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【单选题】
真空中,有一均匀带电细圆环,电荷线密度为 l ,其圆心处的电场强度E 0 和电势U 0 分别为.(选无穷远处电势为零)A.
E 0 =0, U 0 = l / (2 e 0 ) ．
B.
E 0 = l / (2 e 0 r ), U 0 = l / (2 e 0 ) ．
C.
E 0 = l / (2 e 0 r ), U 0 = 0 ．
D.
E 0 =0 , U 0 =0.
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