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【简答题】
带电+q小球位于贴近带电平行板上极板位置时具有的电势能是ε，取下极板电势为零。现将在上极板位置的中性球一分为二成AB两部分，分别带电为+q、-q。将B缓慢移动到贴近下极板位置，（不计AB间的库伦力、重力）则：
A．外力需做功ε； B．外力需做功2ε； C．B部分在下极板时具有的电势能是ε； D．AB两部分增加的电势能为2ε。
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参考解析：
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举一反三
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