電阻串聯時,電阻越大,分到的電壓越大。設串聯後的總電阻=Rt
10k:0.25Vs = Rt:Vs
Rt = 40kΩ
R = 40k-8k-10k-12k = 10kΩ
Vs提供40mW功率
Vs2 / Rt = 40m
Vs2 = 40m × 40k
Vs = 40V
Is = 40/40k =1mA
R 消耗功率 PR=1m × 1m × 10k =10mW