計算電流與電壓

I1 = I2 + I3
Ub1 = U1 + U3
Ub2 = U2 - U3
錯的是這個公式，應是相減不是相加。因為當你立式I1 = I2 + I3時，I3的方向是向下，若考慮右方的電路，電流在這部份和電源Ub2的方向相反。
U1 = I1 x 3300
U2 = I2 x 3300
U3 = I3 x 3300

I1 = I2 + I3
Ub1 = U1 + U3
Ub2 = U2 - U3
U1 = I1 x 3300
U1 = (I2+I3) x 3300
U2 = I2 x 3300
U3 = I3 x 3300

Ub1 = (I2+I3) x 3300 + I3 x 3300
10 = (I2+2I3) x 3300 ___(1)
Ub2 = U2 - U3
15 = I2 x 3300 - I3 x 3300 ___(2)
(1)式減(2)式
-5 = 3I3x3300
I3 = -5.05x10-4A = -0.505mA ___(3)
(3)式代入(2)式
15 = I2x3300 – 0.505x10-3 x 3300
I2 = 4.04mA
I1 = I2+I3 = 4.04 + (-0.505) = 3.54mA
U1 = I1 x 3300 = 3.54x10-3 x3300 = 11.7v
U2 = I2 x 3300 = 4.04x10-3 x3300 = 13.3v
U3 = I3 x 3300 = -0.505x10-3x3300 = -1.67v (負號表示電壓的方向應為下正上負)
這裡求出I3為負值表示最初設這分支的電流是向下，但實際是應為向上。

http://hk.geocities.com/patrickwongh2000/Ans.html

2006-11-01 01:34:23 補充：
Please visit the wesite for the detail analysis

Using kirchoff law, (attention must be paid to the direction of current flow and the direction of the EMF and volt drop across the resistors)

Ub1+U3=U1...................(1)

U2+U3=Ub2...................(2)

I1+I3=I2.........................(3)

then, solve for I1, I2 and I3.