已知:灯泡的电压U1=24V,P1=40W,U=36V,t=5min=300S
U2=U-U1=36V-24V=12V,I=P1/U1=40W/24V=5/3A,R2=U2/I=12V/(5/3A)=7.2欧
P2=U2 I2=12V×(5/3A)=20W,W=P2t=20W×300S=6000J
2 已知:灯泡的额定电压U1=6V,额定电功率P2=6W,实际电功率P2=1.5W
求 :灯泡的电阻R,电源电压U
灯泡的电阻R=U12/P2=(6V)2/6W=6欧,U2=P2 R=1.5W×6欧=9V2 U=3V