如图所示,在以O为圆心的圆形区域内,有一个方向垂直于纸面向外的匀强磁场,磁感应强度大小B=0.1T,圆半径R=
![](http://math.21cnjy.com/mml2svg?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmn%3E10%3C%2Fmn%3E%3Cmsqrt%3E%3Cmn%3E3%3C%2Fmn%3E%3C%2Fmsqrt%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
cm,竖直平行放置的金属板连接在如图所示的电路中,电源电动势E=120V,内阻r=5Ω,定值电阻R
1=5Ω,滑动变阻器R
2的最大阻值为50Ω;两金属板上的小孔S
1、S
2跟O点在垂直于极板的同一直线上,另有一水平放置的足够长的荧光屏D,O点跟荧光屏D之间的距离H=
![](http://math.21cnjy.com/mml2svg?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmn%3E20%3C%2Fmn%3E%3Cmsqrt%3E%3Cmn%3E3%3C%2Fmn%3E%3C%2Fmsqrt%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
cm,现有比荷
![](http://math.21cnjy.com/mml2svg?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmfrac%3E%3Cmi%3Eq%3C%2Fmi%3E%3Cmi%3Em%3C%2Fmi%3E%3C%2Fmfrac%3E%3Cmo%3E%3D%3C%2Fmo%3E%3Cmn%3E2%3C%2Fmn%3E%3Cmo%3E%C3%97%3C%2Fmo%3E%3Cmsup%3E%3Cmrow%3E%3Cmn%3E10%3C%2Fmn%3E%3C%2Fmrow%3E%3Cmn%3E5%3C%2Fmn%3E%3C%2Fmsup%3E%3Cmtext%3EC%2Fkg%3C%2Fmtext%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
的正离子由小孔S
1进入电场加速后,从小孔S
2穿出,通过磁场后打在荧光屏D上,不计离子的重力和离子在小孔S
1处的初速度,问: