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【題目】平行板電容器的兩極板A、B接于電池兩極，一帶正電小球懸掛在電容器內部，閉合開關S，電容器充電，這時懸線偏離豎直方向的夾角為θ，如圖所示，則（）

A．保持開關S閉合，帶正電的A極板向B極板靠近，則θ角增大

B．保持開關S閉合，帶正電的A極板向B極板靠近，則θ角不變

C．開關S斷開，帶正電的A極板向B極板靠近，則θ角增大

D．開關S斷開，帶正電的A極板向B極板靠近，則θ角不變

【答案】AD

【解析】球在電場中平衡，則所受電場力、重力及繩的拉力的合力為零。由平衡條件得tanθ＝，故要判斷θ的變化，只需判斷電場強度E的變化即可。S閉合時，U不變，A向B靠近，d減小，由E＝，可知E增大，θ角增大，故A正確，B錯誤；S斷開，則Q不變，A向B靠近，E不變，則θ角不變，故C錯誤，D正確。

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