如图所示,虚线
![](http://math.21cnjy.com/mml2svg?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmi%3Ea%3C%2Fmi%3E%3Cmi%3Eb%3C%2Fmi%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
左侧空间存在与直流电源相连的两块正对平行金属板,两板间电压恒为U,板长为
![](http://math.21cnjy.com/mml2svg?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmn%3E2%3C%2Fmn%3E%3Cmi%3EL%3C%2Fmi%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
, 板间距离为L,两板间存在匀强磁场,磁感应强度大小为B,方向垂直纸面向里;
![](http://math.21cnjy.com/mml2svg?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmi%3Ea%3C%2Fmi%3E%3Cmi%3Eb%3C%2Fmi%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
右侧空间足够大区域内存在磁感应强度大小为
![](http://math.21cnjy.com/mml2svg?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmn%3E2%3C%2Fmn%3E%3Cmi%3EB%3C%2Fmi%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
, 方向也垂直纸面向里的匀强磁场。S为两板中轴线上的粒子源,能够沿中轴线射出初速度相同的同种带电粒子,沿直线穿过板间区域,从P点进入
![](http://math.21cnjy.com/mml2svg?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmi%3Ea%3C%2Fmi%3E%3Cmi%3Eb%3C%2Fmi%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
右侧区域并运动到M点,
![](http://math.21cnjy.com/mml2svg?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmi%3EP%3C%2Fmi%3E%3Cmi%3EM%3C%2Fmi%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
间的距离为L。保持两板间电压U不变,撤掉
![](http://math.21cnjy.com/mml2svg?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmi%3Ea%3C%2Fmi%3E%3Cmi%3Eb%3C%2Fmi%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
左侧磁场,再次从S射出的粒子最终打在下板上某点A处(图中未画出)。不计粒子重力及粒子间的相互作用,忽略电场和磁场的边缘效应。求:
![](http://tikupic.21cnjy.com/ct20241o/7a/71/7a712b3696ecc44bfae3665df28c39a8.png)