设无穷远处电势为零, 半径为R的导体球带电后其电势为U, 则球外离球心距离为r处的电场强度大小为( )<br>· <img src="/lqwvjeImg/f1/f16abd88b21109cbe245fa7debf50472.png"style="width:36px;height:44px;"><br>· <img src="/lqwvjeImg/c7/c7a722f0a13d39c0552ec233eeb3e214.png"style="width:20px;height:41px;"><br>· <img src="/lqwvjeImg/b5/b52cecc673c3694ab3f2cfeb4e7c7183.png"style="width:31px;height:41px;"><br>· <img src="/lqwvjeImg/6f/6ff392b4d2e0af48a9c6b3aba2ba9338.png"style="width:20px;height:41px;">