题干

如右图,MNP是以MN为直径的半圆弧上的三点,O点为半圆弧的圆心,∠MOP= 60°。电荷量相等、符号相反的两个点电荷分别置于MN两点,这时O点电场强度的大小为E1;若N点处的点电荷移至P点,则O点的场强大小变为E2E1E2之比为(   )

A:1∶2

B:2∶1

C:2∶

D:4∶

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答案(点此获取答案解析)

B

同类题5

    We were standing at the top of a church tower. My father had brought me to this high tower in a small town not far from our home in Rome. I wondered why.
 “Look down, Elsa," Father said. I gathered all my courage and looked down. I saw the square in the center of the village. And I saw several streets leading to the square. "See, my dear," Father said gently. “There is more than one way to the square. Life is like that. If you can't get to the place where you want to go by one road, try another."
    Now I understood why I was there. Earlier that day I had begged my mother to do something about the bad lunches that were served at school. But she refused because she could not believe the lunches were as bad as I said.
    When I turned to Father for help, he would not help. Instead, he brought me to this high tower to give me a lesson. By the time we reached home, I had a plan.
    At school the next day, I poured my lunch soup into a bottle and brought it home. Then I asked our cook to serve it to Mother at dinner. The plan worked perfectly. She tried one spoonful and sputtered. “The cook must have gone mad!” Quickly I told what I had done, and Mother promised that she would take up the matter of lunches at school the next day!
    In the years that followed I often remembered the lesson Father taught me. I began to work as a fashion designer two years ago. I wouldn't stop working until I tried every possible way. Father's wise words always remind me that there is more than one way to the square.