8.一无限长载流圆柱体,半径为R,电流I在圆柱-|||-体的横截面上均匀分布,则在圆柱体内距离中心轴线-|||-为r处的磁感应强度大小为 () 。-|||-(A) =dfrac ({mu )_(0)I}(2pi r)-|||-(B) =dfrac ({mu )_(0)Ir}(2pi {R)^2}-|||-(C) =dfrac ({mu )_(0)I(r)^2}(2pi {R)^3}-|||-(D) =dfrac (sqrt {2)(mu )_(0)Ir}(2pi {R)^2}-|||-二、填空题