dom d(uLIo cos at, 6+ut n dt 2兀 a+ut 00L 6+ut o-LOsin(ot)In cos ot 2丌 a+ ut b+υa+Ut 例2:长直螺线管绕有N匝线圈,通有电流/=C (C为常数且大于零),求感应电动势 t 解:B BdS= Bs dB +6 N N-s Nuns dr Nlos dI dt l dt 1212 L I S dt d m i = − + + = − a vt LI t b vt dt d ln 2 cos 0 0 w − + + = a vt b vt t I L [ sin( )ln 2 0 0 w w + − + a vt v b vt v coswt 例2:长直螺线管绕有N匝线圈,通有电流I 且 (C为常数且大于零),求感应电动势。 C dt dI = B nI = 0 B dS s m = dt dI = −N0nS dt d N m i = − S dt dB = −N dt dI L N 0S 2 = − = BS B 解: