戴维南定理解题步骤

断开R3；等效电压 U = Us2 + R2 压降 = Us2 + R2 * ( Us1 - Us2 )/( R1 + R2 )= 15 + 3( 20 - 15 )/( 5 + 3 ) = 135/8 v；等效电阻 R = R1//R2 = 3 * 5/( 3 + 5 ) = 15/8Ω；Is = U/( R3 + R ) = (135/8)/( 2 + 15/8 ) = 135/31 A 。