问题 计算题

在如图所示的坐标系中,x轴沿水平方向,y轴沿竖直方向。第二象限内存在沿y轴负方向的匀强电场,在第三象限内存在垂直xy平面(纸面)向里的匀强磁场。一质量为m、电量为q的带正电粒子(不计重力),从y轴上的A点以v0的初速度沿x轴负方向进入第二象限,之后到达x轴上x=-2h处的B点,带电粒子在B点的速度方向与x轴负方向成45°角,进入第三象限后粒子做匀速圆周运动,恰好经过y轴上y=-2h处的C点。求:

(1)粒子到达B点时速度大小;

(2)第二象限中匀强电场的电场强度的大小;

(3)第三象限中磁感应强度的大小和粒子在磁场中的运动时间。

答案

解:(1)粒子在第二象限做类平抛运动

由题意可知:粒子到达B点时的速度大小为

(2)在B点,v与x轴负方向成45°

所以:vy=vx=v0

又:vy=qEt/m,t=2h/vx

联立可得匀强电场的电场强度的大小E=mv02/2qh

(3)粒子运动轨迹如图所示

由题意可知

由洛伦兹力提供向心力可得qvB=mv2/R

联立可得磁感应强度的大小

粒子在磁场中的运动时间

阅读理解

If you want to travel with a partner, you may ask for one on the Internet. There are some tips about how to choose a right partner on the Internet:

Unluckily, you can’t ask a person if he has a good feeling. If you ask, everyone will tell you that they do! It is important that you have to get to know your travel partner to find out what kind of feeling he really has.

You can tell a lot about a person’s feeling by the words he uses. Ask him about his worst trip. Is he laughing about it now, or still unhappy? Does his worst trip sound like an adventure to you? If so, he doesn’t match you. Having a good feeling is important for having a good time.

Talk about the hotel you would like to stay in. See if you have the same habit of spending holidays. This is such a quick way to find a partner.

What’s more? Always keep in contact with(联系) your travelling partner when travelling. Don’t simply hang out without telling them, and don’t let them do it to you, either.

小题1:The passage mainly talks about how to find a right partner _________.

A.in class

B.in school

C.on the Internet

D.in the neighborhood小题2: The second paragraph mainly tells us it is important ___________.

A.to find a partner

B.to find out the feeling of the partner

C.to ask if he has a good feeling

D.look for a partner on the Internet小题3:The underline word “adventure” means ___________ in Chinese.

A.冒险

B.安全

C.刺激

D.兴奋小题4:What is the quick way to find if you have the same habit of spending holiday with a       partner?

A.Talk about money.

B.Talk about study.

C.Talk about the place you want to go.

D.Talk about the hotel.小题5:The best title for the passage is __________.

A.Have A Happy Holiday

B.Find a Right Partner for Traveling

C.Don’t Hang Out Alone

D.Make Sure if Has Good Feeling

多项选择题