(2)当灯泡最暗时,U2=U-U1=36V-18V=18V
RL=U1^2/PL=(36V)^2/40W=32.4欧
I=U1/RL=18V/32.4欧=5/9A
P总=UI=36V*5/9A=20W
W=Pt=20W*600s=12000J