Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2015 / S16 / 3 / 2

Seek a constant transverse guiding-centre drift ${\mathbf{v}}_d$ whose magnetic Lorentz force balances the applied force. DefineSince $\mathbf{F}\cdot \mathbf{B}=0$, the vector triple-product identity gives $q{\mathbf{v}}_d\times \mathbf{B}=-\mathbf{F}$. Writing $\mathbf{v}={\mathbf{v}}_d+\mathbf{u}$ therefore reduces the full equation to $m\dot{\mathbf{u}}=q\mathbf{u}\times \mathbf{B}$, the ordinary gyromotion equation. Its transverse orbit is circular in the drifting frame, soAveraging over a gyroperiod removes the transverse oscillatory velocity and leaves this drift, together with any constant parallel velocity. For an electric force $\mathbf{F}=q\mathbf{E}$, this becomes the charge-independent $\mathbf{E}\times \mathbf{B}/B^2$ drift; a charge-independent force instead gives opposite drift directions for opposite charge signs.