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．
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| As waiters,we deal with customers every day and when something goes wrong,somehow we have the ability to keep on going.It's our responsibility to_1_the customer. I remember my first_2_ job was at a fine restaurant.One afternoon,the manager informed all the cooks and waiters that a VIP_3_were coming for lunch.The group were_4 VIPs in the city's fashion industry. After the group arrived and got_5_comfortably,drinks were 6 . Normally,the head waiter would be in charge of wine service._7 ,the manager asked me to serve that afternoon.I had_8_done wine service, nor had I ever served a group of twelve. I arranged 12 wine glasses on the tray and went to the table.As I 9 one of the glasses in front of the first customer,somehow I had 10 my grip (抓,握) and six glasses fell on top of a young woman,and then to the floor and 11 in pieces.I felt frightened and my face turned red.I apologized many times to the 12 and to the rest of the group. Quickly,the manager 13 my aid and helped in the cleanup,and sent out their drinks right away.I didn't understand why,but the group were forgiving me and the young woman 14 informed me it was OK. My manager pulled me to the side and said,"If you know why an accident happened, 15 from it and move on.You still have a table to serve and you can't show your 16 or your disappointment. Mistakes and accidents do happen.Keep your 17 up and be confident.The glasses are replaceable." I understood why he was truly a well 18 restaurant manager within the neighborhood.His 19 relationship with the group made the situation 20 and the group forgave me for that. | ||||
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