如图所示，分别在M、N两点固定放置两个点电荷，电荷量均为+Q ，MN连线的中点为O。正方形ABCD以O点为中心，E、F、G、H是正方形四边的中点，则下列说法正确的是（ ）
A．A点电势低于B点电势
B．正点电荷沿直线从G到H，电势能先增大后减小
C．O点的电场强度为零，电势也为零
D．沿路径A→C→D比沿路径A→B移动同一负点电荷，电场力做的功多
答案：B
题目分析：由于是同种等量电荷形成的电场，根据电场分布的对称性可知A和B两点的电势相等，故选项A错误；在MN的中垂线上，O点的场强为零，电场方向从O点指向外，所以正电荷沿直线从G到H，电场力先做负功再做正功，即电势能先增大后减小，故选项B正确；取无穷远处电势为零，根据顺着电场线方向电势降低可知O点电势最高，故选项C错误；根据电场的对称性可知BD两点的电势相等，所以沿路径A→C→D比沿路径A→B移动同一负点电荷，电场力做的功相同，故选项D错误．
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| We often talk about ourselves as if we have permanent genetic defects (缺陷) that can never be changed. "I'm impatient.""I'm always behind." "I always put things 1 !" You've surely heard them. Maybe you've used them to describe 2 . These comments may come from stories about us that have been 3 for years-often from 4 childhood. These stories may have no 5 in fact. But they can set low expectations for us. As a child, my mother said to me,"Marshall, you have no mechanical (操作机械的) skills, and you will never have any mechanical skills for the rest of your life." How did these expectations 6 my development? I was never 7 to work on cars or be around 8 . When I was 18, I took the US Army's Mechanical Aptitude Test. My scores were in the bottom for the entire nation! Six years later, 9 , I was at California University, working on my doctor's degree. One of my professors, Dr. Bob Tannbaum, asked me to write down things I did well and things I couldn't do. On the positive side, I 10 down, "research, writing, analysis, and speaking." On the 11 side, I wrote, "I have no mechanical skills." Bob asked me how I knew I had no mechanical skills. I explained my life 12 and told him about my 13 performance on the Army test. Bob then asked," 14 is it that you can solve 15 mathematical problems, but you can't solve simple mechanical problems?" Suddenly I realized that I didn't 16 from some sort of genetic defect. I was just living out expectations that I had chosen to 17 . At that point, it wasn't just my family and friends who had been 18 my belief that I was mechanically hopeless. And it wasn't just the Army test, either. I was the one who kept telling myself,"You can't do this!" I realized that as long as I kept saying that, it was going to remain true. 19 , if we don't treat ourselves as if we have incurable genetic defects, we can do well in almost 20 we choose. | ||||
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