Question

题目，所示电路中，当R为何值时获得最大功率？

Answer 1

干路电流为
i = 12/[4R/(4+R)+3]
电阻R两端电压为
U = 12 - 3i = 12 - 36/[4R/(4+R)+3] = [48R/(4+R)]/[4R/(4+R)+3] = 48R/[4R+3(4+R)] = 48R/(7R+12)
于是电阻R功率为
P = U2/R = 2304R/(7R+12)2 = 2304/(49R+168+144/R)
当R = 12/7Ω时，Pmax = 2304/(84+168+84) = 48*48/84*4 = 48*3/21 = 48/7 W

Answer 2

当R=0欧姆的时候，也就是后面进行短路的时候，获得最大功率。