五、图电路中开关S闭合前已处于稳定状态。t=0时开关S闭合, 已知:Us=40V,J=5A,R:=R2=R3=202,L=2H; (1)求一0时的电感电流i1(t)和电压u1(; (2)做出电感电流i(和电压u1(的变化曲线。(15分) i1(0) 1A,i1(0)=i1(0) (R1+R2) i1(∞)=i(Us)+i1(/s)=1+25=3.54 L 3 Ran=(R1+R2)∥R3=40/3τ R20五、图电路中开关S闭合前已处于稳定状态。t=0时开关S闭合， 已知：US =40V，IS =5A，R1=R2=R3 =20Ω，L=2H； （1）求t≥0时的电感电流iL (t)和电压uL (t)； （2）做出电感电流iL (t)和电压uL (t)的变化曲线。（15分） R1 R2 IS R3 S + Us - iL(t) + uL(t) - L 1 , (0 ) (0 ) ( ) (0 ) 1 2 − = + = − + = L L S L A i i R R U i i L () = i L (US )+ i L (I S ) = 1+ 2.5 = 3.5A Req = (R1 + R2 )// R3 = 40 / 3 ( ) 20 3 s R L eq  = = R1 R2 + Us - iL (0- ) R1 R2 IS + Us - iL (∞) R1 R2 R3