解:c B·dS 例1:求线圈中的感应电动势 = Bcos0ds=o ldx Mi=Io sin ot 2 +b 9n=4=「pB=l2xnz T AC uo il a+b O 2兀 tb 0In- sin ot 2兀 , = dt Hlo a+b I cos ot 2元9 a b m S m l = d x dx + = a b a ldx x i 2 0 a il a + b = ln 2 0 x o n d m B dS = = BcosdS ldx x i = 2 0 解: 例1: i = I sint 0 求线圈中的感应电动势 x i B = 2 0 I t a l a b + = ln sin 2 0 0 dt d m i = − I t a l a b + = ln cos 2 0 0