问题 填空题

有一个标有“8V 4W”的小灯泡,如果接在12V的电源上,必须______联一个阻值为______Ω的电阻才能使小灯泡正常发光,此时在这个电阻上消耗的电功率是______W.

答案

灯泡正常发光时的电压为8V,功率为4W,如果接在12V的电源上,应串联一个电阻分压,

∵串联电路中总电压等于各分电压之和,

∴串联电阻两端电压:

UR=U-UL=12V-8V=4V,

∵串联电路中各处的电流相等,

∴根据P=UI可得,灯泡正常工作时电路电流:

I=

PL
UL
=
4W
8V
=0.5A,

根据I=

U
R
可得,串联的电阻阻值:

R=

UR
I
=
4V
0.5A
=8Ω,

电阻消耗的功率:

PR=URI=4V×0.5A=2W.

故答案为:串;8;2.

阅读理解

阅读理解。

     You often hear 3G these days, but do you know the meaning of it? Let me tell you. 3G means the third generation

(第三代) of mobile phones.   In 1995,   the first generation mobile phones came to our life.  We can only use them to

make a call. In 1996, people began to use the second generation mobile phones. Besides(除了) making a call, we can

also send short messages and receive e-mails. Some people also used them to watch news or download music, but it

was slow. What can you do with a 3G mobile phone? It has many functions(功能).   First,   it's a video phone. When

you call your friend, you can not only hear him,but also see him. Second, you can watch TV and quickly surf on the

Internet. Third, mobile wallet function(手机钱包功能).   If you don't have money, you can pay with a mobile phone.

And there are  also  more  functions which make our life convenient(方便).   Now,   do you want to have a 3G mobile

phone?

1. We can only make a call with a __________.   

A. 1Gmobile phone                    

B. 2Gmobile phone

C. 3G mobile phone                    

D. all of the above

2. The first generation mobile phones came to our life in __________.    

A. 1995            

B. 1996            

C. 1997            

D. 1998

3. The meaning of "download" is __________.  

A. 上传            

B.下载            

C.试听            

D.录制

4. We can use a 3G mobile phone to __________. 

A. watch TV                          

B. make a video call      

C. chat on QQ                        

D. all of the above

5. Which one is wrong?   

A. 3G means the third generation of computers.

B. We can send short messages with a 2G mobile phone.

C. 3G mobile phones have many functions.

D. We can watch TV with a 3G mobile phone.

填空题