问题 选择题
如图所示,在x轴上方存在着垂直于纸面向里、磁感应强度为B的匀强磁场.一个不计重力的带电粒子从坐标原点O处以速度v进入磁场,粒子进入磁场时的速度方向垂直于磁场且与x轴正方向成120°角,若粒子从x轴上的P点射出磁场,已知p点与O点的距离为a,则该粒子的比荷和所带电荷的正负是(  )

A.v/aB,正电荷

B.v/2aB,正电荷

C.v/aB,负电荷

D.v/2aB,负电荷

答案

答案:C

阅读理解

阅读理解。

     Jess really felt very happy. When he arrived at his seat in the classroom that morning, he found an invitation (请柬) on his desk. It was from several of his classmates asking him to join them in a camping trip. This was

the first time he had been asked to join in an out-of-school activity. Why were they asking him now? Nobody

seemed to like him. In fact, he had been so lonely that he drowned his feeling with food. As a result, he had

put on a lot of weight and this gave the kids something more to make fun of him.

     Cindy, who was standing near Jess when he read the invitation, went out quickly to tell the others that the

trick (恶作剧) had worked. Everyone was pleased that Jess thought that was true. But there was no camping

trip. The whole thing was made up.

     At first, Cindy thought it was fun. But later, when Jess told her that he was going to buy a sleeping bag with

his savings (积蓄), Cindy had a second idea. She knew that Jess's family had little money, and she hated to see

him spend his savings on something he would never use. Cindy also hated to tell Jess the truth. Her close friends would be angry with her.

     What could she do now?

1. Jess found an invitation to _____.

A. join a camping trip

B. join an indoor activity

C. have a birthday party

D. save money

2. The sentence "he drowned his feeling with food" means " _____ ".

A. he ate a lot to make himself feel less lonely

B. he asked for a lot of food from his classmates

C. he brought his food to his classmates

D. he had a lot of food to put on weight

3. What would happen if Cindy told Jess the truth?

A. Jess would go on the camping trip himself.

B. Jess's family would be angry with Cindy.

C. Cindy might have trouble with her friends.

D. Jess would be thankful to his classmates.

4. If Jess really bought a sleeping bag, _____.

A. everyone else would also buy one

B. it would be the best in the class

C. Cindy would pay for it

D. he would have it for no use

5. From the story we can guess _____.

A. everybody would go camping in the class

B. Jess had few friends in the class

C. Cindy was Jess's best friend in the class

D. Jess joined in many out-of-school activities

单项选择题