现有两个小灯泡，分别标有“6V、1.2W”和“12V、6W”的字样．现这两个灯泡串联起来，为了使其中的一个灯泡持续正常发光，那么加在串联电路两端的电压应该是（ ）
A．9.4伏特
B．10.8伏特
C．18伏特
D．27伏特
由P=UI可得，两灯泡的额定电流分别为：
I1=
=P1 U1
=0.2A，I2=1.2W 6V
=P2 U2
=0.5A，6W 12V
由I=
可得，两灯泡的电阻分别为：U R
R1=
=U1 I1
=30Ω，R2=6V 0.2A
=U2 I2
=24Ω，12V 0.5A
因串联电路中各处的电流相等，且其中的一个灯泡持续正常发光，
所以，电路中的电流为两灯泡额定电流较小的，即I=I1=0.2A，
因串联电路中总电阻等于各分电阻之和，
所以，电源的电压：
U=I（R1+R2）=0.2A×（30Ω+24Ω）=10.8V．
故选B．
请仔细阅读下面的短文，读后按信息表中项目要求，填写下面的表格。 Nowadays more and more people are worrying about their health. But how to keep healthy？Different people may give different answers to this question. In my opinion, it is important to do the following. First, we should take exercise every day. Plenty of exercise helps us build a strong body and makes us work better. That is why more and more people are becoming active in different kinds of sports and exercises. In morning many people get up early and take exercise. Some practice running while others walk or dance to music. In the afternoon, there are also many people doing sports. Some play basketball or volleyball, others play football or badminton. Second, have a healthy diet. Over-eat or too little food is very bad for our health. We should not eat too much or too little for every meal. We should eat healthy food, such as all sorts of grain, green vegetable and fruit. Third, give up bad habits like smoking and drinking. Because they are also bad for people’s health. All above are my thoughts about keeping healthy. If we can do as them, we will become healthier and stronger.
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