two disks(one radius is R,the other radius is 2R) and a flat ring (inner radius is R,outter
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)
答
电场强度与电荷密度有关,电荷密度越大,电场强度也越大
电荷密度跟物体的表面积有关,表面积越小,电荷密度越大
设盘的表面积为上面积与下面积之和即S=(πR^2)*2
R:S1=2πR^2
2R:S2=2(π(2R)^2)=8πR^2
ring:S2=2(π(2R)^2-πR^2)=6πR^2
所以电场强度排列为 ER>Ering>E2R