两个标有“220V、40W”的灯泡串联接入220V电路，则每个灯泡的实际功率（ ）
A．等于40W
B．等于20W
C．等于10W
D．无法判断
由灯泡铭牌可知，灯泡额定电压是220V，额定功率是40W，
把两灯泡串联接入220V电路中，灯泡两端电压等于其额定电压的一半110V，
灯泡实际功率P实=
=U 2实 R
=(
)2U额 2 R
=U 2额 4R
P额=1 4
×40W=10W；1 4
故选C．
| 用框中适当的短语完成句子。 | |
2. My sister is always ill, she _____ does exercise. 3. My parents went to work, so I had to _____ myself. 4. — _____ do you exercise? —Once a week. 5. If I study hard, I believe (相信) I can _____. 6. The young (年轻人) should _____ the old _____ some cleaning. 7. I am a student; I should do my homework _____. 8. I want to play basketball, _____ homework, I will finish it later. 9. It is time for computer, everyone wants to _____. 10. I _____ do my homework. |