请画出等效电路和算出答案如图所示已知R1=5Ω、R2=20Ω、R3=10Ω、R4=2.5Ω、E=12.5V,(电池内阻忽

暂将R从电路移离,

a点电压=12.5v x R2/(R1+R2)=12.5 x 20/25=10v
b点电压= 12.5v x R4/(R3+R4)=12.5 x 2.5/12.5=2.5v
Uoc=Uab= Ua-Ub= 7.5v
等效电阻 (电压源置短路)Req= Rab=(R1//R2) +(R3//R4)
=100/25 + 25/12.5
= 4 + 2 = 6Ω
等效电路



将R接到等效电路的a b 点,i = Uoc/(Req+R)
= 7.5/20 =0.375A 或 375mA.

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