R2<P<R3, fB.dl=4∑1 R3 R2 B.2 - B= 4I(R3-r2) 2(R3-R2)R 2 < r < R3 , B  l =  I  d  0     −− = −  22 23 22 2 0 (( ) 2 R R I r R I B r    2 ( ) ( ) 22 232 2 0 3 r R R I R r B −− =   R 2 R 3 I R 1 I r