4.任意环路 ∵B⊥dl dldi B 1B.dl=o Bdi=,B. di,+ du) ,B du/+f,Bdi=0+uoI B dl dl l . = + l B.  l B.dl ( dl + ) =  l B. dl 4. 任意环路 = 0 +μ o I dl I dl dl B ∵ B dl  B dl l ∴ . = 0