问题 选择题

如图所示电路,当开关S闭合后,L1、L2均能发光,电流表、电压表均有示数。过一会儿,灯都不发光,电流表、电压表的示数均为零,可能发生的故障是(  )

A.L1灯丝断了

B.L1短路

C.L2灯丝断了

D.L2短路

答案

答案:A

题目分析:当开关S闭合后,L1、L2均能发光,电流表、电压表均有示数,说明电路中各部位是好的.过一会儿,灯都不发光,电流表、电压表的示数均为零,如果电路发生短路,则电路中还有电流,不合题意.则电路发生断路.

A、L1灯丝断了,则整个电路中无电流,灯都不亮,电表无示数.符合题意.

B、L1短路,则L2中还有电流,L2会变亮,电表有示数.不合题意.

C、L2灯丝断了,则电压表与电路串联,电压表分担电源电压,有示数.灯L1不亮,电流表无示数.不合题意.

D、L2短路,则电压表也短路无示数.但L1中还有电流,L 1会变亮,电流表有示数.不合题意.

故选A.

点评:要学会使用电流表、电压表判断电路中的故障,要知道不同故障出现的不同情况.

阅读理解

阅读理解。

    The new iPhone 3G should please everyone. Its look and feel are only slightly improved,

but a faster network loads Web pages more quickly, true GPS functionality allows it to easily

find places nearby, and the new $199 price (down from $400) makes it an affordable luxury.

    Before deciding whether to buy, however, make sure you can actually take advantage of

the iPhone 3G's high-speed data network. 3G stands for third-generation, which in non-geek

speak translates to Web pages and mail messages that, ideally, load about three times faster

than on the original iPhone. Even better, 3G coverage enables you to make a phone call and surf

the Web at the same time. That's great, if you live or work in a place where the 3G network of

AT&T (美国电话电报公司) (the sole wireless carrier of the iPhone) is active. That's not so

great in cities like New York where AT&T's cellular coverage is awful. As one of my colleagues

in New York City, who bought the original iPhone, commented, "It's just a toy. You can't make

phone calls on it, so I carry my Verizon phone with me all the time."

    The real fun begins when you tap on the icon called "App Store" and start browsing the hundreds

of add-on applications that have been developed just for the iPhone. You'll find tons of games (I like

JirboBreak, a free game inspired by the Atari classic Breakout) and mobile versions of popular

websites like Pandora, Facebook, MySpace and the New York Times. Most apps will cost you,

but the vast majority are $9.99 or less. The apps work on the old iPhones too, but you'll enjoy them

a lot more on the iPhone 3G because many of the programs, including Yelp (local business reviews),

Whrrl (mobile social networking) and UrbanSpoon (restaurant reviews), use your exact location-

provided by the iPhone 3G's GPS chip-to make recommendations. The apps also load much faster

over the 3G network.

1. The new iPhone 3G has following features except that _____.

A. It looks and feels better

B. It has a faster network

C. It is easy to locate a place

D. It is too expensive for people to buy

2. Why do one of my colleagues carry Verizon phone with him all the time?

A. Because iPhone 3G is just a toy.

B. ecause he prefers Verizon phone better.

C. Because AT&T's cellular coverage is terrible there.

D. Because Verizon phone enables him make a phone call and surf the Web at the same time.

3. Which of the following statements is NOT true according to the passage.

A. App Store will offer you great fun.

B. many apps have been developed for the iPhone.

C. Most apps are very expensive.

D. Pandora, Facebook, MySpace and the New York Times are popular websites.

4. From the passage we know that the attitude of the author toward iPhone 3G is _____.

A. doubtful

B. appreciative

C. optimistic

D. pessimistic

填空题