图中,AB,CD为长直导线,BC为圆心在O点的一段圆弧形导线,其半径为R。-|||-若通以电流 I,则0点的磁感应强度等于(以垂直纸面向里为正向)-|||-dfrac ({mu )_(0)I}(2pi R)(1-dfrac (sqrt {3)}(2)+dfrac (pi )(6)) I-|||-D-|||-dfrac ({mu )_(0)I}(2pi R)(-dfrac (sqrt {3)}(2)-1+dfrac (pi )(6)) I, i R-|||-C-|||-dfrac ({mu )_(0)I}(2pi R)(dfrac (sqrt {3)}(2)-1+dfrac (pi )(6)) A B-|||-6.60°-|||-dfrac ({mu )_(0)I}(2pi R)(dfrac (sqrt {3)}(2)-1-dfrac (pi )(6))