问题 选择题

小明观察到傍晚时候家里的白炽灯比较暗,而在早晨时较亮,他进一步观察发现:傍晚电能表转动比早晨还快一些,这是什么原因呢?(  )

A.傍晚时家里的同盏白炽灯的额定功率比早晨时的小

B.傍晚时家里的同盏白炽灯电阻比早晨的大,实际功率减小

C.傍晚时家里的同盏白炽灯实际电压比早晨的小

D.傍晚时家里用电器的总功率比早晨的小

答案

(1)由于傍晚时分处于用电高峰期,各种电器都在工作,电路电流较大,输电线上损失的电压大,灯泡两端的实际电压较小,小于早晨时灯泡两端的电压,由于灯泡实际电压较小,由P=

U2
R
知,灯泡的实际功率较小,灯泡较暗;

(2)由于傍晚时家用电器总功率较大,早晨家用电器总功率较小,所以傍晚电能表转动比早晨快一些.

故选C.

单项选择题
阅读理解

The world’s biggest Internet search engine,Google,has proven once again to be king of the Web.Users can download“Google Wi-Fi”software and then enjoy free Wi-Fi service available in some US cities since last Tuesday.

Wi-Fi is short for“Wireless Fidelity”(无线上网).It allows laptop computers and personal digital assistants to connect to the Internet at high speed by radio signal.

A person with a Wi-Fi enabled computer can connect to the Internet when near one of the network’s access points.The place covered by one or several access points is called a hotspot.Google has already set up two“hotspots”in the US city of San Francisco which provide a free Wi-Fi service.

The Wi—Fi technology even allows users to enjoy a free Internet phone service.

Some scientists argue that Wi-Fi may replace mobile phone networks.

Wi Fi is still limited to a few cities.where there are“hotspots”.But its capability (容量)of sending information has already done better than that of the mobile phone network,even that of 3G(Third Generation telecommunication technology).Some scientists have started to call Wi-Fi 4G.

In addition to a high speed Internet connection,Wi-Fi has other advantages com pared to mobile phones.Wi-Fi is global.The same Wi-Fi system works in different countries around the world.Different from using cell phones,you don’t need to change computers to use Internet phones when you go to other places and most importantly,many Internet phones cost you nothing at all.

45.Wi-Fi is not available in cities without             

A.a computer                     B.a radio                    C.a hotspot                    D.a cell phone

46.Which of the following is NOT the advantage of Wi-Fi?

A.The high speed Internet connection.

B.Many free Internet phones.

C.Using the same computers for Internet phones when you go to other places.

D.Used in all the US cities

47.Some scientists started to call Wi-Fi 4G,because        

A.it is the product of the Google's 4th generation

B.it is more capable of sending information than the mobile phone network

C.it has been used by more than four generations

D.Wi-Fi will surely take the place of the mobile phone network

48.Which of the following is the best title?

A.Third Generation Telecommunication Technology

B.Free call Service

C.Google Launches Free Wi-Fi Service

D.Google, King of the Web