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
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
，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．q1q2 r2
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．
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| 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. | ||||
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