问题描述：
two disks(one radius is R,the other radius is 2R) and a flat ring (inner radius is R,outter
radius is 2R) each with the same uniform charge Q.Rank the objects according to the magnitude of the electric field they creat ar point P.(P is above the disks and ring,at the same level)
radius is 2R) each with the same uniform charge Q.Rank the objects according to the magnitude of the electric field they creat ar point P.(P is above the disks and ring,at the same level)
问题解答：
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