例: lo a 求:电动势 解: lo a b dx .(x -di-fv-d =-ain5+l<0,b→a 2π Uab=-Eab 例: 求:电动势 解:6aw=×-d b vBdl=vBI, VxB = ->0,a→b B 2n Uab=-Eab 22 例: V I 0 l a l b 求:电动势 解: V V B B dl I 0 l a l b x dx x V B dl = b a L ab ( ) ( ) dx x I VB dl V l l l b a 2 ( 1) 0 0 0 = ln 0, 2 0 0 0 l IV l l b a Uab ab 例: b I r l V a 求:电动势 解: V B dl b a L ab ( ) ( ) = VBdl VBl , b a I r V B = 0, 2 0 r IVl a b l B V Uab ab dla b