解法二 =∫j:dS=12rl R ⅠE R E 2rI P OT R U=JE. di=J R pdr Ip R2 R12l2兀lR1 R I=n=2 IU/ pIn R RI j dS = j2π rl  =     E rl I j = = 2π Rl I E 2π  = 1 2 l n 2π 2π d d 2 1 R R l I l r r U E r R R   =  = =     解法二 R2 l R1 U r 1 2 2π ln R R lU R U I = = 