18.如圖所示,在直角坐標(biāo)系的第Ⅰ象限存在垂直紙面向里的勻強(qiáng)磁場(chǎng),第Ⅳ象限分布著豎直向上的勻強(qiáng)電場(chǎng),場(chǎng)強(qiáng)E=4.0×10
3 V/m,現(xiàn)從圖中M(1.8,-1.0)點(diǎn)由靜止釋放一比荷
=2×10
5 C/kg的帶正電的粒子,該粒子經(jīng)過(guò)電場(chǎng)加速后經(jīng)x軸上的P點(diǎn)進(jìn)入磁場(chǎng),在磁場(chǎng)中運(yùn)動(dòng)一段時(shí)間后經(jīng)y軸上的N點(diǎn)離開(kāi)磁場(chǎng).不計(jì)重力,問(wèn):
(1)若磁感應(yīng)強(qiáng)度B=0.2T,則N點(diǎn)的坐標(biāo)是多少?
(2)若要求粒子最終從N點(diǎn)垂直y軸離開(kāi)磁場(chǎng),則磁感應(yīng)強(qiáng)度為多大?從M點(diǎn)開(kāi)始運(yùn)動(dòng)到從N點(diǎn)垂直y軸離開(kāi)磁場(chǎng)的時(shí)間為多少?