2.r>R B it r=u oI R B=loI r B B b=noIr 2R2 B=kol 打R RI R B r 2. r >R B = 2π r μ 0 I 2 μ π I R 0 B R O r B 2π r =μ 0 I μ B = 2 R Ir 2 π 0 B = 2π r μ 0 I