$ along Z, the force $\vec{F}=q(\vec{v}\times \vec{B})$ lies in the XY-plane and is always perpendicular to $\vec{v}$, so the charge moves in a circle in the XY-plane. (ii) A force perpendicular to the velocity does no work ($\vec{F}\cdot \vec{v}=0$), so the speed and hence the kinetic energy remain constant — only the direction of $\vec{v}$ changes.