Question

圖1所示,y軸右側有垂直紙面向里的勻強磁場,第Ⅱ象限內(nèi)有水平的勻強電場.有一質(zhì)量為m、電荷量為q的帶負電的粒子(不計重力),從P點以初速度v₀沿+y方向射入電場中,OP=L,粒子在電場中運動一段時間后進入磁場,進入磁場時,速度方向與+x方向成30°角,并且恰好經(jīng)過坐標原點O再進入電場中繼續(xù)運動.求:?
(1)電場強度E的大小;?
(2)磁感應強度B的大小.?

圖1

Answer 1

(1)粒子在電場中運動,沿y方向的位移y=v₀t                        ①
速度v_y=v₀                                                                                                                                                                     ②
沿x方向的位移為                         ③
速度                                       ④?
由題意知                                     ⑤
由②⑤得                                     ⑥?
由③④⑥得                                       ⑦
                                              ⑧
(2)由①⑦兩式得出進入磁場時的位置距O點的距離     ⑨
設在磁場中做圓周運動的半徑為R,則有Rcos30°=y/2                  ⑩?
又由于qBv=mv²/R                                                                      11
                                                               12