外圈是闭合电路，电流 = 电源代数和 / 总电阻：I = (12 - 8) / 10 = 0.4AUab = 12 - 5 * 0. 4 = 10VUcd = -10 + Uab = 0Vhttp://wenku.baidu.com/view/1a26938d8762caaedd33d4bc.html
