20|8~2卜英语周报初一答案

书面表达One possible version:Dear Jack,I am happy to know that you are to spend some time in China. I recommend you stay in Beijing, the capital andcultural center of China.First of all, there are a lot of museums where precious relics of historical interest are waiting for you toexplore. Besides, the exceptional Peking Opera performances there are also what you can t miss. And the city isalso best known for its abundance of ancient buildings, which are still well preserved.I hope that you will have a good time in China.Yours sincerely,Li Hua

14.【解析】(1)粒子在磁场中做匀速圆周运动,运动轨迹如图所示,设轨迹半径为r1××人xx·DC由洛伦兹力充当向心力可知qvB=m,解得n=R(2分)粒子在圆外正方形磁场中的轨迹半径也为n2=R粒子恰好不射出正方形区域,则正方形的边长L=2r1+2r2=4R(2分)(2)粒子在圆形磁场中做圆周运动的周期T12TRT1 TR,在圆形磁场中运动时间t=2(2分)粒子在圆形以外的区域做圆周运动的周期T2=2R,在圆形以外的磁场中运动时间t212=UoBR2分)U则再次回到M点的时间t=h1+t2=4xR(2分Uo

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