一个圆形塑料盘半径r=2厘米,表面均匀分布着电荷q=2c.一个宽0.5厘米的圆环与盘面共圆心,环的径向中心在半径r=1厘米处.求环中包含的电荷量（用库仑表示）.
答：均匀分布,电量占总电量的比其实就是环的表面积与总表面积的比.表面积的比等于半径平方的比.环在圆盘的径向占从r=0.75到r=1.25cm的部分.设环上电量为x库仑.
(1.25^2 - 0.75^2) / 2^2 = x / 2
x = 0.5
英语翻译a circular plastic disk of radius r=2cm has a uniformly
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