如图所示的虚线区域内，充满垂直于纸面向里的匀强磁场和竖直向下_爱下问搜题

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问题选择题

如图所示的虚线区域内，充满垂直于纸面向里的匀强磁场和竖直向下的匀强电场．一带电粒子a（不计重力）以一定的初速度由左边界的O点射入磁场、电场区域，恰好沿直线由区域右边界的O′（图中未标出）穿出．若撤去该区域内的磁场而保留电场不变，另一个同样的粒子b（不计重力）仍以相同初速度由O点射入，从区域右边界穿出，则粒子b（　　）

A．穿出位置一定在O′点下方

B．穿出位置一定在O′点上方

C．运动时，在电场中的电势能一定减小

D．在电场中运动时，动能一定减小

答案

a粒子要在电场、磁场的复合场区内做直线运动，则该粒子一定做匀速直线运动，故对粒子a有：Bqv=Eq 即只要满足E=Bv无论粒子带正电还是负电，粒子都可以沿直线穿出复合场区，当撤去磁场只保留电场时，粒子b由于电性不确定，故无法判断从O’点的上方或下方穿出，故AB错误；

粒子b在穿过电场区的过程中必然受到电场力的作用而做类似于平抛的运动，电场力做正功，其电势能减小，动能增大，故C项正确D项错误．

故选C．

完形填空

Do you remember who gets angry because you forget to tidy up your room before going out? Do you remember who feels with your dress from head to feet? This person is your boss nor your friend. It’s funny that you have to to her even though you sometimes don’t like to hear anything she has said. It’s our mother, a great in the world to us.
I always my mother if we have a “long time no see”. But this kind of missing is always quieted down by because I come back home for comfort and but mum always puts more pressure on me. Of course, I know she about me too much and just has a lot of words to tell me. But the turns to be pressure. On the other hand, I hurt her and make her feel lonely even though I am her. Then what can we do? This problem is really hard to deal with. I think not I have this problem. Here are my ideas. When we find there are some unhappy things going to , shall we just stop for 30 ? Give me a chance to rest in your arms and we sing a childish song together just like when I was ? I need your love indeed but without telling me to do as you wish me to. And then let’s go out for a walk hand in hand. Tell me what’s in your gently and slowly. I will feel better if you don’t just pour them out.

A．dissatisfied

A．description

同学们喜欢的NBA球星中有许多都是黑人，他们的祖先来自非洲大陆。历史上与非洲黑人大批来到北美有关的史实是（）

A．三角贸易

B．独立战争

C．南北战争

D．非洲独立年