问题 选择题

如图甲所示,Q1、Q2为两个被固定的点电荷,其中Q1带负电,a、b两点在它们连线的延长线上.现有一带负电的粒子以一定的初速度沿直线从a点开始经b点向远处运动(粒子只受电场力作用),粒子经过a、b两点时的速度分别为va、vb,其速度图象如图乙所示.以下说法中正确的是(  )

A.Q2一定带负电

B.Q2的电量一定大于Q1的电量

C.b点的电场强度一定为零

D.整个运动过程中,粒子的电势能先减小后增大

答案

A、从速度图象上看,可见a到b做加速度减小的减速运动,在b点时粒子运动的加速度为零,则电场力为零,所以该点场强为零.Q1对负电荷的电场力向右,则Q2对负电荷的电场力向左,所以Q2带正电.故A错误,C正确.

    B、b点场强为零,可见两点电荷在b点对负电荷的电场力相等,根据F=k

q1q2
r2
,b到Q1的距离大于到Q2的距离,所以Q1的电量大于Q2的电量.故B错误.

    C、整个过程动能先减小后增大,根据能量守恒电势能先增大后减小.故D错误.

故选C.

完形填空
完型填空.
     When you are in England, you must be careful in the streets because traffic drives on the left. __1__
you cross a street, you must look to the right first and then left.
     If the traffic lights are __2__, the traffic must stop. Then the people on the foot can cross the road
__3__.If the traffic lights are green, the traffic can go on, people on foot __4__cross.
In the morning and in the evening, when people go to or from work, the streets are very __5__.Traffice
is most dangerous then.
     When you go by bus in England, you have to be carefully, too. Always remember that traffic moves
on the __6__. So you must be careful. Have a look first, __7__ you will go the wrong __8__.
     In many English cities, there are big buses __9__ floors. You can sit on the __10__ floor. From there
you can see the city very well. It's very interesting.
(       )1. A. Before
(       )2. A. black
(       )3. A. care
(       )4. A. can  
(       )5. A. busy
(       )6. A. left
(       )7. A. because
(       )8. A. street
(       )9. A. in
(       )10. A. first
B. After
B. red
B. careful
B. could
B. big
B. right
B. or
B. park
B. or
B. second
C. Between
C. g reen
C. carefully
C. must not
C. safe
C. front
C. so than
C. road
C. with
C. third
D. Unit l      
D. yellow      
D. more careful
D. have to    
D. wide        
D. back        
D. either      
D. way        
D. on          
D. fourth      
名词解释