一点电荷q（q>0）置于一接地导体平面之上，在上半空间中存在一均匀电场<img src="https://nimg.ppkao.com/2019-04/wangjing/2019042216145239885.jpg?sign=d7b8339ae011898a9e95e94a5a5fa239&t=62d6952e" />，问点电荷在x为何值时所受的电场力为零。<br /> <img src="https://nimg.ppkao.com/2019-04/wangjing/2019042216144417064.jpg?sign=4a153da5192f9aa6396b2b63def011cc&t=62d6952e" />