Past exam of the mathematics course of the University of Cambridge / 2011 / ia / Paper 4 / 9B / a / Solution

Take the scalar product of Newton's second law with the velocity $\dot{\mathbf{r}}$. The cross product is perpendicular to its first factor, soConsequentlyThis is conservation of energy: the sum of kinetic energy and potential energy stays fixed. The magnetic-type Lorentz force $\dot{\mathbf{r}}\times \mathbf{B}$ does no work, because it is perpendicular to the instantaneous velocity. It may turn the trajectory but cannot directly change its speed, regardless of the spatial variation of $\mathbf{B}$.