Uc(0+) = Uc(0-) = Is * (R1+R2) = 16V
S 闭合后：
用戴维南定理化简：
Uab = Is * R1 - Us1 = -2V
Rab = R1 = 2Ω
Uc(∞) = -2V
τ = Rab * C = 10
三要素法：
Uc = -2 + （16 +2）e^-t/10
= -2 + 18e^-0.1t
