2.环形螺线管的磁场 B.dL= Bdi cos0° Bp, dl=b it r =40NVI(N:匝数) 尺 HoNT r B ri R2. r R1 R2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B 2. 环形螺线管的磁场  l B.dl = Bdl cos 0 0  l B  =B 2π r l = dl =m NI (N :匝数) 0 B = 2π r μ 0NI I I B r R1 R2 0