= Guiding-centre drift
{title2=$\mathbf v_d=\mathbf F\times\mathbf B/(qB^2)$}
For a constant perpendicular force in a uniform <magnetic field>, write the particle velocity as $\mathbf v=\mathbf v_d+\mathbf u$. Choosing $q\mathbf v_d\times\mathbf B=-\mathbf F$ leaves the ordinary <gyromotion> equation for $\mathbf u$. The gyroperiod-averaged transverse velocity is therefore the displayed drift. An electric force gives the same drift for both charge signs; a charge-independent applied force gives opposite drifts.
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