英语周报高考（XN）2022-2022第12期答案

Dear HenryI am glad to hear from you and thank you for your concern about my school lifeRecently, I have been preparing for the approaching college entrance examination, which isof great importance for me. Every day, I have to finish tons of assignments. It is a little bittough, but I am determined to leave no stone unturned to enter one of my dreamuniversitlesThank you again and best wishes to you!ours.

17.【解析】(1)如图所示,设粒子的轨迹半径为r,粒子的速度为v时恰不从外边界飞出由几何关系可得R2+r2=(2R-r)2(2分)由向心力公式得gnB=mz(2分)联立解得v=39BR(1分)(2)设粒子以速度v=39BR出射时在磁场内做圆周运动的半径为r1,圆心为O1,OO1连线与粒子的发射方向的夹角为B1由几何关系可得=tanB(1分)粒子在磁场内做圆周运动的周期T=B(1分)可知粒子从内边界进入磁场到第一次返回磁场内边界所经历的时间180°+261360T(1分)可得t(1分)(3)由题意可知,另一出射速度为v的粒子从内边界进入磁场到第一次返回磁场内边界所经历的时间t24π(1分)3qB 6qB设该粒子在磁场中做圆周运动的轨迹半径为n2,圆心为O2,OO2连线与粒子的发射方向间的角度为02,如图所示xxxxx应有t280°+202360°T(1分)由几何关系可得合=tan2分)由洛伦兹力提供向心力可得r2=(1分)联立解得v=BR(1分)

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