如图所示，为正电荷Ｑ的电场，Ａ、Ｂ是电场中的两点，将电量为q=5×10－8库仑的点电荷（试探电荷）置于Ａ点，所受电场力为2×10－3牛，则下列判断正确的是：
Ａ、将点电荷ｑ从Ａ点移走，则该点的电场强度为零
Ｂ、将电量为q的负点电荷放于A点，A点场强大小为4.0×104N/C，方向指向Ｂ
Ｃ、将电量为2q的正点电荷放于Ａ点，A点场强大小为8.0×104 N/C，方向指向Ｂ
Ｄ、Ｂ点处的电场强度小于4.0×104N/C
：BD
：
题目分析：A、A点的场强由场源电荷Q决定，与电荷ｑ无关，A错
B、A点的电场强度由场源电荷Q决定，
，方向为电场线的方向，指向Ｂ，所以B对
C、A点的场强由场源电荷Q决定，与放入其中的电荷无关，A点场强大小仍为
，方向指向Ｂ，C错
D、电场线的疏密程度表示场强的大小，B处比A处稀疏，所以Ｂ点处的电场强度小于，D对
故选BD
点评：点电荷形成的电场只与场源电荷有关，与试探电荷无关，电场线的疏密程度代表场强的大小，电场线的方向代表场强的方向。
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| My father woke me up early one summer morning announcing, "Get up. You're going with me to cut grass." The idea 1 my father actually thought I was big enough to help him in his 2 made me feel proud and 3 . From sunup to sundown my father, my younger brother and I 4 in the large yards in a rich part of Atlanta, Georgia. By the end of the day I was tired out, but felt good. I had 5 a hard day's labour and had earned $6. One day my father found some leaves I'd 6 and pulled me aside.  "Clear away these leaves," he said 7 , "and don't make me have to tell you to do it again." The 8 was clear. Today I value the importance of doing a job 9 the first time. It will never 10 to impress the person you are working for. After two years my father told me and my brother that he felt we were 11 enough to do lawns(草坪) on our own. Every Saturday , we 12 out early in the morning with the same desire and drive we had gained while working 13 our father. Taking care of lawns was not exciting 14 high-paying, but that didn't matter. It taught me that any job is a good job and that 15 I was paid was more than I had before. A newspaper reporter once asked me how someone 16 possible live on a forty-hour- a -week minimum(最低的) pay. "My father never worked just forty hours a 17 , and neither have I." I replied. "If you're only working forty hours, you probably don't want to do 18 better than you're doing." In every job I've 19 -from doing lawns to washing dishes-I have learned something that helped me in my next job. If you 20 hard enough, you can learn from any job you do. | ||||
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