如图所示：在真空中，有一半径为r的圆形区域内充满垂直纸面向里的匀强磁场，磁感应强度为B，一带电粒子质量为m，电量为q，以某一速度由a点沿半径方向射入磁场，从c点射出磁场时其速度方向改变了60度，（粒子的重力可忽略）试求
（1）该粒子在磁场中运动时间t
（2）粒子做圆周运动的半径R
（3）粒子运动的速度
（1）（2）
（3）
题目分析：设圆周运动半径为R
（1）粒子做匀速圆周运动，洛伦兹力提供向心力
即
而
由上述公式有
则粒子运动时间
（2）由几何关系有
（3）由（1）得
点评：本题难度较小，由于洛伦兹力提供向心力，所以粒子做的是匀速圆周运动，处理的步骤是：先找圆心、后求半径，必要的时候要借助几何关系求解
When I was a boy, every holiday that I had seemed wonderful. My parents took me by train or by car to a hotel by the 26 . All day, I seem to remember, I played on the sands with strange 27 children. We made houses and gardens, and 28 the tide destroy them. When the tide went out, we climbed over the rocks and looked down at the fish in the rock-pools. In those days the sun seemed to shine always 29 and the water was always warm. Sometimes we left beach and walked in the country, 30 ruined houses and dark woods and climbing trees. There were sweets in one’s pockets or good places where one could 31 ice creams. Each day seemed a life-time. 32 I am now thirty-five years old, my idea of a good holiday is much the same as it was. I 33 like the sun and warm sand and the sound of 34 beating (拍打) the rocks. I no longer wish to 35 any sand house or sand garden, and I dislike sweets. 36 , I still love the sea and often feel sand running through my fingers. Sometimes I 37 what my ideal(理想的) holiday will be like when I am old . 38 I want to do then, perhaps, will be to lie in bed, reading books about 39 who make houses and gardens with sands, who watch the incoming tide, who make themselves 40 on too many ices creams.
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