The magnitude is F = qvB sinθ, θ being the angle between v and B. Send the field across the page (→) and the force pops straight out of it (⊙ / ⊗), scaling with sin θ. Send the field through the page (⊗ / ⊙) and the force lies in the plane, square to v — the very push that will curl the charge into a circle overleaf. Hit play to launch the charge and flip the field to feel the difference.

Fire a charge straight across a uniform field and that perpendicular force becomes a perfect centripetal pull: the charge loops at radius r = mv/(qB). The astonishing part is the clock — the period T = 2πm/(qB) has no v in it at all. Speed the charge up and its circle grows in exact step, so every lap takes the same time. Flip the charge or the field and watch the sense of rotation reverse.