如图所示，真空中的A、B两点上固定着两个带正电的点电荷Q1、Q2、Q1=4Q2．C为A、B连线的中点．现引入第三个点电荷q，为能使q处于平衡状态，q应放在（ ）
A．AC之间
B．CB之间
C．AB线段左侧的延长线上
D．AB线段右侧的延长线上
因为Q1、Q2为固定的正电荷，只要放入的电荷受到的合力为0即可，通过受力分析可知，既可以放入正电荷，也可以放入负电荷；
有库仑定律，对q有
k
=kQ1q r 21
①Q2q r 22
Q1＞Q2 ②
有①②的
=r1 r2
＜1，故B正确，ACD错误；Q2 Q1
故选B
Stuart and Steve were twin brothers. Stuart loved to play basketball. But Steve loved to read books. One day Stuart fell and broke his leg he was playing basketball. When they took him to the hospital, the doctor said he wouldn’t be able to play for six months. Steve went him and brought him books on basketball. At first Stuart wasn’t going to read them. Then he began to read them and that there were ways he could play basketball better. He began to think that books weren’t so . When Stuart’s leg started getting better, Steve would help him by going for walks with him. Stuart’s doctor said he could start playing basketball again. Then Stuart showed Steve how to toss (投) for baskets. Steve found that he enjoyed it. Soon Stuart was ready to basketball games. Steve went to games and enjoyed himself. From then on they practiced basketball and read books together. Now you can see, when happens, something good may also happen. Steve showed Stuart about books and Stuart showed Steve about basketball. So please showing others something that you like to do and they may show you something that they like to do. you can find new things to do.
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