问题
不定项选择
如图所示，一半径为R的圆形区域内有垂直于纸面向里的匀强磁场，一质量为m，电荷量为q的正电荷(重力忽略不计)以速度v沿正对着圆心O的方向射入磁场，从磁场中射出时速度方向改变了θ角。磁场的磁感应强度大小为
A. 
B. 
C. 
D. 
答案
答案：B
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如图所示，一半径为R的圆形区域内有垂直于纸面向里的匀强磁场，一质量为m，电荷量为q的正电荷(重力忽略不计)以速度v沿正对着圆心O的方向射入磁场，从磁场中射出时速度方向改变了θ角。磁场的磁感应强度大小为
A.
B.
C.
D.
答案：B
| 完形填空。 | |||
| Time spent in a bookshop can be enjoyable. __1 you go to a good shop, no assistant will come near to you and say, "Can I help you?" You __2 buy anything you don't want. You may try to find out. __3 the book you want is. But if you fail, the assistant will lead you there and then he will go away. It seems that he is __4 selling any books at all. There is a story which tells us about a good shop. A medical student __5 a very useful book in the shop, 6 it was too expensive for him to buy. He couldn't get it from the library, either. So every afternoon, he went there to read _ 7 at a time. One day, however, he couldn't find __8 _ from its usual place and was leaving when he saw an assistant signing to him. To his surprise, the assistant pointed to the book __9 ,"1 put it there so as not to be sold out," said the assistant. Then he __10 the student to go on with his reading. | |||
| ( )1. A. If ( )2. A shouldn't ( )3. A. what ( )4. A surprised at ( )5. A. bought ( )6. A. but ( )7. A. little ( )8. A. the book ( )9. A. in a bookshop ( )10. A. left | B. Because B. needn't B. which B. not surprised at B. found B. and B. a little B. the shop B. in his hand B. let | C. When C. mustn't C. where C. interested in C. read C. so C. few C. the assistant C. in a corner C. helped | D. But D. can't D. when D. not interested in D. took D. or D. a few D. the shopkeeper D. in his pocket D. taught |