如圖所示,質(zhì)量為m電荷量為q的帶電粒子,重力不計(jì),由靜止開(kāi)始經(jīng)兩板間電壓為U的加速電場(chǎng)加速,又經(jīng)磁感應(yīng)強(qiáng)度為B的勻強(qiáng)磁場(chǎng)偏轉(zhuǎn)后落到圖中D點(diǎn),求: