恰好从右侧慧芦穗穿出:R1^2 = (R1-d)^2 + L^2。qv1B = mv1^2/R1。得v1 = qB (d^2+L^2) /2md。恰好从左侧穿出:R2 = d/2。qv2B = mv2^2/R2。得v2 = qBd/2m。故v>qB (d^2+L^2) /2md 或 v<前卜qBd/哗穗2m。