灯泡L1标有“8V 4W”,L2标有“8V 2W”字样,现把它们串联接到某电源上,闭合开关后只有一个灯泡正常发光,另一个发光很暗,则该电源电压为_ V.

问题描述:

灯泡L1标有“8V 4W”,L2标有“8V 2W”字样,现把它们串联接到某电源上,闭合开关后只有一个灯泡正常发光,另一个发光很暗,则该电源电压为______ V.

灯泡L1的电阻:R1=

U 21
P1
=
(8V)2
4W
=16Ω,额定电流I1=
P1
U1
=
4W
8V
=0.5A;
灯泡L2的电阻:R2=
U 22
P2
=
(8V)2
2W
=32Ω,额定电流I2=
P2
U2
=
2W
8V
=0.25A;
所以电源电压U=I2(R1+R2)=0.25A×(16Ω+32Ω)=12V.
故答案为 12.